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. 2022 Nov 16;61:e70. doi: 10.6620/ZS.2022.61-70

Larval Development of the Mangrove Fiddler Crab Austruca albimana (Kossmann, 1877) (Crustacea: Brachyura: Ocypodidae) Under Laboratory Conditions

Austin AJ Kumar 1,*, Ali M Al-Aidaroos 1,*
PMCID: PMC9810992  PMID: 36644630

Abstract

Larvae of the mangrove fiddler crab Austruca albimana (Kossmann, 1877), hatched from an ovigerous female collected from the mangroves of Sumariat, Jazan Province, Saudi Arabia in the southern Red Sea, were reared in the laboratory. Four zoeal and a megalopal stages were recorded, and their morphological features are described herein for the first time. The setations of the cephalothoracic appendages of the zoeas of A. albimana and their congeners exhibit several variations that help differentiate larvae of this genus easily from other meroplankton. However, a character of phylogenetic significance –minute spines on the forks of the telson of pleon –is common to larvae of this genus. These minute spines were studied with the aid of scanning electron microscope images. There were five common morphological features between A. albimana and other fiddler crab megalope, including Minuca burgersi, Leptuca uruguayensis and Leptuca thayeri. These features were a deflexed front, rounded to obtuse frontal margin, seven-segmented antennal flagellum, unsegmented endopod of maxilla and three cincinnuli on the endopods of pleopods. Two zoeal morphological features described in this study and other studies (i.e., the absence of lateral spines on carapace [vs. their presence in species of Uca, Afruca and Ocypode in the Ocypodinae] and the presence of a maximum of four pairs of inner setae on the telson of pleon [vs. presence of more than four pairs of setae in species of Uca, Afruca and Ocypode]) support the taxonomic amendment of transferring Uca spp. and Afruca spp. crabs from Gelasiminae to Ocypodinae.

Keywords: Fiddler crabs, Gelasiminae, Megalopa, Ocypodinae, Red Sea, Zoea

BACKGROUND

Fiddler crabs inhabit muddy, muddy-sandy or muddy-intertidal zones, especially in the mangroves of tropical and subtropical regions (Crane 1975). In Saudi Arabia, mangroves have a discontinuous distribution along the 1,700-km Red Sea coast (Saifullah 1996). Three species of fiddler crabs have been recorded from the mangroves of Saudi Arabia: Cranuca inversa (Hoffmann 1874), Austruca albimana (Kossmann, 1877) and Tubuca alcocki Shih, Chan & Ng, 2018. The most studied Tubuca alcocki has been mentioned in several ecological reports of the Red Sea (e.g., Por and Dor 1975; Tubbs and Hogarth 1986; Vine 1986; Mandura et al. 1987).

Austruca Bott, 1973 widely distributed in the Indo-West Pacific, includes 13 species (Shih et al. 2009 2016; Rosenberg 2019; Shih and Poupin 2020). Austruca albimana is distributed from the Red Sea to the south-eastern Persian Gulf and has sympatric coexistence with two congeners, Austruca annulipes (H. Milne Edwards, 1837) and Austruca iranica Pretzmann, 1971. In the southern Red Sea of Saudi Arabia, A. albimana co-occurs with C. inversa and T. alcocki in the mangroves of Jazan, and with C. inversa in Sumariat mangroves, Jazan (Kumar 2019).

Although several taxonomic studies of fiddler crab zoea have been conducted (Zhang and Shih 2022), studies from the western Indian Ocean are few in number (Table 1). Anger (1991) and Spivak and Cuesta (2009) studied the effects of salinity, particularly the cumulative effect of salinity and temperature, on the development of brachyuran larvae. Insecticides such as fenoxycarb and dieldrin affect the development of xanthid larvae (Epifanio 1971 1972 1973; Cripe et al. 2003). In highly polluted sites at Piles Creek, Linden, New Jersey, USA, the survival rate of Minuca pugnax in the early benthic stages (Smith, 1870) was found to have been affected (Bergey and Weis 2008). This is a matter of concern because the reduced survival of early benthic stage crabs shrinks the population size of the fiddler crabs in mangroves, since as at this stage crabs settle into the intertidal areas inhabited by older conspecifics (Armendariz 2005). This reduction may severely affect the health and survival of mangroves as their burrows promote nutrient turnover (Bertness 1985) and accelerate decomposition of organic matter by supplying oxygen to the anoxic mangrove subsurface organic sediments (Mokhtari et al. 2013).

Table 1.

Morphological descriptions of the first zoeas of fiddler crabs available from the western Indian Ocean

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Phylogenetic studies of brachyuran crabs revealed that larval morphological data rather than adult morphological data concur more with molecular data (Hultgren et al. 2009). In the phylogeny of fiddler crabs, the larval morphological features also play a crucial role (Shih et al. 2016).

Comprehensive identification keys of brachyuran larvae are necessary to identify meroplankton up to the species level and to understand spatial-scale biodiversity (Bento 2017; Clark and Paula 2003). Most available descriptions are inaccurate and do not meet requirements for comparative studies (Clark et al. 1998). On the Red Sea coast of Saudi Arabia, only the larvae of C. inversa have been described (Al-Aidaroos 2013), and among the 13 species of Austruca, only the larval stages of A. iranica (Hashmi 1968; Ghory and Siddique 2006) (as A. annulipes) (see Apel 2001) and Austruca lactea have been partially described (Terada 1979).

In this study, all the larval stages of A. albimana are described and compared with the larvae of other fiddler crabs. Some taxonomically significant morphological features found in those larvae are also discussed in detail.

MATERIALS AND METHODS

An ovigerous female of A. albimana was collected on 8 March 2018 from the mangroves of Sumariat (17°29'15.3312"N, 42°15'9.7452"E), Jazan Province, Saudi Arabia (Fig. 1). The crab was reared in an ESPEC walk-in type environmental chamber at the Faculty of Marine Sciences, King Abdulaziz University at 28°C under a 12-h light/dark photoperiod. Soil collected from the habitats of fiddler crabs was sprinkled at the base of the container, and the crab was found feeding on organic matter. The larvae hatched on 17 March 2018. Six batches of 50 larvae each were reared in 1-L containers, each with 800 mL of seawater with a salinity of 35 psu at 28°C. All zoeal stages were fed with rotifers (50/mL). Water was changed every other day. Antibiotics were not used.

Fig. 1.

Fig. 1.

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Austruca albimana (Kossmann, 1877), dorsal view of the berried female.

Ten larvae from each stage were preserved in 70% ethanol and dissected using a Leica M80 microscope in polyvinyl lactophenol. A Leica 6000B phase-contrast stereomicroscope equipped with camera lucida (5–40× objective lenses) was used for drawings and setal counting. The following measurements were made. For zoeal stages, CL: distance from the base of the rostral spine to the posterior margin of the carapace; RDL: distance between the tips of the dorsal and rostral spines. For the megalopa: CW: maximum distance across the carapace; CL: maximum distance along the carapace. Setal counts were made from the proximal to distal portions. Setal numbers are represented for appendages from basis to endopod (Clark et al. 1998; Clark and Cuesta 2015). The first zoea has been morphologically described completely, and, for the following stages, only changes from the previous stages were noted. The berried female crab (CW, 9 mm) and complete larval stages were deposited into the King Abdulaziz University Museum (KAUMM 879–884).

For the study involving scanning electron microscopy (SEM), the pleons of zoeas II and IV were fixed in 2.5% glutaraldehyde followed by washing three times with sodium cacodylate. Subsequently, the samples were dehydrated successively in 30%, 50%, 70%, 90% and 100% ethyl alcohol. The appendages were air-dried and placed on a thin layer of conductive carbon tape stuck on the specimen stub. Afterwards, the pleons were coated with a thin layer of gold using an Auto Fine Coater for 30 seconds at 30 mA and approximately 3.5 Pa (K550X, 50/60 Hz; Quorum Technologies Ltd, Ashford, UK). Thereafter, the specimen stub was loaded into the microscope and SEM images were taken at 2–5 kV (FEI Quanta 250, FP 2012/14, Czech Republic) (Hayat 2000).

RESULTS

Family Ocypodidae Rafinesque, 1815

Austruca albimana (Kossmann, 1877)

(Figs. 1–15)

Zoea I

Size (mean ± sd): CL = 0.3 ± 0.05 mm; RDL = 0.59 ± 0.09 mm.

Carapace (Fig. 2a): Smooth; rostrum short, dorsal spine small, curved posteriorly, smaller than rostrum; lateral spines absent; ventral margin without setae; eyes sessile.

Fig. 2.

Fig. 2.

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Austruca albimana (Kossmann, 1877), lateral view of carapace: a) zoea I; b) zoea II; c) zoea III; d) zoea IV; e) dorsal view of carapace of megalopa.

Antennule (Fig. 3a): Endopod absent; exopod unsegmented with 2 long, 3 short aesthetascs.

Fig. 3.

Fig. 3.

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Austruca albimana (Kossmann, 1877), antennule: a) zoea I; b) zoea II; c) zoea III; d) zoea IV, e) megalopa.

Antenna (Fig. 4a): Biramous. Protopodal process bilaterally spinulate up to distal margin; exopod with two terminal simple setae.

Fig. 4.

Fig. 4.

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Austruca albimana (Kossmann, 1877), antenna: a) zoea I; b) zoea II; c) zoea III; d) zoea IV; e) megalopa.

Mandible (Fig. 5a): Incisor with five blunt teeth, molar process present, posterior margin with three teeth; palp absent.

Fig. 5.

Fig. 5.

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Austruca albimana (Kossmann, 1877), mandible: a) zoea I; b) zoea II; c) zoea III; d) zoea IV; e) megalopa.

Maxillule (Fig. 6a): Epipod seta absent; coxa with 2 subterminal plumodenticulate, 3 terminal cuspidate setae; basis with 5 terminal setae (4 cuspidate, 1 plumodenticulate); endopod 2-segmented, distal segment with two plumodenticulate terminal setae.

Fig. 6.

Fig. 6.

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Austruca albimana (Kossmann, 1877), maxillule: a) zoea I; b) zoea II; c) zoea III; d) zoea IV; e) megalopa.

Maxilla (Fig. 7a): Coxa bilobed, proximal lobe with 2 cuspidate, 1 plumodenticulate setae, basis with 2 plumodenticulate setae; basis bilobed, proximal segment with 4 cuspidate terminal setae, distal segment with 3 plumodenticulate terminal setae; endopod bilobed with 1+2 terminal plumose setae; scaphognathite with 4 plumose setae.

Fig. 7.

Fig. 7.

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Austruca albimana (Kossmann, 1877), maxilla: a) zoea I; b) zoea II; c) zoea III; d) zoea IV; e) megalopa.

First maxilliped (Fig. 8a): Basis with 2+2+3+2 sparsely plumose setae; endopod 5-segmented with 2 (plumose), 2 (plumose), 1 (plumose), 2–3 (plumose), 5 (2 plumose subterminal, 3 plumodenticulate terminal) setae; exopod 2-segmented, distal segment with 4 natatory plumose setae.

Fig. 8.

Fig. 8.

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Austruca albimana (Kossmann, 1877), first maxilliped: a) zoea I; b) zoea II; c) zoea III; d) zoea IV; e) megalopa.

Second maxilliped (Fig. 9a): Basis with 1+1+1+1 plumose setae; endopod 3-segmented with 0,0,5 (3 simple, 2 plumodenticulate setae); exopod bisegmented with 4 terminal plumose setae.

Fig. 9.

Fig. 9.

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Austruca albimana (Kossmann, 1877), second maxilliped: a) zoea I; b) zoea II; c) zoea III; d) zoea IV; e) megalopa; f) third maxilliped of zoea IV; g) third maxilliped of megalopa.

Pleon (Fig. 10a): Five somites; somite II with pair of anteriorly directed dorsolateral processes; somite III with pair of posteriorly directed dorsolateral processes; somite IV broader than others, posterolateral margin overlapping with Vth somite; somite I without seta; somites II–V with pair of posterodorsal simple setae.

Telson (Fig 10a): Inner margin with 3 pairs of setae; minute spines on lateral sides of forks of telson.

Fig. 10.

Fig. 10.

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Austruca albimana (Kossmann, 1877), dorsal view of pleon: a) zoea I; b) zoea II; c) zoea III; d) zoea IV; e) fork of the telson of zoea IV; dorsal view of pleon: e) megalopa.

Zoea II

Size (mean ± sd): CL = 0.35 ± 0.02 mm; RDL = 0.8 ± 0.05 mm.

Antenna (Fig. 4b): Endopod bud present.

Mandible (Fig. 5b): Incisor with six blunt teeth, all margins of molar process toothed.

Maxillule (Fig. 6b): Basis with 5 cuspidate, 1 plumodenticulate setae; epipod seta present.

Maxilla (Fig. 7b): Coxa bilobed, proximal lobe with 1 cuspidate, 4 plumodenticulate setae, distal lobe with 3 plumodenticulate setae; basis bilobed, proximal segment with 1 cuspidate, 5 plumodenticulate setae, distal segment with 1 cuspidate, 3 plumodenticulate setae; scaphognathite with 8 plumose setae.

First maxilliped (Fig. 8b): Endopod with 2,2,1,3,6 (2 subterminal plumose, 4 terminal plumodenticulate) setation; exopod with 6 natatory setae.

Second maxilliped (Fig. 9b): Exopod with 6 natatory setae.

Pleon (Figs. 10b; 13 a, b): Pleopod buds appear.

Zoea III

Size (mean ± sd): CL = 0.57 ± 0.05 mm; RDL = 0.9 ± 0.09 mm.

Carapace (Fig. 2c): Small hairs on anterodorsal margin.

Antennule (Fig. 3c): Three aesthetascs, two spines. Antenna (Fig. 4c): Endopod about one half length of protopod.

Mandible (Fig. 5c): Incisor with around eight blunt teeth, posterior margin of molar process with large tooth, other margins smooth.

Maxillule (Fig. 6c): Coxa with 2 cuspidate, 2 plumodenticulate, 1 simple setae; basis with 7 cuspidate, 2 plumodenticulate setae.

Maxilla (Fig. 7c): Proximal segment of coxa with 4 plumodenticulate, 1 cuspidate setae, distal segment with 3 cuspidate setae; proximal segment of basis with 3 cuspidate, 2 plumodenticulate setae, distal segment with 2 cuspidate, 3 plumodenticulate setae; scaphognathite with 13 plumose setae.

First maxilliped (Fig. 8c): Coxal seta; basis with 2,2,3,3 plumose setae; exopod with 8 plumose natatory setae.

Second maxilliped (Fig. 9c): Exopod with 8 plumose natatory setae.

Pereiopods (Fig. 11a). Segmentation incomplete.

Fig. 11.

Fig. 11.

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Austruca albimana (Kossmann, 1877), pereiopods: a) zoea III; b) zoea IV; c–g) megalopa: c) cheliped; d–g) first to last ambulatory legs.

Pleon (Fig. 10c): Six somites; somites III–V with well-developed posterolateral processes; pleopod buds more developed.

Telson (Fig. 10c): Inner margin with four pairs of setae.

Zoea IV

Size(mean±sd):CL=0.6±0.05mm;RDL=1.4 ± 0.1 mm.

Carapace (Fig. 2d): Twelve pairs of ventral setae.

Antennule (Fig. 3d): Eight aesthetascs arranged in two groups (4+4).

Antenna (Fig. 4d): Endopod longer than exopod.

Mandible (Fig. 5d): Incisor with eight teeth, all margins of molar process toothed.

Maxillule (Fig. 6d): Two epipodal setae; coxa with 2 cuspidate, 3 plumodenticulate (1 subterminal, 2 terminal) setae; basis with 3 terminal cuspidate, 6 plumodenticulate setae (3 subterminal).

Maxilla (Fig. 7d): Proximal lobe of coxa with 5 plumodenticulate setae, distal segment with 3 cuspidate setae; proximal lobe of basial with 2 cuspidate, 4 plumodenticulate setae, distal segment with 3 cuspidate, 2 plumodenticulate setae; scaphognathite with 24 plumose setae (20 Plumose, 4 simple).

First maxilliped (Fig. 8d): Exopod with 10 plumose natatory setae.

Second maxilliped (Fig. 9d): Exopod with 10 plumose natatory setae.

Third maxilliped (Fig. 9f): Present, not well developed.

Pereiopods (Fig. 11b): Well developed, segmented.

Pleon (Figs. 10d, e, 14a, b, 15): Somite I with 5 dorsal setae; pleopods well developed.

Megalopa

Size: CW = 0.86 mm; CL = 1.1 mm.

Carapace (Fig. 2e): Longer than broad; sparsely setose dorsally, posterolateral margins with few setae; rostral spine truncated, naked; eyes stalked.

Antennule (Fig. 3e): Peduncle 3-segmented with 3,1,0 plumose setae; endopod small, unsegmented with 2 plumose, distal setae; exopod three-segmented with 0, 6 (aesthetascs), 4 (3 aesthetascs, 1 seta) setation.

Antenna (Fig. 4e): 3-segmented peduncle with 1,1,1 plumose setae; flagellum 7-segmented with 0,0, 2,0,2,2,2 setae, segments 1–6 with plumose setae, 7th segment with simple setae.

Mandible (Fig. 5e): Palp two-segmented; distal segment with 3 terminal simple setae.

Maxillule (Fig. 6e): Coxa with 8 cuspidate, 6 plumodenticulate setae; basis with 5 cuspidate, 2 plumose and 2 plumodenticulate setae; endopod bisegmented, distal segment terminates with seta; single protopod plumose seta.

Maxilla (Fig. 7e): Coxa bilobed, proximal segment with 13 setae, distal segment with 5 setae, all plumodenticulate; basis bilobed, proximal segment with 7 simple setae, distal segment with 6 simple, 1 plumose setae; endopod unsegmented without setae; scaphognathite with 51 plumose setae.

First maxilliped (Fig. 8e): Coxa, basis with 5, 6 plumodenticulate setae, respectively; endopod with 2 subterminal setae; exopod 2-segmented, proximal segment with two distal setae, distal segment with 3 setae, all plumose; epipod with 7 long setae.

Second maxilliped (Fig. 9e): Endopod 4-segmented with 0, 0, 4 (all plumodenticulate), 7 (4 cuspidate, 3 plumodenticulate) setation; exopod 2-segmented, distal segment with 4 plumose setae.

Third maxilliped (Fig. 9g): Protopod naked; endopod 5-segmented with 9 (5 plumodenticulate, 4 simple), 7 (4 simple, 3 plumodenticulate), 2 (plumodenticulate), 6 (plumodenticulate), 5 cuspidate setae; exopod 3-segmented without setation, epipod with 20 (18 simple, 2 plumose) setae.

Pereiopods (Fig. 11c–g): Inner margins of fused basis, ischium, merus, superior border of carpus of chelae with 1 or 2 spines each; fingers cross each other; inner margins of segments of pereiopods II–IV with few spines; coxa of Vth pereiopods with three stout plumose setae; superior margins of carpus and propodus with 1 or 2 spines; tips of dactyl with 3 long setae.

Pleon (Fig. 10f): Six somites; somites I, III, IV with 1 pair of setae each, somite II with 2 pairs of setae, one each on anterior and posterior borders, V and VI with two pairs of posterior setae each; somite III with 3 pairs of setae; somite V with well-developed lateral processes.

Telson (Fig. 10f): Tip rounded: 5 pairs of plumose setae; three pairs anteriorly, two pairs posteriorly.

Pleopods (Fig. 12a–d): Endopods of somites I–V with three cincinnuli each, pleopod of somite VI without endopod; exopods of pleopods of somite II–VI with 13S, 12S, 14S, 12S plumose setae, respectively.

Uropod (Fig. 12e): Exopod with 9 plumose setae, protopod with plumose seta.

Fig. 12.

Fig. 12.

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Austruca albimana (Kossmann, 1877), megalopa; a–e) pleopods II–VI.

Fig. 13.

Fig. 13.

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Austruca albimana (Kossmann, 1877), SEM images of the telson of zoea II; a) at magnification 6000×; b) at magnification 10000×.

Fig. 14.

Fig. 14.

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Austruca albimana (Kossmann, 1877), SEM images of the telson of zoea IV; a) at magnification 10000×; b) at magnification 20000×.

Fig. 15.

Fig. 15.

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Austruca albimana (Kossmann, 1877), SEM images of the telson of zoea IV at magnification 40000×.

DISCUSSION

Four zoeal stages and one megalopal stage were recorded during the larval development of A. albimana. Some fiddler crabs exhibit more than four zoeal stages: A. lactea and L. thayeri (five) (Terada 1979; Anger et al. 1990) and Minuca burgersi (six) (Rieger 1998a).

Several environmental factors have been thought to influence the number of zoeal stages in brachyuran crabs, e.g., salinity, temperature, photoperiodicity, prophylactic usage and food availability (Gardner and Northam 1997; Gardner and Quintana 1998; Al-Aidaroos et al. 2014). Crabs living in highly specialised habitats exhibit abbreviated development to reduce the chances of larval dispersion (Rabalais and Gore 1985). Although this statement has proven true in A. albimana—it inhabits a specialised ecosystem, mangroves and has just four zoeal stages—other fiddler crabs of this habitat have up to six zoeal stages (Anger et al. 1990; Rieger 1997 1998a b; Batisteli 2003). Temperature is another important factor in determining the number of larval stages of crustaceans (Anger 2001). In five species of Coenobita (hermit crabs), the total zoeal stages tended to decrease with increasing temperature (Hamasaki et al. 2020). In our study, the zoeas were reared at a higher temperature, 28°C, which generally might cause a reduction in zoeal stages.

In this study, the zoea I of A. albimana can be well differentiated from those of the other two congeneric species. The antennule of A. albimana has five aesthetascs, while the other two species have two aesthetascs and a spine. Both the coxa and the basis of maxillule of A. albimana, A. iranica (Hashmi 1968) and A. lactea (Terada 1979) have five setae each. However, the zoea I of A. iranica described by Ghory and Siddique (2006) had only four setae (Table 2). Similarly, intrageneric variations can be seen in the setations of the maxilla and maxillipeds I and II of the first zoeas of Austruca. These variations can help identify the larvae of Austruca spp.

Table 2.

Morphological differences between the first zoeas of Austruca albimana (Kossmann, 1877) (present study) and other related fiddler crabs. *Ghory and Siddique 2006; †Hashmi 1968; ‡Anger et al. 1990; +De Souza et al. 2013

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One important character, several minute spines on the forks of the telson, has been found in the zoeas of all Austruca spp. (Hashmi 1968; Terada 1979; Ghory and Siddique 2006; this study; Figs. 12–14). This characteristic suggests that this genus is monophyletic. It has long been understood that larval morphological features reflect phylogenetic relationships (Rice 1983; Hultgren et al. 2009).

Several morphological features are similar among

the first zoeas of different species in Gelasiminae: presence of two aesthetascs and a spine in the antennule (A. iranica, A. lactea, Gelasimus hesperiae, L. thayeri, M. burgersi and T. alcocki); presence of two setae on the exopods of antenna (A. iranica, C. inversa, G. hesperiae and T. alcocki); presence of five setae on each of the coxa and basis of the maxillule (A. iranica, A. lactea, C. inversa, G. hesperiae, L. thayeri, Leptuca uruguayensis, M. burgersi, Minuca mordax, Minuca rapax, Minuca vocator and T. alcocki); presence of 5+4 setal pattern on the basis of maxilla; presence of 2+2+3+2 and 2,2,1,2,5 setation on the basis and endopod of maxilliped I, respectively and the occurrence of 0,0,5 setation on the endopod of maxilliped II (Table 2).

Bento and Paula (2018) have listed some morphological features of the first zoeas of four species of fiddler crabs collected from the western Indian Ocean, A. occidentalis (as A. annulipes) (see Naderloo et al. 2016), Gelasimus hesperiae (as G. vocans) (see Crane 1975), T. urvillei and Paraleptuca chlorophthalmus. The larvae of Bento and Paula (2018) and our study have several common morphological features (Table 4). That is, the ratio of the length of the dorsal spine to the length of the carapace is 1/3 for A. annulipes, G. hesperiae and T. urvielli, and the setation on the basis of first maxilliped is 2+2+3+2 for A. albimana, G. hesperiae and T. urvielli. Likewise, minute spines are found in the forks of telson of A. annulipes, A. albimana and G. hesperiae. These morphological similarities in early zoeal stages are common in several brachyuran larvae at the genus level, and variations become more apparent in the later stages (De Souza et al. 2013).

Morphological descriptions are fewer for later zoeal stages because of high mortality during larval rearing; this hampers better larval comparisons. Twelve posterolateral setae are seen on the ventral margin of the carapace of the fourth zoeal stage of A. albimana (this study), and these setae are present from zoea III onwards in L. thayeri (4 in zoea III, 6 in zoea IV) and from zoea II onwards in M. burgersi (1 in zoea II, 2 in zoea III, 7 in zoea IV) (Anger et al. 1990; Rieger 1998a). In the fourth zoeal stage, the number of plumose setae on the scaphognathite of the maxilla is higher for Austruca sp. (24 in A. albimana, 17–20 in A. lactea) and Leptuca sp. (24 in L. uruguayensis) (Table 3).

Table 3.

Morphological differences between the second, third and fourth zoeas of Austruca albimana (Kossmann, 1877) (present study) and other related fiddler crabs

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MXPI, first maxilliped; FTA, forks of the telson; CL, length of carapace; ND, no data; RDL, the length of carapacial dorsal spine; SANT, size of antenna; SRS, rostral spine length.

Two morphological features observed in the larvae of A. albimana (this study) and other fiddler crabs, viz. the absence of lateral spines on the carapace and the presence of a maximum of four pairs of inner setae on the telson during the entire larval development, are suggested to have taxonomic significance at the subfamily level. These characteristics provide additional evidence indicating that species of Uca and Afruca belong to the subfamily Ocypodinae. The carapacial lateral spines are absent in the larvae of the subfamily Gelasiminae but present in those of Afruca, Uca and Ocypode species (Terada 1979; Rieger 1996 1997 1998a b; Kakati 2005; Negreiros-Fransozo et al. 2009; Spivak and Cuesta 2009; Jiang et al. 2014; this study). Moreover, a maximum of four pairs of inner setae are present on the telson of the zoeas of Gelasiminae even at the sixth stage (Terada 1979; Rieger 1996 1997 1998a; this study). Species of Uca, Afruca and Ocypode, however, have more than four pairs of setae on the telson of the later zoeal stages. This is a consistent feature similar to the presence of carapacial lateral spines (Kakati 2005; Negreiros-Fransozo et al. 2009; Spivak and Cuesta 2009; Jiang et al. 2014).

Only a few morphological descriptions are available for the megalopal stage of Gelasiminae (Table 4). Availability of more larval descriptions is a prerequisite for better larval comparisons (Rieger 1998a). A deflexed front, rounded to obtuse frontal margin, seven-segmented antennal flagellum, unsegmented endopod of maxilla and three cincinnuli on the endopod of pleopod are some of the characteristics common to the megalopae of Gelasiminae (Table 5). The morphology of the megalopae of other species of Gelasiminae should be studied further to confirm those characteristics as the key characteristics of this subfamily.

Table 4.

Comparison of some of the morphological features of the first zoeas of the four species of fiddler crabs from the western Indian Ocean with Austruca albimana (Kossmann, 1877) (present study)

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Table 5.

Comparison of the morphological characteristics of the megalopae of Austruca albimana (Kossmann, 1877) (present study) and other related fiddler crabs

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The megalopae of Gelasiminae can be easily distinguished from each other morphologically (Table 5). The posterolateral margins of A. albimana have a row of setae (this study) and this setation is extended to the posterior border in M. burgersi (Rieger 1998a), while this setation is absent in other species of Gelasiminae. Taxonomists should be cautious when considering this characteristic for comparing morphology of brachyuran larvae as it might have been overlooked in several previous works (Clark et al. 1998; De Souza et al. 2013). Austruca albimana is the only species with three segments in the exopod of the third maxilliped. Negreiros-Fransozo et al. (2009) reported the presence of a spine on both the basis of the chelipeds and third pereiopods of Uca maracoani. These spines are absent in the megalops of Gelasiminae (Anger et al. 1990; Rieger 1998a; Armendariz 2005; Negreiros-Fransozo et al. 2009; this study).

CONCLUSIONS

In this study, four zoeal and one megalopal stages were recorded in the larval development of A. albimana. In the different zoeal stages of the species of Gelasiminae, genus-level characteristics could not be identified, as no suites of characteristics are common to the congeners, and similar morphological features existed at the intergeneric level. However, exclusively in the different zoeal stages of Austruca spp., there are minute spines on the forks of the telson. This character is suggested to be of phylogenetic significance. Two morphological features were found in the zoeas of all fiddler crabs: (1) absence of lateral spines on the carapace; (2) presence of a maximum of four pairs of inner setae on the telson, even in the sixth-stage larvae. The congeners of fiddler crabs can easily be differentiated at the megalopal stage based on suites of morphological characteristics. Several morphological features are common to the megalopae of fiddler crabs. To confirm those features as key characteristics of this group, more extensive studies are necessary.

Acknowledgments

This project was funded by the Deanship of Scientific Research (DSR) at King Abdulaziz University, Jeddah, under grant no. G-478-150-38. The authors, therefore, acknowledge DSR with thanks for their technical and financial support. Thanks are also due to Prof. Dr. Dieter Waloszek, Guest Professor, Lund University, Lund, Sweden for providing a crucial reference. The authors thank Mr. C. Suresh Babu and Mr. C. Senthil Babu, JAZADCO, Jazan Province and Mr. Radha Krishnan Palassaeri, Red Sea Aquaculture Company, Sumariat, Jazan Province for their warm hospitality during this study. Thanks to Mr. Abdullah Salawati, King Fahad Medical Research Centre, Jeddah, Saudi Arabia for his help in taking SEM images. Thanks also to Mr. Reny P. Devassy, Faculty of Marine Sciences, King Abdulaziz University, Jeddah for artwork assistance. We acknowledge the helpful comments on the manuscript from two anonymous referees.

Footnotes

Authors’ contributions: A.A.J. Kumar cultured larvae, dissected specimens, did the camera Lucida drawings and prepared the manuscript. Ali M. Al-Aidaroos organised the project and specimen collection and edited the manuscript.

Competing interests: The authors have no conflicts of interest to declare.

Availability of data and materials: Accession numbers are included in the manuscript.

Consent for publication: Both the authors agree to submit the manuscript to Zoological Studies and to publish if accepted.

Ethics approval content to participate: Not applicable.

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