Defenses of crabs include camouflage, considered here, and HIDE AWAY and LIMB AUTOTOMY considered in other sections. Defense by claws may be such an obvious defense that no west-coast studies have been done on it; however, information on MECHANICS OF CRUSHING is extensive and can be found in its own section.
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The main topic of this section on camouflage is active decoration, while PASSIVE DECORATION, COLORATION, and BEHAVIOUR are found in their own sections.

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Active decoration

Research study 1

photograph of decorator crabs Pugettia gracilis bearing bits of green algae Ulva sp. on their carapacesMany spider crabs, including Pugettia spp., Scyra acutifrons, Oregonia gracilis, and Loxorhynchus crispatus have organisms attached to and/or growing 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 crabs Pugettia gracilis in the photograph on the Right are holding bits of brown and green algae in hooked setae protruding from its carapace. It is not known whether it has photograph of the foliate spider crab Mimulus foliatus has variable colour patterns,  including this brightly demarcated individual photographed at Bamfield, B.C. courtesy Iain McGaw, U Nevada. Most  specimens are much more drab than this and the function of what appears to our  eyes to be disruptive coloration, is not knownmuscular control over these hooks. Whether camouflaging actually works in defense in P. gracilis or any other west-coast spider crab appears not to have been investigated. Photograph of Mimulus on Left courtesy Iain McGaw, U Nevada.


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.

Research study 2

photographs of decorator crab Pugettia gracilis with attached algae, and close view of a seta or hook used to attach algal bitsEarly uncertainty over whether decorating by spider crabs is done mainly by mechanical or chemical (adhesive) means is clarified by a researcher at the Allan Hancock Foundation, University of Southern California, Los Angeles.  Decoration in Loxorhynchus crispatus, for example, involves attaching selected bits of algae, bryozoans, sponges, and so on to rows of hooked setae on the dorsal surface of the exoskeleton.  Electron-micrographical scannings reveal the fine structure of the setae, including median groove, proximal region bearing spirals, and distal region bearing spinules (see photographs).  Ablation of the setae prevents a crab from decorating itself, but removal of glands of the first-maxilliped feeding appendages has no effect.  Since these or similar glands have been thought by other researchers to provide an adhesive material for attachment, the results suggest that attachment is strictly mechanical.  Whether small muscles are involved in the gripping, or just elasticity of the setae is not mentioned by the author.  Wicksten 1978 Trans Am Microsc Soc 97 (2): 217.

NOTE some individuals are used as their own controls, in that one side is stripped of its hooked setae, while the other side is left intact.  Individuals treated in this way could still decorate normally on the untreated side, indicating no serious after-effects of the ablation treatments on a given individual

NOTE  tests on possible adhesive chemicals added to filter paper being chewed by a crab reveals no presence of acid mucopolysaccharides or proteins, both suggestive of the presence of adhesive secretions

Above: close view of hooked setae on exoskeleton
gripping bits of yellow sponge and red algae; below:
view of median groove and spiraling on seta

Research study 3

schematic showing behavioral repertoire of a decorator crab leading to attachment of an algal bitdrawings The same author describes decorating behaviour in spider crabs Loxorhynchus spp.  in more detail.  In total, the author studies 296 L. crispatus and 115 L. grandis from live collections and museum holdings over a 5yr period, and in addition does several in situ SCUBA-diving sessions.  The most common decorating materials for both species include red algae (about 14 species), bryozoans (4 species), hydroids (2-3), and sponges (4).  The behaviour used by both species when decorating is similar, following the schedule shown on the Left.  Details of acquisition, manipulation, and attachment are shown in the drawings.  If a piece does not readily attach it may be re-treated by the mouthparts several more times before being abandoned.  Loxorhynchus crispatus attaches about 40 different decorating species, 4-fold more than L. grandis, the difference being thought by the author to reflect the richer biodiversity found in the former’s usual habitats.  With respect to reproductively mature individuals of both species, only females of L. crispatus appear to decorate.  Some of this is passive, however, representing settlement of spores and larvae of various species.  This category of decoration is readily identifiable by its location on the carapace, separate from areas of hooked setae.  An experiment in which individual crabs are blinded by painting the eyestalks only temporarily affects decorating activity, indicating that selection of materials for decorating is not primarily visual.  Other than noting that each species tends to utilise material “in harmony” with its background, the author does no experiments specifically on camouflaging function of the decorations.  Therefore, much work still needs to be done on this topic.  Finally, as both dead and living materials are used indiscriminately by the crabs, there appears to be no mutualistic relationship between host and decoration as reported for other crustaceans, such as hermit crabs.  Wicksten 1979 Crustaceana Suppl No 5: 37.

Research study 4

What happens when a decorator crab Loxorhynchus crispatus moults?  Does it reclaim the old decorations for the new carapace or does it just discard the lot?  Research at Hopkins Marine Station, Pacific Grove shows that 4 of 8 aquarium-held individuals retain the old exuvia through the hardening period (24h), then pick off and re-use the useful decorations.  Of the remaining 4 individuals, 2 appeared to have decorated their new exoskeletons with bryozoans from the exuviae, while the other 2 abandoned their exuviae.  Wicksten 1975 Crustaceana 29 (3): 315.

Research study 5

drawing of crab Oregonia gracilis with decorationsdrawing of decorator crab Loxorhynchus crispatusA researcher at Texas A & M University, College Station provides details of camouflaging behaviour for several species of west-coast decorator crabs.  For a species such as Oregonia gracilis, the process of decoration involves the same steps as shown for Loxorhynchus in Research Study 3 above.  The crab uses its sharp chelae to slice off suitable lengths of camouflaging materials, such as sponges, bryozoans, and hydroids.  These are then roughened at their intended attachment ends by the mouthparts, then rubbed against an area of hooked setae on the exoskeleton (in the example shown, on the rostrum) until they become entangled or impaled.  There appears to be no glue involved, just the elastic tension of the setae.  Decorating behaviour may occur throughout the life a species or, for the moss crab, Loxorhynchus crispatus, may terminate at the onset of maturity (see drawing above Right).  The author believes that at this point in its life cycle, the crab may be large enough to withstand attack by most predators.  Some species such as Scyra acutifrons may actively decorate but at the same time act as passive hosts for settling larvae of bottom-dwelling invertebrates such as sponges and bryozoans (see photographs below).  Apart from passive camouflage from potential predators, other functions of the behaviour may include disguise for closer approach to prey, and provision of tools for active defense, such as a branches of hydroids containing functional stinging cells or pieces of sponges or tunicates containing toxic chemicals. Wicksten 1980 Sci Amer 242 (2): 146.  Drawings courtesy author and Scientific Amer, Inc.

  Some other examples of west-coast decorator crabs are shown below (photo of Scyra acutifrons top row, middle, courtesy Ron Long, SFU, Burnaby, British Columbia):
photograph of an unidentified spider crab with several attached sea anemones that may provide both stinging protection as well as camouflage
This unidentified spider crab has several attached sea anemones that may provide stinging protection as well as camouflage. Does the crab detach an anemone and then place it, or does a planula settle and grow?
photograph of crab Scyra acutifrons with a cover of sponges, tunicates, bryozoans, and possibly hydroids courtesy Ron Long, SFU, Burnaby, British Columbia
Scyra acutifrons with a cover of sponges, tunicates, bryozoans, and possibly hydroids. As with the anemone-bearing crab it is not known the extent to which these growths are actively or passively attained
photograph of a decorator crab that is using material from its immediate habitat for most effective camouflaging
Decorator crabs tend to use material from their immediate habitat for most effective camouflaging. If they change their habitat, then over time their covering growths also change
photograph of crab Scyra acutifrons with growths mainly of colonial tunicates and perhaps sponges
Scyra acutifrons with growths of colonial tunicates and perhaps sponges 0.6X
photograph showing close view of head of crab Scyra acutifrons, showing at least one colonial tunicate and possibly some sponges
Close view of head of this same Scyra, showing at least one colonial tunicate and possibly some sponges
photograph of crab Oregonia gracilis - an "active" decorator like P. gracilis, tending to decorate with things "at hand"
Oregonia gracilis is an "active" decorator like P. gracilis, and tends to decorate with things "at hand" 0.3X
photograph of an unidentified species of crab bearing a colour-matched ensemble of bryozoans, sponges, and possibly hydroids
This crab, whatever it is, has a colour-matched ensemble of bryozoans, sponges, and possibly hydroids
photograph of a crab Scyra acutifrons showing how its decorations tend to match the growths of its habitat; here, colonial tunicates
Scyra acutifrons showing how its decorations tend to match the growths of its habitat; here, colonial tunicates
photograph of an unidentified spider crab bearing mostly growths of healthy hydroids
This unidentified spider crab bears mostly growths of healthy hydroids
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photograph of the the undersurface of a helmet crab Cryptolithodes typicus as it walks. The expanded carapace likely does double duty, both for camouflaging and physical protection; photo taken from a video

CLICK HERE to see a video of the undersurface of a helmet crab Cryptolithodes typicus as it walks. The expanded carapace likely does double duty, for both camouflaging and physical protection.

NOTE  the video replays automatically

Research study 6

Adult kelp crabs Pugettia producta never seem to adorn themselves with algae or other camouflaging material, and a study at Bodega Bay Laboratory, California shows, in fact, that the requisite setal gripping structures are present only in the juveniles. For whatever reasons of selective benefit, they are lost during the moult to adulthood.  Questions posed include: 1)  do juveniles in the field prefer a certain algae for decoration, 2) if preferences exist, do they relate to availability of certain species, and 3) are similar preferences exhibited by laboratory-held drawing of head of decorated crab showing details of hooked setaespecimens?  Results show that of 60 juveniles collected in the field, 82% are decorated most commonly with branched reds such as Plocamium pacificum.  Ten seaweed species in total are recorded.  The author notes poor correlation between species present in the field and ones selected for adornment, so selection of a particular alga for decoration seems likely to be based on factors other than just its availabilitiy.  Finally, in the laboratory Pugettia prefers brown kelps Macrocystis integrifolia and Egregia menziesii over other other species and these species then tend to be the ones most commonly used for decoration.  The author discusses the function of the decorating behaviour.  Visual or chemical camouflage are obvious possibilities, but the relative size of the gripping setae is much smaller in P. producta than in other decorating species, photograph of kelp crab Pugettia producta Kristin Hultgrenso the potential effectiveness of this would also seem to be small (see drawing on Right and photograph on Left).  Another idea, based on the fact that some species of Pugettia will eat their decorations in times of food scarcity, is that the bits may act as a reserve food supply ,but for the same reason  this idea seems to have little practical merit.  Further research on this topic may be justified.  Mastro 1981 Crustaceana 41 (1): 64.  Photograph courtesy Kristin Hultgren, Smithsonian Institution, Washington.


Juvenile kelp crab Pugettia producta showing relatively small surface
area of algal decorations.  Compare with extent of camouflaging of
other species of decorator crabs shown in the photo series above 0.5X

Research study 7

A review of decorating behaviour in spider crabs (F. Majoidae) including several west-coast species already considered in earlier Research Studies, combines results from the author’s own studies as well as those of other researchers.  What follows is a general summary of these studies:

  1. large species of majoid crabs tend not to decorate.  Does this indicate a common evolution of  size-refuge from predation?
  2. sand-dwelling and deep-water species tend not to decorate.   Does this owe to scarcity of decorating materials, or lack of light (i.e., no selection for visual camouflaging from predators)?
  3. in daylight many majid species tend to be inactive, often sitting out in full view of predators.  The only moving parts are the flicking of the 2nd antennae under the rostrum, and the rostrum is often the part most heavily decorated.
  4. while many predators of majids exist (sea otters, fishes, octopuses, lobsters, and so on), field reports of predation are extremely rare.
  5. experiments in which decorated versus non-decorated crabs are offered to predators either in the field or laboratory are uncommon and often cursory, and the results are often inconclusive or contradictory.  For example: i) 2 sea otters in Monterey Bay reject both treatment types when offered as food, ii)  when offered to fishes such as rockfishes, sculpins, cabezons, and sheepshead, both treatment types are either ignored or taken in the mouth then spit out, iii) a laboratory-held Octopus bimaculatus that previously has eaten other types of crabs rejects a decorated Loxorhynchus crispatus, and iv) 3 wolf eels in the Monterey Bay Aquarium either ignore or reject an offering of 2 decorated L. crispatus.
  6. the flesh of several majid species is known to be edible to various sea-star, fish, turtle, and crab predators.

So, is the decoration functioning in camouflage protection in these crabs, either visual or chemical, or is some other function being served?  To test this will require a series of well-designed experiments using several species of potential predators, with tightly controlled unequivocal research protocols, and including further tests of selectivity of materials by the crabs under different experimental conditions.  For example, does the pattern of decoration differ in the presence of predators that may be mostly visual (fishes), mostly chemotactile (octopuses at night), or a combination?  In such circumstances, might there be competition for certain more useful decorating materials by the crabs?  Wicksten 1993 Crustaceana 64 (3): 314.

Research study 8

schematic graph showing percentage cover of hooked setae on the carapaces of numerous world species of majoid crabsphotographs showing different dietarily induced colours in kelp crabs Pugettia producta
Questions that should be on everone’s mind by now are: 1) what is the relationship between body colour and active decoration in kelp crabs (the only spider-crab taxon to have colours), 2) are there sex differences in extent of decoration/colour camouflaging, 3) what is the relationship between number of hooked setae on the carapace and body size in spider crabs, and 4) what is the phylogeny of decorative ability in spider crabs? Answers are provided in a review by University of California, Davis researchers. First, although kelp crabs decorate much less than other spider-crab species, they seem to compensate by having more potentially camouflaging colours (see photographs of Pugettia producta above). Second, although data are available for only 3 species1 out of the total, females are found to bear significantly2 more hooked setae than males, and there is a significant interaction with body size for 2 of the 3 species (see graphs below; note for H. coarctatus and O. gracilis that males exhibit stronger decreases in decoration as they grow larger than do females). Third, there is a strong negative correlation between the number of hooked setae and adult body size for about half the species examined. This suggests that active decoration using hooked setae may be more beneficial for younger stages, with older stages relying more on heavier exoskeleton and stronger absolute crushing strength of claws for defense. Finally, phylogenetic positioning of 32 species based on ribosomal-RNA typing bears little apparent relationship with percentage hooked-setal cover. To some extent this can be seen by a glance at the presentation on the right3, where several species of Pugettia range from zero hook cover to “high-minimal” hook cover (with another kelp-crab species, Mimulus foliatus, this group of Pugettia species represents the most complete taxon in the phylogenetic tree presented by the researchers). If colour is included, one Pugettia species P. dalli decorates more and changes colour the least , while P. producta decorates the least and changes colour the most. Like all good studies, this one raises more questions than it answers. Hultgren & Stachowicz 2009 The Am Nat 173 (5): 566. Photographs of Pugettia producta courtesy John Stachowicz.

graph comparing male and female majoid crabs with respect to percentage cover of hooked setae on carapaceNOTE1 if only 3 data sets show separate data for males and females, are we to believe that the researchers responsible for providing these data did not bother to separate the sexes, or perhaps had incomplete or too small data sets? If sexes have been ignored or possibly inconsistently intermixed, how confident can the authors be about the accuracy of the data they present here?

NOTE2 the authors appear to have incorrectly plotted these setal-area vs. body-size data. An area-based measurement plotted against a linear measurement would be expected to scale exponentially; thus, a straight line as shown in the graphs would result only after log-log transformation of data on each axis. A semi-log transformation of just the Y-axis data (setal area) as the authors have done would help to “visually” straighten the line, but the resulting data might not support subsequent statistical analysis

NOTE3 these data include only setae on the top of the carapace; leg setae are present in many species but, for technical reasons (e.g., missing legs), they are not included by the researchers