Decorator Crab Genus: Loxorhynchus Species: crispatus ITIS: 98478 Primary Common Name: Decorator crab Common names (s): Moss crab, masking crab Similar Species: Scyra acutifrons General grouping: (pull down on site) Crabs, barnacles, shrimp, lobster Geographic Range Range description: Northern California, Redding rock (Humboldt County) to Isla Natividad (Baja, Southern California). Brief range description: (this should always be included in above description) Redding rock (Humboldt Co.) to Isla Natividad (Baja California) Habitats (pull down on site) Habitat notes: The decorator crab can be found locally on rocks, pilings, and kelp holdfasts; below low-tide line to water 600 ft. (183 m) deep. Occurs in sublitoral; low intertidal areas on semi protected rocky coasts in crevices; often heavily decorated with hydroids, sponges, and algae. Large individuals are often are found clinging, head down, on vertical walls and pilings. Abundance Relative abundance: Scarce in low intertidal zone on rocky shores of protected outer coast, much more com-mon on subtidal pilings, kelp holdfasts, and rocks to 183 m depth. Species Description: The Decorator crab, Loxorhynchus crispatus, is pear shaped, and covered with growth. It is a grayish-brown color, covered with short, brownish hairs, and has white fingertips. Carapace is widest in the rear third; side margin without spines; beak notched at the tip, thick, moderately long, bent down slightly; sharp spine above and one beside eye. Pincers long, slender; fingers short; 2nd pair of walking legs longest, 5th is the shortest. L. crispatus is generally a slow-moving crab, at least during the day and when left alone. But at night it is more active, and if harassed it is quite capable of scuttling across the bottom at a respectable pace, or scampering off of a ledge and para-chuting down with outstretched legs. Until it moves, however, one may never suspect that the crab is even there. Distinctive features: To some animals the accidental growth of algae or sessile animals on their shells seems to be a source of danger, presumably because the weight and the water resistance of such growth might inhibit their movement or ability to cling to the substratum. Others tolerate such growths, and still others, such as L. crispatus, go to the extreme of augmenting the natural growths by planting hydroids, algae, sponges, etc. on their backs. Although mask-ing may serve the dual purpose of concealment from predators, and prey, L. crispatus uses masking primarily for defense, as its diet consist largely of sessile animals. Key Feature Decorator crabs attach masking materials to their bodies by wedging or impaling them among hooked setae on the back and legs. Size: Carapace width males: up to 8.8 cm (3.5 in.) Carapace width females: up to 6.8 cm (2.7 in.) Carapace length: up to 10 cm (3.9 in.) Natural History General natural history: L. crispatus uses hooked setae on their back and legs, to attach masking ma-terials to their bodies. Crabs that have had these hooks removed experimentally cannot attach materials until their next molt, when a new set of hooks are produced. Masking materials are normally removed from the cast-off exuvia of the molt and recycled in re-decorating the crab’s new exterior. Large crabs, especially males over 8 cm in carapace width, apparently no longer actively decorate their backs, but may nevertheless be cov-ered by various growths that have settled and grown there without assistance. Other sen-sory bristles inform the crab about the status and position of its decorations. These mate-rials remain alive on the crab and are held in position by special hooked setae. Many of these interactions are symbiotic, in which anemones and sponges benefit from constant water currents via the crabs walking motion, and are able to take advantage of leftovers of the crab’s meal. Excess decoration may increase energy costs such as mobility, feeding, and escape strategies. However, it has been shown that algae on their carapace can serve as an alternate food source. Relative growth techniques are very useful in maturation studies. The relative size of claws is also an aid to recognition of social systems. Males with particularly large claws are generally dominant, and males of different sizes and claw development may adopt very different behavioral reproduction strategies. Discontinuity of growth is one of the most singular aspects of the lives of arthropods. Ecdysis, is the process of shedding the old skin, called the exuvia, and the accompanying increase in size only takes a few mi-nutes. However, the entire interval between molts represents a dynamic, cyclical process. Following ecdysis the animal is soft, until the cuticle gradually hardens as different areas of the exoskeleton calcify sequentially. Before its new shell hardens, the crab absorbs wa-ter and expands to a size larger than before the molt. While the new shell is solidifying, the crab hides from predators. Post molt ends with the deposition of a thin, membranous layer. The premolt period, is signaled by the separation of the epidermis from the cuticle. A mi-totic burst, expanding the number of epidermis cells, precedes the deposition of new cuti-cular structures. First, new setae are organized using the previous cuticle as a template. After the new setae are organized, pre-exuvial layers are secreted over the general body surface. Molting is imminent if the epimeral suture of a crab is visibly split. Exuviae can be distinguished from empty remains of a dead animal, by the absence of pigment from the corneas of the eyestalks of the exuvia. Discovery of an intact fragile exuvia suggests that a very soft, recently molted animal may be nearby hiding. The pre-vious owner of the exuvia can by identified by matching the details of pigment patterns of the exuvia, with the soft animal. Comparison of the soft animal and its exuvia demon-strates the growth increment per molt for animals of that particular size. Decapods are able to cast off (autotomize) their limbs under stress, and then later regene-rate the appendages at subsequent molts. Autotomy is easily demonstrated by squeezing the basal segments of an appendage. A specific muscle is stimulated, which slices through a cuticular apodeme, severing the limb. The autotomized limb is severed at a preformed breakage plane. Upon autotomy, a flap of skin seals over the severed limb base so that barely a drop of blood is lost. The regenerating limb forms in a bud, which protrudes from the stump of the autotomized limb. Recently regenerated appendages are somewhat smaller than normal limbs, but this size discrepancy is no longer apparent after several molting cycles. Predator(s): O. rubescens, fish, and some marine mammals. Prey: L. crispatus is a generalist omnivore that feeds on drift kelp, and a variety of both living and dead sessile invertebrates, such as worms and mollusks. Feeding behavior: (click on site) Feeding behavior notes Crabs are omnivores, feeding primarily on algae, and taking any other food, includ-ing molluscs, worms, other crustaceans, fungi, bacteria and detritus, depending on their availability and the crab species. For most crabs, a mixed diet of both animal and plant matter results in the greatest fitness and fastest growth. Algae on the animal’s carapace can also serve as an alternate food source. Reproduction L. crispatus reproduces sexually. Upon reaching sexual maturity, most if not all decapods undergo a molt of puberty to attain their adult morphology. Ovaries of deca-pod Crustacea lie dorsally to other organs in the carapace and the anterior part of the ab-domen. Eggs that are recently laid rest in a gelatinous mass on the crabs pleopods (swimming legs). Within one day, an outer membrane forms around each egg and a thin strand attaches it to the pleopodal setae. Young eggs are generally evenly pigmented. As the yolk is gradually displaced by the growing embryo, a transparent area appears in the egg; in advanced embryos, eyespots and larval chromatophores may also be recognized. Broods that are about to hatch, often have a grayish cast since all the brightly colored yolk has been absorbed. Information about the reproductive state of male decapods is rel-atively difficult to obtain, dissection and microscopic examination are often required. Most crustaceans deposit a single brood in an instar (stage). Conservation Issues Decorator crabs are a main food source for some fishes, including croakers and cabezon. Presently the population of decorator crabs is not in danger; however, pesticides, oil runoff, chemicals, and paint solvents threaten the crabs’ habitats. As stewards of the oceans, we must carefully dispose of hazardous materials like these or, better yet, use environmentally safe products.
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