The diversity of invertebrate visual opsins spanning Protostomia, Deuterostomia, and Cnidaria

Abstract

Across eumetazoans, the ability to perceive and respond to visual stimuli is largely mediated by opsins, a family of proteins belonging to the G protein-coupled receptor (GPCR) superclass. Lineage-specific gains and losses led to a striking diversity in the numbers, types, and spectral sensitivities conferred by visual opsin gene expression. Here, we review the diversity of visual opsins and differences in opsin gene expression from well-studied protostome, invertebrate deuterostome, and cnidarian groups. We discuss the functional significance of opsin expression differences and spectral tuning among lineages. In some cases, opsin evolution has been linked to the detection of relevant visual signals, including sexually selected color traits and host plant features. In other instances, variation in opsins has not been directly linked to functional or ecological differences. Overall, the array of opsin expression patterns and sensitivities across invertebrate lineages highlight the diversity of opsins in the eumetazoan ancestor and the labile nature of opsins over evolutionary time.

1. Introduction

Opsins are a family of G protein-coupled receptors (GPCRs) that, among other functions, mediate vision across Eumetazoa. Opsin proteins consist of seven transmembrane helices characterized by a lysine in the seventh helix. This lysine serves as the attachment site for a vitamin A-derived, light-sensitive chromophore that, when covalently bound to an opsin protein, comprise a functional visual pigment. Upon exposure to light, a change in the conformational state of the chromophore, typically 11-cis-retinal to all-trans retinal, drives the activation of the visual pigment (for reviews, see Shichida and Matsuyama, 2009; Terakita, 2005; Tsukamoto and Terakita, 2010).

Visual pigments are sensitive to spectral compositions of light, which are defined by the maximally absorbed wavelength (λ_max). While the chromophore alone has a λ_max between 377 and 400 nm (Hubbard et al., 1953; Tsukida et al., 1980), the interaction of the chromophore with different opsin proteins leads to a wide range of sensitivities, ranging from 330 nm (ultraviolet light) to 600 nm (red light) (e.g., Bok et al., 2014; Briscoe and Chittka, 2001). Apart from various indirect mechanisms that affect visual sensitivity (e.g., screening pigments, oil droplets etc.), λ_max values are largely determined by the distinct interactions between the chromophore and the associated opsin protein.

Phylogenetically, opsins can be divided into several distinct clades (Arendt et al., 2004; Plachetzki et al., 2007; Plachetzki et al., 2010; Porter et al., 2012; Ramirez et al., 2016). Here, we briefly introduce the opsin clades that we discuss throughout this review:

• C-opsins: Canonical c-type opsins (hereafter referred to as c-opsins) are characteristically observed in ciliary photoreceptors in the eyes of deuterostomes. Ciliary photoreceptors hyperpolarize in response to light using a Gt (transducin)-mediated signaling cascade and photo-bleach after photoactivation, breaking the association between the opsin and the bound chromophore (see Shichida and Matsuyama, 2009; Terakita, 2005; Tsukamoto and Terakita, 2010).

• R-opsins: Canonical r-type opsins (hereafter referred to as r-opsins) are characteristically observed in rhabdomeric photoreceptors in the eyes of protostomes. Rhabdomeric photoreceptors depolarize in response to light using a Gq-type signal transduction pathway and are typically bistable, retaining the chromophore in both the native (dark) and photoactivated (light) states (see Shichida and Matsuyama, 2009; Terakita, 2005; Tsukamoto and Terakita, 2010).

• Cnidops: In contrast to r- and c-type opsins, which have broad taxonomic representation in expression patterns, the cnidops clade is composed entirely of cnidarian and ctenophore opsins (Liegertová et al., 2015; Plachetzki et al., 2007; Porter et al., 2012). Cnidops are expressed in retinal ciliary photoreceptors, CNS neurons, and gonad cells (Liegertová et al., 2015; Plachetzki et al., 2007; Suga et al., 2008). Phototransduction, in at least some cnidops, is mediated via a Gs-coupled signaling pathway (Koyanagi et al., 2008b), although cnidops do not universally employ a Gs-signaling cascade (Liegertová et al., 2015).

• Group 4/Tetraopsins: The Group 4 opsins or tetraopsins include RGR/retinochromes/peropsins, Go-opsins, and neuropsins (Porter et al., 2012; Ramirez et al., 2016). The opsins within this clade form a monophyletic group with representatives in protostomes and deuterostomes (Ramirez et al., 2016). Among these opsins, we will focus on Go-opsins. Go-opsins function in both ciliary and rhabdomeric photoreceptors (Gühmann et al., 2015; Kojima et al., 1997) and employ a Go-mediated transduction cascade, leading to hyperpolarization in response to light when expressed in ciliary photoreceptors or depolarization when expressed in rhabdomeric photoreceptors (Gühmann et al., 2015; Kojima et al., 1997).

• Xenopsins: Xenopsins have been identified in several lophotrochozoan species (Döring et al., 2020; Passamaneck et al., 2011; Ramirez et al., 2016; Rawlinson et al., 2019; Vöcking et al., 2017). Xenopsins are associated with ciliary photoreceptors and employ a similar signaling cascade as c-opsins. However, xenopsins can also be co-expressed with r-opsins in photoreceptor cells that have features of both rhabdomeric and ciliary photoreceptors (Döring et al., 2020; Rawlinson et al., 2019; Vöcking et al., 2017).

Within these major opsin clades, c- and r-opsins form distinct phylogenetic groups that generally correspond to the wavelengths of light that they confer sensitivity to. For r-opsins, these are the long-wavelength (LW), medium-wavelength (MW), and short-wavelength (SW)/ultraviolet (UV) sensitive clades (Porter et al., 2007, 2013). In some lineages (see Sec. 2), duplications of the SW/UV opsin clade form two distinct clades of SW/blue (B) sensitive and ultraviolet (UV) sensitive opsins (Briscoe and Chittka, 2001; Koyanagi et al., 2008a). However, even within opsin clades, a large diversity of visual pigment sensitivity exists. In this review, we discuss the diversity of opsin expression and sensitivities across invertebrate lineages (see Fig. 1 for lineages discussed) and potential ecological and behavioral functions of these changes in vision across species.

Fig. 1.

The invertebrate lineages that are the focus of this review with illustrations of representative species for each group.

2. Visual opsins in arthropods

Arthropoda is divided into three major extant subphyla: Pancrustacea (Insecta + Crustacea), Chelicerata, and Myriapoda. We focus on patterns of opsin expression in Insecta, Crustacea, and Chelicerata (Fig. 1) as studies of myriapod opsins are limited (but see Chipman et al., 2014; Fleming et al., 2018; Kenny et al., 2015).

Arthropods use r-opsins for vision. The arthropod ancestor likely possessed three to five r-opsins, with one UV-sensitive opsin and the remainder sensitive to light in the visible spectrum (Henze and Oakley, 2015; Koyanagi et al., 2008a), representing opsins from the SW, MW, and LW opsin clades (Henze and Oakley, 2015). Opsin duplications and losses diversified patterns of opsin expression within Arthropoda (e.g., Guignard et al., 2022; Henze and Oakley, 2015). For example, while extant insects lack the MW opsin clade that is still present in crustaceans and chelicerates, the insect SW/UV clade has diversified to form distinct SW and UV sensitive clades (Fig. 2) (Henze and Oakley, 2015; Koyanagi et al., 2008a).

Fig. 2.

Phylogeny illustrating the relationship between invertebrate opsins genes. Vertebrate c-opsin genes are also included to illustrate their relationship to invertebrate deuterostome opsins (i.e., Ciona intestinalis Ci-opsin1). Color coding within the Arthropod UV/SW, MW, and LW opsin groups insect (blue), crustacean (orange), or chelicerate (grey) lineages. Grey filled circles indicate nodes with >95% bootstrap support, unfilled circles indicate nodes with 90–94% bootstrap support, and circles with dashed outlines indicate nodes with 80–89% bootstrap support. For a full phylogeny including outgroup taxa and bootstrap values, see Supplementary Fig. 1.

In some cases, Arthropod opsin expression, visual sensitivity, and visual behaviors have been particularly well-studied. Below, we discuss opsin expression and potential roles of opsins in the ecology and behavior of these groups.

2.1. Insecta

Insects typically possess at least one copy of the UV, SW, and LW opsin classes (Briscoe and Chittka, 2001; Guignard et al., 2022), conferring sensitivity to UV (~350 nm), blue (~440 nm), and green (~530 nm) wavelengths of light, respectively (Briscoe and Chittka, 2001). Opsin losses and duplications generated diversity within insect opsins, with duplications occurring in all insect opsin classes and losses of the UV, SW, or both in some groups (Guignard et al., 2022).

2.1.1. UV opsins

Most insects possess a single UV opsin copy, with losses only noted in the orders Trichoptera (caddisflies) and Siphonaptera (fleas) (Guignard et al., 2022). UV opsin duplications occurred in some families within Ephemeroptera (mayflies), Hemiptera (true bugs), Raphidioptera (snakeflies), Coleoptera (beetles), Lepidoptera (moths, butterflies), and Diptera (flies) (Almudi et al., 2020; Briscoe et al., 2010; Feuda et al., 2020; Guignard et al., 2022; Lord et al., 2016; Sharkey et al., 2017). UV sensitivity is driven by a single amino acid polymorphism, with a lysine residue in position 90 relative to bovine rhodopsin, while asparagine or glutamate is in this position for blue sensitive opsins (Salcedo et al., 2003).

The typical λ_max of the UV opsin is 330 nm–400 nm (Fig. 3), although the UV opsin can also confer sensitivity to blue light. An example comes from Coleoptera, where some species are sensitive to 420–460 nm light, despite lacking a SW sensitive opsin across the order (Fig. 3C) (Lord et al., 2016; Sharkey et al., 2017). Several amino acid substitutions in the coleopteran UV opsin correspond to known tuning sites in invertebrate and vertebrate species, suggesting conserved mechanisms of opsin tuning across diverse taxa (Lord et al., 2016).

Fig. 3.

Insect phylogeny with representative species for (D) Diptera, (L) Lepidoptera, (C) Coleoptera, (H) Hymenoptera, and (O) Odonata showing the r-opsin classes and λ_max values of recorded visual sensitivities. Number and type of opsins identified from each species is shown below the species name. The color of the squares illustrates an approximation of spectral sensitivity between the UV (dark purple) and visible spectrum, relative to other values. When the opsin associated with a specific sensitivity is known, it is listed below the λ_max value. Asterisks (*) indicate λ_max values come from a different species of the same genus, a cross (†) symbol indicates that the reported λ_max value is the midpoint between a measured range, and a dashed border around a square indicates sensitivity recorded from ocelli. The relationship between insect orders were inferred from Ishiwata et al. (2011) and Misof et al. (2014) and relationships between lepidopteran species were inferred from Timmermans et al. (2014). For λ_max references, see Supplemental Table 2.

Specific behaviors involving UV sensitivity include detection and discrimination of flowers (Dyer and Chittka, 2004; Horth et al., 2014), polarized light vision (Brines and Gould, 1979; Schwind, 1991), and orientation (Möller, 2002). UV opsin duplications are often associated with visually guided behavior, including flower detection, hunting, and mate choice (Almudi et al., 2020; McCulloch et al., 2017; McCulloch et al., 2016; Sharkey et al., 2017). For example, the evolution of UV-yellow coloration, based on the pigment 3-hydroxykynurenine (3-OHK), is correlated with the UV duplication event in Heliconius (Fig. 3L, Heliconius erato UVRh1 and UVRh2) (Briscoe et al., 2010). Modeling suggested that the UV duplication functions in conspecific recognition by improving discrimination of 3-OHK coloration from the non-3-OHK based coloration of heterospecific individuals (McCulloch et al., 2017). This corresponds with behavioral observations showing that male H. erato prefer 3-OHK yellow coloration over non-3-OHK yellow coloration in choice assays (Finkbeiner et al., 2017).

2.1.2. SW opsins

An SW opsin is broadly maintained across Insecta, as a recent, widescale study identified SW opsin homologs in over 60% of families (Guignard et al., 2022). Duplications of the SW opsin occurred in several orders, including Lepidoptera and Odonata (dragonflies, damselflies), which will be discussed below. Duplications of the SW opsin are particularly abundant in Paleoptera (Odonata and Ephemeroptera), where most families surveyed have at least 2 SW opsins (Futahashi et al., 2015; Guignard et al., 2022).

The insect SW opsin is typically sensitive to 430–500 nm light (Fig. 3). Amino acid substitutions responsible for sensitivity shifts have been identified from some species of butterflies using site-directed mutagenesis. Spectral tuning in Lycaenid and Pierid butterflies, which have independent duplications of the SW opsin, is influenced by amino acid substitutions at sites 116 and 177 (relative to squid rhodopsin) (Liénard et al., 2021; Wakakuwa et al., 2010). The Pierid Pieris rapae, has a violet-sensitive (PrV: λ_max = 425 nm) and blue-sensitive (PrB: λ_max = 450 nm) SW opsin with Ala116 + Tyr177 and Ser116 + Phe177, respectively. Double PrB mutants (Ser116Ala + Phe177Tyr) display a short-wavelength shift with approximately the same sensitivity as the PrV opsin (Wakakuwa et al., 2010). In the Lycaenid Eumaeus atala, which has a blue-sensitive SW opsin (BRh1: λ_max = 435 nm) with Ala116 and Try177 and blue-green-sensitive SW opsin (BRh2: λ_max = 500 nm) with Ser116 and Phe117, the reverse substitutions (Ala116Ser + Tyr177Phe), along with additive effects of additional tuning sites, explains the blue-shift in the sensitivity of BRh2 (Liénard et al., 2021).

Duplications of the SW opsin are hypothesized to function in sexual selection in some lepidopterans. In male Pieris butterflies, filtering of the duplicated SW opsin via pigments within the eye is thought to aid in spectral discrimination among females and males (Fig. 3L) (Arikawa et al., 2005). In Lycaenid butterflies, the presence of two SW opsins has been hypothesized to improve discrimination of the blue wing coloration characteristic of many Lycaenid species (Sison-Mangus et al., 2006). Accordingly, there is high overlap between the λ_max of the duplicated SW opsin and reflectance measurements of wing coloration from E. atala butterflies (Liénard et al., 2021), suggesting a role for opsin duplication in the detection of color signals.

Patterns of SW opsin expression changes over developmental stages may also correspond with the saliency of visual signals. For example, the eyes of adult dragonflies express multiple SW opsin copies (three SW opsin genes expressed in Sympetrum frequens and Orthetrum albistylum) while only one SW opsin gene is expressed in nymph eyes (Futahashi et al., 2015). This may correspond to the increased relevance of visual signals for adult dragonflies, which are active flyers and rely on vision for predation and mate detection (Corbert, 1999). Consistent with this, an SW opsin has not been identified in several orders that inhabit low-light environments, and may therefore rely less on vision for their behavior (Guignard et al., 2022; Jadhav and Sharma, 2012; James et al., 2016; Liu, 2021; Vas et al., 1999; Wright et al., 2013).

2.1.3. LW opsins

The insect LW clade can be divided into the LW2a and LW2b opsin groups. LW2a opsins are primarily expressed in the compound eye while LW2b opsins are typically ocelli-specific (Guignard et al., 2022). All insect orders studied have at least one copy of the LW2a opsin, while the LW2b is found in about half of insect orders studied (Guignard et al., 2022). Duplications of LW2b have been identified from some Brachycera (Diptera) and Odonata (Futahashi et al., 2015), although in Drosophila melanogaster, one LW2b copy (Rh1) is expressed in the compound eye. Duplications of the LW2a opsin class are more common, occurring broadly in Odonata and Ephemeroptera, and several families within Hemiptera, Coleoptera, Lepidoptera, Diptera, Strepsiptera (twisted-wing parasites), and Thysanoptera (thrips) (Guignard et al., 2022).

LW sensitivities typically vary from 500 nm to >560 nm (Fig. 3). The sensitivities of LW opsins that are expressed in the ocelli tend to differ from the LW opsins in the compound eye (Aksoy and Camlitepe, 2018; Feiler et al., 1988; Goldsmith and Ruck, 1958; Henze et al., 2012; Menzel and Blakers, 1976; Salcedo et al., 1999). In honeybees (Apis mellifera), LW opsins expressed in the compound eyes have sensitivity with λ_max = 540 nm while LW opsins expressed in the ocelli have sensitivity to 490 nm light (Fig. 3H) (Goldsmith and Ruck, 1958; Menzel and Blakers, 1976). In D. melanogaster, Rh2, which is specifically expressed in the ocelli, shares a close phylogenetic relationship with Rh1, which is expressed in the compound eye (Feiler et al., 1988; Mismer et al., 1988; O’Tousa et al., 1985; Pollock and Benzer, 1988). Both Rh1 and Rh2 opsins are phylogenetically grouped with insect LW opsins (Fig. 2) (Guignard et al., 2022), but have short-wavelength shifted sensitivities (Rh1 λ_max = 478 nm; Rh2 λ_max = 420 nm) relative to the green-sensitive LW opsin (Rh6, λ_max = 508 nm) that is expressed in the compound eye (Fig. 3D) (Feiler et al., 1988; Salcedo et al., 1999).

Tuning sites for LW opsins have been identified through experiments in D. melanogaster (Salcedo et al., 2009). Substitutions between Ser and Ala at site 292 (relative to bovine rhodopsin) shifts sensitivity for Rh1 and Rh6. For Rh1, Ala292Ser resulted in a short-wavelength shift in sensitivity, while the reverse substitution in Rh6 shifted sensitivity to longer wavelengths (Salcedo et al., 2009). Similar substitutions between Ala and Ser at this site also affect spectral tuning in many vertebrate species (for review see Hagen, Roberts, & Johnson, this issue). In contrast, sites important for generating differences in vertebrate green- and red-sensitive opsins are invariant in the spectrally diverse LW opsins of the butterfly Papilio glaucus (see Papilio xuthus for sensitivity of a related species, Fig. 3L), suggesting differences in LW tuning mechanisms between insects and vertebrates (Briscoe et al., 2010).

The role of the LW opsin in insect behavior varies across functions performed by the compound eyes and ocelli. For example, the ocelli function in flight regulation, polarized light detection, navigation and orientation, and circadian timing of activity (Guignard et al., 2022; Honkanen et al., 2018; Narendra et al., 2016; Schwarz et al., 2011a,b; Schwarz et al., 2011; Taylor et al., 2016; Wunderer & Jan De Kramer, 1989). In addition to the LW opsin, the ocelli also often express a UV-sensitive opsin (Chappell and Devoe, 1975; Eaton, 1976; Guignard et al., 2021; Henze et al., 2012; Sontag, 1971). Expression of LW and UV opsins in the ocelli may enable discrimination between the predominately green foreground and the sky (Möller, 2002). For LW2b opsins expressed outside of the ocelli, functions include motion vision in D. melanogaster (Heisenberg and Buchner, 1977; Yamaguchi et al., 2008) and light detection in male fig wasps, Ceratosolen solmsi, which lack ocelli (Wang et al., 2013).

LW opsin duplication is also associated with taxa that have aquatic life stages. LW duplications are common in the orders Odonata (Fig. 3O) and Ephemeroptera and in the suborder Culicomorpha (mosquitoes and lake flies) (Futahashi et al., 2015; Guignard et al., 2022), which have aquatic nymph and terrestrial adult stages. In Culicidae and Odonata, differences in LW opsin expression are observed between nymph and adult stages (Futahashi et al., 2015; Jenkens and Muskavitch, 2015). LW opsin is more highly expressed relative to UV and SW opsin in nymphs, and subsets of LW opsins are differentially expressed between nymphs and adults (Futahashi et al., 2015; Jenkens and Muskavitch, 2015). These differences may be due to a LW-light skew in the aquatic environments inhabited by nymphs (Futahashi et al., 2015; Levine and MacNichol, 1982).

LW opsin duplications also occurred in the family Gerridae (water striders) (Guignard et al., 2022). LW opsins in the water strider Gerris buenoi are predicted to be short wavelength shifted, accounting for the blue-light sensitivity recorded from this species (Armisén et al., 2018) considering that no SW opsins have been identified in this group (Guignard et al., 2022). Like duplications of the UV opsin class following loss of SW opsins in Coleoptera, LW opsin duplications may counteract loss of other opsin classes. In support of this idea, nearly a quarter of duplications within the LW2a/LW2b clades are from families that lack an SW opsin (Guignard et al., 2022).

Extended spectral sensitivities of LW opsins into yellow-red wavelengths function in visual behaviors. For example, Papilio butterflies possess three LW opsin copies, with sensitivity to 515 nm, 530 nm, and 575 nm light (Fig. 3L) (Arikawa et al., 1999; Arikawa et al., 1987; Kitamoto et al., 1998). Visual sensitivity to red wavelengths improves detection of green coloration, improving detection of healthy host plants for egg laying by females (Kelber, 1999). Detection of red light likely benefits the flower pollinating beetle Pygopleurus israelitus, by improving detection of red flowers (Martínez-Harms et al., 2012).

2.2. Crustacea

Crustaceans possess opsins from the SW/UV, MW, and LW opsin clades (Fig. 2). The SW opsins are sensitive to light in the ultraviolet to violet-blue part of the spectrum (λ_max = 330 nm–450 nm) (Fig. 4) (Bok et al., 2014; Cronin et al., 1994; Cummins and Goldsmith, 1981; Horch et al., 2002; Porter et al., 2013; Wald, 1968). The MW and LW opsins confer sensitivity to blue (~480 nm) and blue-green (500–530 nm) light, respectively (Fig. 4) (Crandall and Cronin, 1997; Cronin and Marshall, 1989; Porter et al., 2013; Sakamoto et al., 1996). Opsin duplications have been observed in many crustaceans and, in some cases, there have been large opsin gene expansions (e.g., Porter et al., 2020; Zhang et al., 2019).

Fig. 4.

Crustacean phylogeny with representative species for (D) Decapoda, (S) Stomatopoda, and (B) Branchiopoda showing the r-opsin classes and λ_max values of recorded visual sensitivities. Number and type of opsins identified from each species is shown below the species name. The color of the squares illustrates an approximation of spectral sensitivity between the UV (dark purple) and visible spectrum, relative to other values. When the opsin associated with a specific sensitivity is known, it is listed below the λ_max value. Asterisks (*) indicate λ_max values come from a different species of the same genus and a cross (†) symbol indicates that the reported λ_max value is the midpoint between a measured range. Relationships between Crustacea were inferred from Chang and Lai (2018) and Wolfe et al. (2019). For λ_max references, see Supplemental Table 2.

Compared to insects, less is known about the roles of specific opsins and opsin duplications in behavior in other arthropods. As such, we present the roles of opsin expression in crustaceans in a single section (Sec. 2.2.3) following the description of opsin diversity within the SW, MW, and LW clades.

2.2.1. SW/UV opsins

SW opsins have been identified in various crustaceans, including species from the orders Stomatopoda (mantis shrimp), Decapoda (crabs, lobsters), and Argulidae (fish lice), and from the class Branchiopoda (fairy shrimp) (Brandon et al., 2017; Colbourne et al., 2011; Henze and Oakley, 2015; Kashiyama et al., 2009; Kingston and Cronin, 2015).

The SW/UV opsin clade in Crustacea is sensitive to UV and violet-blue light (Fig. 4). In Daphnia pulex and Daphnia magna, which have duplicated SW/UV opsins with predicted sensitivities to UV (λ_max ≈ 350 nm) and blue (λ_max ≈ 430 nm) light (Fig. 4B) (Brandon et al., 2017; Smith and Macagno, 1990), amino acid differences were observed in sites associated with spectral tuning in other species (Brandon et al., 2017). Of particular interest is a Ser in the blue sensitive opsin and an Ala in the UV sensitive opsin at position 116, which similarly causes a blue shift in sensitivity in the butterfly P. rapae (Brandon et al., 2017; Wakakuwa et al., 2010).

2.2.2. MW and LW opsins

MW or LW opsins are broadly present in crustaceans (Biscontin et al., 2016; Crandall and Cronin, 1997; Henze and Oakley, 2015; Kingston and Cronin, 2015; Porter et al., 2009; Rajkumar et al., 2010; Sakamoto et al., 1996; Zhang et al., 2019, 2021).

Notable examples of MW and LW duplications in Crustacea have been observed in the Pacific white shrimp Litopenaeus vannamei (Decapoda), stomatopods (e.g., Neogonodactylus oerstedii, Fig. 4S, and species of branchiopods (e.g., Daphnia magna, Fig. 4B). Seven MW and 33 LW opsin genes (in addition to 2 SW) have been identified in L. vannamei, 10 MW and 20 LW opsin genes (in addition to 3 SW) have been identified in N. oerstedii (Porter et al., 2020), and as many as 19 MW opsin genes (sometimes termed long-wave A/B) have been identified in Daphnia species (Brandon et al., 2017; Colbourne et al., 2011; Zhang et al., 2021). In some species with abundant opsin classes, visual sensitivity is likewise diverse, as in N. oerstedii and D. magna (Fig. 4) (Bok et al., 2014; Cronin and Marshall, 1989; Smith and Macagno, 1990). In other cases, opsin duplications do not expand spectral sensitivity. In the brachyuran crab Hemigrapsus sanguineus, two MW opsin classes appear to be sensitive to the same wavelength of light (Fig. 4D) (Sakamoto et al., 1996).

Amino acid changes that shift spectral sensitivity have been identified from several species of crustaceans (Brandon et al., 2017; Crandall and Cronin, 1997; Porter et al., 2020). Many of these substitutions have been implicated in spectral tuning in other species, including vertebrates (Brandon et al., 2017; Porter et al., 2020; Salcedo et al., 2009; Takahashi and Ebrey, 2003; Wakakuwa et al., 2010), suggesting conserved effects of amino acid substitutions on spectral sensitivities across diverse lineages.

2.2.3. Opsins, ecology, and behavior

While many crustaceans are capable of dichromatic vision based on the expression of two different opsins with distinct sensitivities, some species possess numerous spectrally distinct opsins suggesting that these organisms are capable of polychromatic vision (Fig. 4). The role of numerous opsin duplications in color vision is perhaps best studied in stomatopods. Despite possessing 12 distinct color channels, stomatopod vision is generally course, as individuals do not discriminate color differences between 12 and 25 nm (Thoen et al., 2014). These data suggest that stomatopods do not use their abundant opsin classes for color vision using conventional color-opponency mechanisms, but may rather recognize color by incorporating signals from each of their 12 distinct color channels via temporal signals generated by scanning eye movements (Thoen et al., 2014).

In crustaceans, environmental light conditions appear to affect patterns of opsin expression. In L. vannamei, proportionally more LW opsins are expressed in adults (Zhang et al., 2019), which prefer to feed in yellow-red lighting environments (Luchiari et al., 2012) while larvae express higher levels of MW opsin genes, coinciding with the higher availability of green light in planktonic habitats (Zhang et al., 2019). Similarly, stomatopod species inhabiting shallower water, which has greater light availability and spectral richness than deep water, express more opsin classes and have a broader range of spectral sensitivity than species inhabiting deeper waters (Porter et al., 2013, 2020). In contrast, there was no correlation between habitat differences and variation in spectral sensitivity between several species of crayfish (Crandall and Cronin, 1997), suggesting that environmental conditions may not universally influence visual sensitivity.

2.3. Chelicerata

Chelicerates contain opsins from the SW/UV, MW, and LW opsin clades (Fig. 2) (Battelle et al., 2014, 2015, 2016; Morehouse et al., 2017). Corresponding visual sensitivities have been recorded with λ_max ≈ 350 nm (Battelle et al., 2014; Nagata et al., 2012; Walla et al., 1996), λ_max = 525 nm (Battelle et al., 2014), and λ_max = 480–520 nm (Knox et al., 2003; Nagata et al., 2012; Walla et al., 1996) (Fig. 5). Opsin expression has been best studied in Araneae (spiders) and Xiphosura (horseshoe crabs), which will be the primary focus of this section.

Fig. 5.

Chelicerate phylogeny with representative species for (A) Araneae and (X) Xiphosurida showing the r-opsin classes and and λ_max values of recorded visual sensitivities. Number and type of opsins identified from each species is shown below the species name. The color of the squares illustrates an approximation of spectral sensitivity between the UV (dark purple) and the visible spectrum, relative to other values. When the opsin associated with a specific sensitivity is known it, is listed below the λ_max value. For λ_max references, see Supplemental Table 2.

2.3.1. UV opsins

Opsins from the SW/UV opsin class are commonly expressed in Chelicerate species, conferring sensitivity to UV light (Fig. 5). As SW/UV opsins in chelicerates confer sensitivity to UV light (Battelle et al., 2014; Nagata et al., 2012), we will refer to opsins in the SW/UV opsin clade as UV opsins within this section.

Expression of one UV opsin in the eye of the horseshoe crab, L. polyphemus, has been well-characterized (Fig. 5X). Opsin transcripts for three additional opsins belonging to the SW/UV clade have also been identified suggesting additional complexity (Battelle et al., 2016). A UV opsin has also been identified from many species of Araneae spiders, as well as representative species from Scorpiones (scorpions) and Ixodida (ticks) (Morehouse et al., 2017). However, in a survey of 13 species spanning Aranea, about 30% of species were not found to contain opsins from the SW/UV clade (Morehouse et al., 2017). It is still unclear whether these differences represent true losses or simply results of methodological limitations. For example, a SW/UV opsin duplication has been identified from the well-studied jumping spider Hasarius adansoni (Fig. 5A)(Nagata et al., 2012), despite previous studies that identified only one SW/UV opsin copy from this species (Koyanagi et al., 2008a).

2.3.2. MW and LW opsins

Though a MW opsin is expressed in the horseshoe crab L. polyphemus (Fig. 3X)(Battelle et al., 2015, 2016), no MW opsin has been recovered from the majority of other chelicerate species studied to date, with the exception of one species of spider (Morehouse et al., 2017). In contrast, the LW opsin clade is commonly found in chelicerates, with many duplications (Battelle et al., 2015, 2016; Morehouse et al., 2017). LW duplications occurred in Xiphosurida (Battelle et al., 2015, 2016), many Araneae (Koyanagi et al., 2008a; Morehouse et al., 2017), and the scorpion Mesobuthus martensii (Morehouse et al., 2017). In Araneae, one copy of the LW opsin is sensitive to green light (Rh1: λ_max = 520 nm) while the second is sensitive to blue light (Rh2: λ_max = 480 nm) (Fig. 5A) (Nagata et al., 2012). The Rh1 and Rh2 opsins share about 60% amino acid sequence similarity (Koyanagi et al., 2008a), although specific changes that may underlie sensitivity changes have not been identified to date.

2.3.3. Opsins, ecology, and behavior

The roles of opsin expression are different among chelicerate species. Three spectrally distinct opsins across two classes are found in some Araneae, while three opsin classes confer limited spectral sensitivity to UV and green light in Xiphosurida (Fig. 5). Despite the presence of three spectrally distinct opsins in some spiders, color vision in the acute ‘principal’ eyes of spiders relies only on sensitivity to UV and green light (Blest et al., 1981; DeVoe, 1975; Nagata et al., 2012). Trichromatic vision is nevertheless possible via spectral filtering of green-sensitive opsin, resulting in sensitivity to longer-wavelengths (λ_max = 630 nm) (Zurek et al., 2015). UV signals also influence the attractiveness of male jumping spiders Cosmophasis umbratical (Lim et al., 2008), supporting a role of UV sensitivity in sexual behaviors. There is no evidence of color vision in Xiphosurids like L. polyphemus/horseshoe crabs, but dramatic changes in opsin expression in response to light and an internal circadian clock suggest that opsins function in altering photoresponses (Battelle et al., 2015).

Opsin expression appears to correspond to light environment in many chelicerates. In Aranea, UV opsins are absent in most nocturnal species (Morehouse et al., 2017), in line with opsin loss associated with low-light environments in other species (Futahashi et al., 2015; Jacobs, 2013; Sharkey et al., 2017; Sondhi et al., 2021). In contrast, LW opsin expansion is noted in several nocturnal chelicerates. Numerous LW opsin copies are found in species of nocturnal spiders and horseshoe crabs (Battelle et al., 2016; Morehouse et al., 2017). In L. polyphemus, high expression of LW opsins at night (Battelle et al., 2013; Katti et al., 2010) may contribute to increased sensitivity of the lateral eyes to low light (Barlow et al., 1977), aiding in the detection of mates during nocturnal spawning (Barlow et al., 1982).

2.4. Summary of Arthropod opsin expression

Expression of the SW, MW, and LW opsin classes occurs in distinct patterns among arthropod lineages. Despite this, there are several similarities noted between groups as well. UV and MW (or LW) opsins are often present together (Figs. 3–5), meaning that most arthropods have the capacity for dichromatic vision. Many opsin duplications are associated with increased spectral classes, allowing for increased color vision capabilities, and the amino acid substitutions responsible for tuning shifts are sometimes conserved among lineages. Insects represent some of the best studied arthropods in terms of linking opsin expression to behavior and ecology, with some studies in insects finding specific functions attributed to opsin properties. General patterns across arthropods suggest that changes in opsin expression are associated with differences in light environment and that visual sensitivity often appears tuned to environmental lighting conditions.

3. Visual opsins in Lophotrochozoa

Lophotrochozoans are a monophyletic group of protostomes that includes cephalopods, bivalves, and polychaetes (Fig. 1). Rhabdomeric and ciliary photoreceptors have been identified in the visual structures of lophotrochozoans (e.g., McReynolds and Gorman, 1970) expressing many diverse opsin types (Bok et al., 2017a,b; Bok et al., 2017; Kojima et al., 1997). In this section, we discuss r-opsins, c-opsins, Go-opsins, and xenopsins in Mollusca (bivalves, cephalopods) and Polychaeta (annelid worms). Because there are lineage-specific patterns of opsin co-expression between r-, c-, and Go-opsins, we outline expression of these opsin types together. We then discuss emerging data on xenopsins and their roles in behavior.

3.1. R-opsin, C-opsin, and go-opsins

Rhabdomeric photoreceptors, expressing two r-opsins and a Go-opsin, have been found in the larval eyespots and pigment-cup eyes of the polychaete Platynereis dumerilii (Arendt et al., 2002, 2004; Ayers et al., 2018; Gühmann et al., 2015; Randel et al., 2013). The P. dumerilii genome contains a second Go-opsin, however expression of this gene has not been characterized experimentally (Gühmann et al., 2015). R-opsins have also been identified from the rhabdomeric photoreceptors of mollusks (Fig. 6L), with two r-opsins identified in many bivalves (Serb et al., 2013) and one r-opsin identified across several cephalopod species (Chung and Marshall, 2016). A second r-opsin has been identified in the cephalopod Sepia officinalis, although expression of this opsin has not been observed in the eye (Bonadè et al., 2020).

Fig. 6.

Phylogeny of (L) lophotrochozoan, (D) invertebrate deuterostome, and (C) cnidarian species showing opsin classes with known or putative visual functions and λ_max values of recorded visual sensitivities. Number and type of opsins identified from each species is shown below the species name. The color of each square illustrates an approximation of spectral sensitivity relative to other values. When the opsin associated with a specific sensitivity is known, it is listed below the λ_max value. The § symbol by the Placopecten magellanicus Go-opsin indicates that this opsin type is assumed to be expressed in the ciliary photoreceptors for this species based on expression of Go-opsin in the ciliary photoreceptors of other closely related species (Kingston et al., 2017; Kojima et al., 1997). Relationships between Lophotrochozoa, Deteruostoma, and Cnidaria were inferred from Valentine and Collins (2000) and relationships within Lophotrochozoa was inferred from (Bleidorn, 2019). For λ_max references, see Supplemental Table 2.

Ciliary photoreceptors are also found in polychaetes and bivalves (Barber et al., 1967; Bok et al., 2016; Lawrence and Krasne, 1965; McReynolds and Gorman, 1970). However, the opsins expressed in ciliary photoreceptors differ across groups. While c-type opsins are expressed in ciliary photoreceptors of the radiolar eyes in the polychaetes Spirobranchus corniculatus and Acromegalomma interruptum (Bok et al., 2017a,b; Bok et al., 2017), Go-opsin is expressed in the ciliary photoreceptors of scallops (Kojima et al., 1997).

Opsins appear to play important behavioral and ecological roles in lophotrocozoans. Sessile and eyeless bivalve species generally have fewer r-opsin copies compared to mobile species with eyes (Serb et al., 2013), suggesting functional benefits of multiple opsins in visual tasks and navigation. In P. dumerilii, expression of one of the r-opsins corresponds with the beginning of positive phototaxis behavior in early-stage larvae (Randel et al., 2013). Expression of a second r-opsin occurs at the onset of photoreceptor differentiation in later-stage larvae, suggesting distinct roles of each opsin in development (Randel et al., 2013). In the marine annelid Capitella teleta, r-opsin mediates the strong phototaxis responses of larvae (Neal et al., 2019), which is suggested to aid in dispersal for many marine invertebrates with planktonic, free-swimming larvae (Randel and Jékely, 2016).

Visual sensitivity may also be tuned relative to the light environment. In bivalves, species living in blue-dominated waters are sensitive to shorter wavelengths than species inhabiting green-shifted environments (see A. irradians and P. magellanicus, Fig. 6L) (Speiser et al., 2011). In the polychaeta S. corniculatus, visual sensitivity (λ_max = 464 nm) also provides a good match to downwelling environmental light (Fig. 6L) (Bok et al., 2017a,b) and may help with the detection of overhead predators (Nilsson, 1994). In cephalopods, differences in spectral sensitivities match environmental light conditions among decapodiform species (see Nautilus pompilius, Euprymna tasmanica, and Sepiola atlantica Fig. 6L) (Chung and Marshall, 2016).

3.2. Xenopsins

Xenopsins are found in mollusks, brachiopods, rotifers, flatworms, and annelids (Bonadè et al., 2020; Döring et al., 2020; Passamaneck et al., 2011; Ramirez et al., 2016; Rawlinson et al., 2019; Vöcking et al., 2017). In the annelid Malacoceros fuliginosus, xenopsin is co-expressed with r-opsin in the larval eyes (Döring et al., 2020). Identification of a c-opsin sequence from M. fuliginosus makes this the first organisms known to possess r-opsin, c-opsin, and xenopsins together (Döring et al., 2020). Though the roles of xenopsin in vision are not well understood, studies in the bryozoan Tricellaria inopinat, (Fig. 6L), show that xenopsins are involved in phototactic behavior (Döring et al., 2020).

3.3. Summary of Lophotrochozoa opsin expression

Lophotrochozoan vision is characterized by a diversity of opsin types. The use of c-opsin and Go-opsin for vision in Lophotrochozoa contrasts with r-opsin-mediated vision from arthropods and suggests that a diverse array of opsins and photoreceptor types were present in the protostome ancestor. Like in arthropods, visual sensitivity appears to be tuned to environmental lighting conditions and, in some cases, the saliency of visual cues may also explain some opsin duplications.

4. Visual opsins in invertebrate deuterostomes

An analysis of invertebrate deuterostome opsin evolution illustrates the similarities and differences in visual opsin expression between these species, vertebrates, and protostome groups. In this section, we discuss opsin expression and functions in Echinodermata and Tunicata (Chordata). Visual opsin evolution in vertebrates is discussed by Hagen et al. (this issue).

R-, c-, and Go-opsins have been identified in echinoderms and tunicates (e.g., Porter et al., 2012). However, in each case, only a single opsin type has been characterized for a functional role in vision (r-opsin in echinoderms and c-opsin in tunicates), which we discuss below.

4.1. R-opsin

R-opsin (Sp-Opsin4) appears to be the opsin that mediates vision in echinoderms, although c-opsins and Go-opsins have also been identified within this group (Agca et al., 2011; Delroisse et al., 2013; Lesser et al., 2011; Raible et al., 2006; Ullrich-Lüter et al., 2011; Ullrich-Lüter, D’Aniello and Arnone, 2013). Despite the absence of eyes, photoreceptor cells on the tube feet of the purple sea urchin (Strongylocentrotus purpuratus) confer directional vision via the localized expression of Sp-Opsin4, which is likely sensitive to blue light based on tests of phototaxis response (λ_max = 450 nm, Fig. 6D) (Ullrich-Lüter et al., 2011). Sp-Opsin4 is also expressed in the optic cushions (optic organs that play a role in phototaxis) of the sea star Asterias rubens, further supporting a role for r-opsin in direction vision in echinoderms (Delroisse et al., 2013; Ullrich-Lüter et al., 2011).

Rhabdomeric photoreceptors have been identified in other deuterostomes (Brandenburger et al., 1973; Gorman et al., 1971; Pantzartzi et al., 2017; Watanabe and Yoshida, 1986). However, the use of r-opsin-expressing rhabdomeric photoreceptors in vision appears to be unique in echinoderms, whereas r-type opsins function in non-visual roles in many deuterostomes (Lacalli, 2004; Panda et al., 2005; Provencio et al., 1998; but see Ecker et al., 2010).

4.2. C-opsin

Though r-opsins have been identified in tunicates (Porter et al., 2012), a single c-opsin identified in ciliary photoreceptors provides putative visual function in larval Ciona intestinalis (Inada et al., 2003; Kusakabe et al., 2001; Kusakabe and Tsuda, 2007). Expression of the c-opsin (λ_max = 500 nm) within the ocellus is necessary for the photic behavior of larval Ciona (Inada et al., 2003; Kusakabe and Tsuda, 2007) (Nakagawa et al., 1999) (Fig. 6D). The C. intestinalis c-opsin groups phylogenetically with other deuterostome c-opsins (Fig. 2) and is sister to the vertebrate ancient (VA) opsins identified from teleost fish (Kusakabe et al., 2001; Soni et al., 1998; Soni and Foster, 1997).

4.3. Summary of invertebrate deuterostome opsin expression

Specific opsin types are associated with vision in the two invertebrate deuterostome groups considered here. While echinoderms use a r-opsin to mediate vision, like many protostome species, this contrasts with other deuterostomes that employ c-opsins. Despite differences in opsin type, the spectral sensitivity of the invertebrate deuterostome opsin may function similarly by providing maximal sensitivity to the blue-light environments they inhabit.

5. Cnidops

Cnidops are an opsin class found only in cnidarians and ctenophores (Liegertová et al., 2015; Plachetzki et al., 2007; Porter et al., 2012) that share a close phylogenetic relationship to deuterostome c-opsins and lophotrochozoan opsins (Fig. 2). Nevertheless, the relationships between cnidops and other opsin classes remain unclear (Bielecki et al., 2014; Feuda et al., 2012; Porter et al., 2012; Ramirez et al., 2016).

5.1. Cnidops diversity and functions

Cnidops expression varies greatly among cnidarians (Bielecki et al., 2014; Liegertová et al., 2015; Plachetzki et al., 2007; Suga et al., 2008). cDNAs for two distinct opsins have been identified in the eyeless species Podocoryne carnea, compared to eighteen opsin sequences identified in Cladonema radiatum, seven of which are expressed in the eyes (Suga et al., 2008). In Tripedalia cystophora, eighteen opsin sequences were identified in the rhopalium, the sensory structures containing the eyes (Liegertová et al., 2015). It is unclear if these opsins form visual pigments with distinct spectral sensitivities with physiological evidence suggesting monochromatic vision in the blue-green part of the light spectrum (Fig. 6C) (Coates et al., 2006; Garm et al., 2007).

Opsins often function in extra-ocular photoreception in Cnidaria, with cnidops identified from structures that lack any apparent visual function and from species that lack eyes (Bielecki et al., 2014; Gornik et al., 2021; Suga et al., 2008). For example, a subset of T. cystophora cnidops are expressed in the gonads and tentacles (Liegertová et al., 2015). In the jellyfish Clytia hemisphaerica, a cnidops expressed in the gonads mediates oocyte maturation and spawning behavior (Artigas et al., 2018). Increased expression of Anthozoan specific opsins (which fall outside of the cnidops clade) in gonadal tissue of Aiptasia suggests that opsins may function generally in spawning behavior for cnidarians (Gornik et al., 2021). The large number of opsin genes identified in Cnidaria, even in eyeless and sessile species (e.g. 63 opsin genes identified in Hydra magnipapillata; Suga et al., 2008), indicates an important role of extraocular photoreception in physiological functions, and could contribute to the expansion of opsin genes in these species (for a review see Garm and Ekström, 2010).

6. Conclusion

Invertebrate visual opsins are extremely diverse. While distinct patterns of opsin expression exist across distantly related species, even within closely related lineages, patterns of opsin expression and visual sensitivities vary greatly. The roles of changes to opsin sequence on tuning have been identified in some cases (e.g., Salcedo et al., 2009; Wakakuwa et al., 2010), although there is still much to learn about the roles of sequence changes on spectral tuning. Similarly, the roles of spectral sensitivity variation have been studied within some groups, with findings suggesting general roles of spectral tuning in response to increased saliency of visual cues and environmental light conditions. Exciting advances in genomic techniques have already begun to identify new mechanisms underlying protostome opsin evolution and function (e.g. Liénard et al., 2021; Neal et al., 2019), providing an interesting avenue for future research on invertebrate opsin evolution. Overall, the diversity of opsins spanning protostomes, invertebrate deuterostomes, and cnidaria (this review), and vertebrates (Hagen et al., this issue) highlights the diversity of opsins present in the eumetazoan ancestor and sheds light on the different behavioral and ecological contexts that opsin diversity may influence these groups.

Data availability

No data was used for the research described in the article.