An unusual defensive behaviour has been described for  mole crabs Emerita analoga around La Jolla, California.  When placed on dry sand after handling, a crab will often lie for a moment on its back in what is termed by the author a “death feint”.  However, from a survival point of view this doesn’t make much sense, for a potential predator like a bird would simply snatch up its prey, whether if pretends to be dead or not.  Another possibility, though, is that by lying still the crab may camouflage itself against the speckly sand for long enough to evade detection. If it lies for too long in the hot sun, though, the author notes that the crab may die. The behaviour is interesting and may repay further investigation.  Mead 1917 Univ Calif Publ Zool 16: 431. 

Mole crab Emerita analoga
camouflaged on speckly sand? 0.6X

Many spider crabs, including Pugettia spp., Scyra acutifrons, Oregonia gracilis, and Loxorhynchus crispatus have organisms attached to and/or gowing on their carapaces that appear to act as camouflage.  In some cases these camouflagings result from settlement of spores and larvae; in others, from active attachment behaviour of the crab.  Passive buildup of growths is greater with increasing age as moulting frequency decreases.  Also, in many species there is a final or terminal moult which, if the species' exoskeleton is receptive to settlement of larvae and spores, leads to an even greater build-up of cover.  The drawing above Left shows the pattern of hooks extending from the carapace of Pugettia gracilis into which the crab actively inserts bits of algae and other material. It is not known whether it has muscular control over these hooks. The photograph shows two P. gracilis on a piece of kelp, both with attached bits of green algae Ulva sp. Whether camouflaging actually works in defense in P. gracilis or any other west-coast spider crab appears not to have been investigated.

The foliate spider crab Mimulus foliatus has variable colour patterns,
including those of this individual photographed at Bamfield, B.C. Most
specimens are much more drab than this and the function of what
appears to our eyes to be disruptive coloration, is not known 1.5X.
Photo courtesy Iain McGaw, U Nevada

There is much documentation in the scientific literature that features of size, mass, and volume of shells are involved in selection by hermit crabs, but a study in Vancouver, British Columbia shows also that how well a shell blends in with its background is important to the hermit crab Pagurus hirsutiusculus. The author first paints Chlorostoma funebralis shells with either black or white enamel paint, then presents them to crabs without shells. Shell-less crabs readily enter the new shells.  The author maintains 10 crabs in white shells for 1wk and 10 crabs in black shells for 1wk, then removes the crabs from their shells.  After 20min, each crab is presented with 5 empty black shells and 5 empty white shells on brown sand, and allowed to select a shell.  Nineteen of the 20 crabs select black shells, showing that previous shell colour has no significant effect on the new shell colour selected.  Do these black shell-preferring crabs also prefer dark areas?  This is tested in a T-maze that has an entrance corridor (light blue colour) leading into either a black cubicle or a white cubicle.  When allowed to crawl down the corridor, shell-less crabs exhibit no significant preference for one colour of cubicle over the other.  Crabs in black shells, however, select the black cubicle, and crabs in white shells select the white cubicle, in each case 88% of the time.  Other tests show that on a black/white checkerboard, crabs in black shells spend 75% of the time on black squares, while crabs in white shells spend 58% of the time on white squares (both are statistically significant).  The results suggest that P. hirsutiusculus spends most ot their time on a background that matches their current shell colour.  Partridge 1980 Bull Mar Sci 30: 914.

NOTE  the author employs a novel method of getting a hermit-crab out of its shell that is more stress-free for the participants than other methods. Simply collect crabs with shells that are too small for their occupants.  Hold a shell out of water and the crab may abandon it

One thinks of Dungeness crabs Cancer magister as having clean, unfouled carapaces, but this is likely to be true only of young crabs that moult several times per year.  Some old crabs are richly adorned with growths.  A study of Cancer spp. in Barkley Sound, British Columbia records 29 epibiont species of 10 phyla.  Cancer gracilis is least adorned (33% of 553 individuals examined), C. magister is intermediate (49% of 959 individuals), and C. productus is most adorned (68% of 1227 individuals).  A sample of the data (for C. productus) is shown graphically on the Right. Male crabs are larger than females and have correspondingly more epibionts. Barnacles, especially the subtidal species Balanus crenatus (representing >90% of all barnacle epibionts), are most common, and occur mainly on the upper carapace surface.  Algae (11 genera) are also common and are found mostly on the antennae (>80% of all algal infestations are in this location).  Other less numerous taxa include tube-dwelling polychaetes, bryozoans, hydroids and, rarely, tunicates, molluscs, and sponges.  Most epibiont speces are suspension-feeders, and attachment to a crab’s exoskeleton provides an elevated position with access to clean seawater. 

The presence of algae mainly on the antennae is thought by the author to owe to their position close to the urinary openings, a source of nitrogen-rich ammonia that could be an attractant for settling zoospores.  Is there any advantage to an old crab in having abundant epibionts on its body?  One obvious suggestion is camouflage especially, as noted by the author, to hide the antennae, which protrude (and flick) above the sand when a crab is buried.  Otherwise, a camouflage benefit for cancrid crabs in this area may be slight because the epibiont coverage is never very dense.  McGaw 2006 J Crust Biol 26: 85. Photos courtesy Iain McGaw, U Nevada.

A study in Kodiak Island, Alaska on epibionts on male Tanner crabs Chionoecetes bairdi reveals 39 different epibiont taxa.  Seven of these occur on >50% of the 98 crabs studied.  These include barnacles, amphipods, tubeworms, bryozoans, and other oganisms such as fungi.  Twelve species occur in the branchial chamber, including tubeworms, bryozoans, flatworms, and clams.  Older crabs have more epibionts. The authors discuss the positive effects of the epibionts, including camouflage from predators, and negative effects, including energy costs of transporting often heavy loads of epibionts, especially calcareous forms such as barnacles.  Effects on the crabs of organisms inhabiting their branchial chambers and gills are unknown.  Moulting is a way for crabs to relieve themselves of the burden of epibionts.  However, while male Chionoecetes continue to moult throughout their lives, females reach a terminal moult at sexual maturity, thus losing this ability.  Dick et al. 1998 J Crust Biol 18: 519.

NOTE  recently, researchers at the University of Alaska have shown through eyestalk-ablation experiments that both male and female snow crabs Chioneocetes opilio reach a terminal moult.  Tamone et al. 2005 Integr Comp Biol 45: 166.

Chionoecetes bairdi in an aquarium tank, epibiont-free

Despite the common occurrence of camouflage in marine invertebrates, tests of its functional significance are rare.  In crabs, camouflaging occurs in 3 main ways: colour, decoration¹, and growths² on the exoskeleton.  Colours involve pigment deposits and, because colour change requires moulting, it occurs on a timescale of weeks or months (chromatophores apparently contribute little to colour change in most adult crabs).  In contrast, decoration changes can occur within hours or days.  The adaptive significance of the first 2 strategies is examined by researchers at the Bodega Marine Laboratory of 3 species of epialtid kelp crabs co-occurring in central California kelp beds: Pugettia producta, P. richii, and Mimulus foliatus.  All 3 species inhabit and feed on red algae and kelp during at least part of their lives, with P. productua moving sequentially from intertidal red-algal beds to subtidal red-algal beds and then into the kelp canopy as they grow older.  The authors first investigate whether body colours differ with habitat.  They find that all 3 species are, indeed, significantly more red in red-algal habitats and more amber (i.e., kelp-coloured) in kelp habitats. The extent of colour change, moreover, is reciprocally related to the degree of decoration.  For example, P. producta, which decorates the least (23% in field individuals), has the highest magnitude of colour change, while P. richii, which decorates most (82%), has the lowest magnitude of colour³ change.  The third species, M. foliatus, is intermediate in both decoration (48%) and colour change.  The reciprocal correlations of colour and decoration mirror the effectiveness of each strategy in reducing predation.  Thus, in field predation experiments with these 2 species, where pairs of constrastingly coloured (P. producta) or combinations of coloured/decorated (P. richii) crabs are tethered to kelp or other algae in monofilament-line harnesses⁴, colour camouflage is found to reduce predation (presumably by fishes) on P. producta, while decoration camouflage is found to principally reduce predation on P. richii.  Hultgren & Stachowicz 2008 Oecologia 155: 519; photograph of P. richii courtesy Ron Long, SFU, Burnaby, B.C.  

NOTE¹  decoration occurs in only a few taxa, and involves attaching bits of algae or other debris to the body by means of hooked setae scattered about the carapace

NOTE²  growths on the exoskeleton occur during long moult-intervals in species that do not reach a terminal moult; in those that do reach a final moult, such as most or all decorator crabs, the carapace can become quite cluttered.  As much of this last camouflaging strategy tends to be happenstance, only the first 2 types of camouflaging are considered in the present study

NOTE³  distinctive white or white-patterned morphs of M. foliatus (see Research Study 2 above) are apparently uncommon in the study area and so are not used in the experiments

NOTE⁴  tests of tethering efficacy show that harnessed crabs never escape (4d lab tests, 2wk in predator-free tidal channels, 3d in subtidal cages); hence, any missing crab is assumed to have been killed and eaten by predators 

Kelp crab Pugettia producta
in an aquarium tank 0.6X

Juveniles of red rock-crabs Cancer productus exhibit a wide range of colour patterns.  For example, in a spring/summer collection of 163 juveniles from intertidal shores¹ in Barkley Sound, British Columbia, researchers from the Bamfield Marine Sciences Centre are able to resolve 6 colour types and 5 pattern types, as shown in the accompanying photograph. The colour proportions do not vary significantly among 4 collection sites, but they do vary with season.  By autumn, and with increased body size, the vivid colours and patterns are increasingly replaced by the rust-red coloration of the adult (occurring at around 40mm carapace width; see image 11).  Juveniles fed for 7mo on diets of eithr mussels or shrimps show no change in colour/pattern at the next moult².  In other laboratory experiments, juveniles exhibit no significant preferences for different backgrounds, substrata (see photograph of experimental arena below), or light levels.  Thus, neither camouflaging nor diet appear to be involved in selection of the colour morphs.  Instead, the authors speculate that disruption of formation of search images in a yet to be identified visual predator³(s) may be the explanation.  Thus frequency-dependent selection by this certain predator will lead to the more abundant variants being spotted and eaten more often than the rarer variants.  This favours survival of the latter until they, in turn, become the abundant, more easily spotted ones.  The explanation is clever and plausible, and readers of this interesting paper will hope that there is more to come. Krause-Nehring et al. 2010 Zoology 113: 131. Photographs courtesy the authors.

NOTE¹  collecting sites are typically underneath rocks or in eelgrass beds, buried in sand or gravel

NOTE²  moult frequency during the 7-mo feeding experiment varies from 0.6-1.3, depending upon ration size

NOTE³  known visual predators of juvenile C. productus include dogfishes, halibuts, sculpins, wolf eels, rockfishes, minks, birds, and octopuses

One type of experimental arena used in preference
tests with different colour morphs of Cancer
productus. The arena is 43cm in length