Table 1.

Behaviours and reactions of hermit crabs.

Category/sub-category	Criteria
Visibility
Visible	Hermit crab, its shell or parts thereof visible. This includes shell largely covered by other organisms (e.g. crab under brittle star aggregation, i.e. outline recognisable, no exit tracks).
Non-visible	Neither crab nor its shell or parts thereof visible (e.g. hidden behind a multi-species clump)
Location
On sediment	Crab/shell located on sediment or on bivalve shells on sediment
Elevated	Crab/shell on multi-species clump (no contact with sediment)
Locomotion
No locomotion	No displacement
Minor	Displacement < 1 shell length or < 1 body length (if crab outside shell)
Major	Displacement ≥ 1 shell length or ≥ 1 body length (if crab outside shell)
Turn	Turning movement without displacement
Body movement	Crab movement without displacement (retraction into or stretching out of shell; appendage movements: chelipeds/legs; slight shell movements if crab not visible)
Body posture
Normal	Biologically normal body postures. One or more of the following visible: eyes, cheliped(s), leg(s), antennae. If crab not visible, shell aperture slightly elevated (i.e. not flat on sediment surface)
Extended	Soft (posterior) part of carapace (and occasionally also anterior part of abdomen) visible
Out	Crab fully emerged/shell abandoned
Shell position
Upright	Aperture facing down
Overturned	Aperture facing up
Interaction	Visible interaction of crab, its shell, or a symbiont with another organism. Includes:
	•organism on top of shell,
	•hermit crab on organism that shows reaction,
	•interaction not directly visible but reconstructed based on track and/or reactions between two images.
	Excludes organisms used by crab solely as a substrate, e.g. crab climbs up on or over a sponge or ascidian