Marine Invertebrates of Bermuda Decorator Crab (Macrocoeloma trispinosum nodipes) By Kaitlin Baird Dr. James B. Wood and Kim Zeeh - Editors Abstract  Taxonomy  Habitat  Ecology  Recent Research  Commercial Importance  Bermuda Laws  Personal Interest  References  Links

Abstract


Macrocoeloma trispinosum nodipes is the only species of decorator crab found in Bermuda and is a member of the Majidae family. These abundant crabs can be found within the rocky intertidal zone, within the shallow bays of Bermuda (Sterrer 1986). These crabs are characterized by their amazing ability to camouflage themselves with organisms in their environment such as corals, anemones, sponges, and various algae (Woods and McLay 1994). This species' anatomy, reproductive strategy, and feeding methods are characteristic of all true crabs (Jones and Shimlock 1992). The larval form is characterized by a planktonic zoea (Marques et al. 2003). Research into this camouflaging behavior is a subject of interest to scientists looking at the different purposes of its extensive covering. Too much decoration may heighten energy costs such as mobility, feeding, and escape strategies (Woods and McLay 1994). However, it has been shown that algae on their carapce can serve as an alternate food source (Woods and Mclay 1994). Although this species has no documented commercial importance in Bermuda, many species of decorator crab are often used in the aquaria trade. This particular species is not protected by any specific marine laws in Bermuda.


Taxonomy


Phylum: Arthropoda
  Class: Crustacea
    Subclass: Malacostraca
      Order: Decopoda
        Family: Majidae



Habitat


Macrocoeloma trispinosum nodipes, commonly known as the decorator crab, is found in shallow bays and is the only decorator crab in the family Majidae to appear in Bermuda (Sterrer 1986). This crab thrives in shallow water areas, in less than 20 m, in part because of its unique camouflage abilities (Sterrer 1986). In general, decorator crabs inhabit tropical and subtropical waters (Jones and Shimlock 1992). They display high site fidelity and have low mobility because they must remain near their substratum for protection from predation (Stachowicz and Hay 1999).

Macrocoeloma trispinosum nodipes uses pieces of its environment such as algae, coral, bryozoans, sea squirts and sponges to decorate the carapace and legs of its exoskeleton, creating its own microhabitat (Woods and McLay 1994). This characteristic of attaching selected materials to various parts of their bodies has been seen to both have camouflage and alternate food source benefits (Woods and McLay 1994). These materials remain alive on the crab and are held in position by special hooked setae (Woods and Mclay 1994). Many of these interactions are symbiotic, in which anemones and sponges benefit from a constant water current via the crabs walking motion, and are able to take advantage of leftovers of the crabs meal. The invertebrates and algae in turn allow Macrocoeloma trispinosum nodipes to blend in perfectly with its environment to avoid predation (Jones and Shimlock 1992). Once attached this camouflage seems to grow by itself (Colin 1978). In many cases the crab is nearly undistinguishable from the bottom, with anterior and lateral portions of the carapace densely covered (Colin 1978). Shallow bays of the rocky intertidal zone of Bermuda such as Whalebone Bay and Bailey’s Bay, are perfect environments for these decapods to hide among the rubble, where unless they move, they are perfectly camouflaged (Sterrer 1986).

Ecology


Reproduction

The decorator crab lives singly during most of the year, until the arrival of mating season from May to Spetember (Carmona-Suarez 2003). Courtship behaviors have been seen in mating pairs, but it is believed that chemical cues play an important role in acquiring a mate (Kilar and Lou 1986). The male pleopods are highly modified for clasping the female and transfering sperm (Kilar and Lou 1986). The male's sperm lack flagella, so sperm are packaged into spermatophores, which are inserted into the female's oviduct. The fertilized eggs are transported and attached to bristles on the female abdomen. The abdomen plate covers the eggs and often bulges outward (Kilar and Lou 1986). During the brooding stage, maternal care is given to the eggs by frequent aeration and removal of foreign bodies. When the eggs hatch, the larvae become planktonic in the water column (Kilar and Lou 1986).

The life cycle of the decorator crab begins with a zoea larvae, and drifts through the water column as plankton (Marques et al. 2003). As the juvenile crab grows in size, it goes through a series of molts (Marques et al. 2003). At each molt more segments are added posteriorly and the feathered limbs are replaced by clawed limbs (Marques et al. 2003). The megalops stage more closely resembles the adult decapod crab.

Anatomy

Although decorator crabs such as Macrocoeloma trispinosum nodipes, tend to remain in one place, they do have to move to feed and reproduce (Kilar and Lou 1986). The characteristic sideways movement of crabs is accomplished by using striped skeletal muscles attached to a chitinous exoskeleton (Stachowicz and Hay 2000). Decorator crabs are usually characterized by their long, spindly legs. The chelipeds, or pinchers, are modified for feeding; the other four pairs of legs are for walking (Woods and McLay 1994).

Feeding

Macrocoeloma trispinosum nodipes feeds in a manner conserved throughout the Decopoda order of the class Malacostraca (Sterrer 1986). The five most anterior segments of the crab’s body are fused as a head and five pairs of appendages mark the position of these segments (Woods and McLay 1994). These head appendages have become highly modified to aid in feeding process.

Decorator crabs possess six basic mouth parts (Thanh et al. 2005). The most posterior three, the maxillipeds, are not head appendages but are the first three appendages of the thorax (Thanh et al. 2005). They are used to cover and protect the inner most mouth parts as well as to manipulate larger pieces of food. Once food has been obtained via the large pincers, or chelipeds, it is passed to the posterior-most maxilliped, which moves food into the other mouth parts. The inner- most mouth part, the mandible, is the main agent of food maceration (Thanh et al. 2005). The other mouth parts, the maxillae, are used to rip food while held by the mandible. Finally, once the food has been reduced to an appropriate size, it is passed into the mouth. The mouth leads to the esophagus, which in turn opens to a specialized stomach (Thanh et al. 2005). The stomach is lined with an exoskeleton shaped like teeth that line an area of the stomach known as the gastric mill. This muscular stomach can grind food into a pulp using the teeth along with strong muscle contractions (Thanh et al. 2005).

The diet of the decorator crab is mainly omniverous, consisting of both algae and small invertebrates. (Kilar and Lou 1986). The long spindle pincers do not have the adaptations for crushing bivalve shells or other crabs exoskeleton's (Woods and Mclay 1994). These crabs use the longer pincers to pluck small animals from crevices and are scavengers of the sea floor (Woods and McLay 1994). Tropical species of decorator crab have a diet consisting mostly of algae (Stachowicz and Hay 2000).

Recent Research


Decorator crab research has centered on the areas of camouflage and dietary preference. In a study done by Kilar and Lou (1986) it was found that a decorator crabs' feeding selection is not independent of decorating activity. Recent research shows this decorating behavior is rooted in both availability of algae and other resources, and the nutrients and biomass of algae (Kilar and Lou 1986). It was shown in this study that by allocating the carapace flora between food and camouflage, the crab can decrease its energy expenditure and risk of predation to obtain food and minimize overall movement (Kilar and Lou 1986). It has also been found that these decorations of seaweed and invertebrates such as sponges and polychaetes can provide chemical defenses against fish and other predators, which find these organisms toxic or distasteful (Duffy and Hay 1994).

Some researchers have found that decorator crabs will use algae on their carpace as an alternate food source (Woods and Mclay 1994). In a study done by Woods and McLay (1994) the decorator crab, Notomithrax ursus utilized an algal mask as a source of food. When the crabs were starved of an alternate food source they ate an average of 4.1% of the mask per day, but when given an alternate food source of mussel flesh daily they only consumed 2.8% (Woods and McLay 1994). Kilar and Lou (1984) demonstrated that if a decorator crab has no algal canopy or way to conceal its movement, the crab is subjected to increased predation pressures. By limiting its movement with too much decoration the crab, however decreases its exposure to potential food (Kilar and Lou 1986). The delicate balance between food storage and camouflage techniques of the decorator crab are still yet to be fully understood.

Commercial Importance


Although Macrocoeloma trispinosum nodipes has no documented comercial importance, they do protect localized patches of coral, in bay zones, by keeping algal populations under control, along with other herbivores (Stachowicz 1999). Decorator crabs from around the world are frequently used in the aquaria market. Some of the most expensive and popular decorator crabs cover themselves in colorful anemones. Divers and aquarists alike are fascinated by their choices for disguise.


Bermuda Laws


This species does not have specific special laws that protect it in Bermuda. However, the 29 Marine Protected areas of Bermuda prohibit the collection of this decorator crab along with other marine life. These protected areas fall under the Fisheries Act of 1972 (Wood and Jackson 2005).


Personal Interest


I find decorator crabs fascinating because of the great lengths they will go to camouflage themselves. I think the most interesting part is, why are they doing it? Is it a chemical defense to make themselves inedible by covering themselves with impalpable algae, is it a sunscreen? Is it an anti-predator or predator mechanism? One of the reasons I wanted to use this species is that there are no published studies done on Macrocoeloma trispinosum nodipes in Bermuda.

I want to investigate a species that has advanced adaptations and has evolved with the reef environment. I was really fascinated that with the use of their chelipeds they are able to cut algae and sponges to the exact length to cover their appendages, and I wondered how they have evolved this behavior, as well as the ability to adhere these items to their exoskeleton. I have decided to study this in further detail in my research project at the Bermuda Biological Station for Research.

References


Carmona-Suarez, C. 2003. Reproductive biology and relative growth in the spider crab Maja crispata. Scientica Marina 67: 75-80.

Colin, P. caribbean reef invertebrates and plants. England. T.F.H. Publications inc. 1978.

Duffy, J. and M. Hay. 1994. Herbivore resistance to seaweed chemical defense: the roles of mobility and predation risk. Ecology 75: 1304-1319

Jones, B., and M. Shimlock. 1992. Masters of Deception. Sea Frontiers 38: 30-40.

Kilar, J. and R. Lou. 1986. The subtleties of camouflage and dietary preference of the decorator crab, Microphtys bicorntus Latreille. J. Exp. Mar. Biol. Ecol. 101: 143-160.

Kilar, J. and R. Lou. 1984. Ecological and behavioral studies of the decorator crab, Microphrys bicornutus: a test of optimal foraging theory. J. Exp. Mar. Biol. Ecol. 74: 157-167.

Marques, F., Pohle, G., and L. Vrbova. 2003. The larval stages of Macrocoelema diplacanthum, and a review of Mithracine phylognentic aspects. Journal of Crustacean Biology 183: 187-200.

Stachowicz, J. and M. Hay. 2000. Geographic variation in camouflage specialization of a decorator crab. The American Naturalist 156: 58-71

Stachowicz, J. and M. Hay 1999. Reducing predation through chemically mediated camouflage: indirect effects of plant defenses on herbivores. Ecology 80: 495-509.

Sterrer, Wolfgang., ed. Marine Fauna and Flora of Bermuda: A Systematic Guide to Identification of Marine Organisms. New York: John Wiley and Sons, 1986.

Thanh, P., Keiji, W., Michiko, S. and S. Yoshihisa. 2005. The effect of resource availability, predators, conspecific and heterospecifics on the decorating behavior of the majid crab Tiarinia cornigera. Marine Biology 147: 1191-1199

Wood, J. and K. Jackson. 2005. Bermuda, In: Caribbean marine Biodiversity., Miloslavich, P. and E. Klein Eds. De Stech Publications Inc. pp 19-35.

Woods, C. and C. McLay. 1994. Use of camouflage materials as a food store by the spider crab Notomithrax ursus. New Zealand Journal of Marine and Freshwater Research 28: 97-104


Links

Decorator Crab
Crab data sheet (Teachers)
Decorator Crabs!
Aquarium Information
Commercial Species of Decorator Crab
Cool Pictures of other species!