Loxorhynchus crispatus

Geographic Range

Moss crabs, also known as decorator crabs, are found along the southwest coast of North America from Redding Rock, California to Natividad Island (Baja, California).

• Biogeographic Regions
• nearctic nearctic
  • native native

Habitat

Moss crabs are marine crustaceans that are most commonly found in the middle and deeper reef regions of kelp forests. They may also be found in rocky areas or on underwater structures in the epipelagic zone, up to 183 m deep. This species prefers habitat with plants or other cover material, which provides protection from open water predators.

• Habitat Regions
• temperate temperate
• saltwater or marine saltwater or marine

• Aquatic Biomes
• coastal coastal

Physical Description

Moss crabs are large spider crabs. Average carapace width ranges from 68 mm (female) to 88 mm (male), with a maximum carapace length of up to 100 mm. The carapace is pear-shaped (pointed anteriorly) and is covered in short hairs, called setae, which are used in attaching various materials for camouflage. Two rows of hooked setae are present on the rostrum, as well as a sharp spine above and next to each eye. The walking legs and carapace areas near the gills and stomach are also covered in setae. Camouflage materials are placed in the setae rows, keeping them firmly attached. While it is not usually visible, due to being covered with attached items, the carapace color is typically dark green, brown, or gray-brown. One spine and a dome-shaped structure are present on the hepatic region. These crabs have four pairs of walking legs, located on each side of the shell, and one pair of chelae (claws) located anteriorly. Walking leg length is usually very similar or equal to the carapace width, with the second pair of walking legs being the longest. In addition to their larger carapace width, males also have much larger chelae than females.

• Other Physical Features
• ectothermic ectothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• male larger

Development

Moss crab eggs are attached to setae on their mother's pleopods; eggs may be as large as 0.55 mm in diameter. As embryos grow, they consume egg yolk and, as eggs near hatching, they turn gray in color. These crabs begin life as protozoea larvae. After approximately 30 days, they develop into planktonic zoeae, swimming with limb-like cephalothorax structures, which will develop into the antennae and mandibles. A zoea completes two additional developmental stages, lasting about 18 days, before metamorphosing into a megalops, resembling a small adult. After additional molts, it becomes a juvenile crab, with an average carapace width of 1.5 mm, and eventually develops into a sexually mature adult. Development for this species has not been extensively studied, but some research suggests that the length of time of larval development is dependent on water temperature, with colder temperatures causing slower development and lower reproductive output.

• Development - Life Cycle
• metamorphosis metamorphosis

Reproduction

The mating system of moss crabs has not been described, however the mating system of a closely related spider crab, ( Libinia spinosa ) may be similar. Male spider crabs have two pairs of gonopods of different sizes located anterio-ventrally. The larger pair is inserted into a female’s gonopores (vulvae) during copulation. There are six different types of setae on a male's gonopods, which many aid in positioning during copulation. Rosette glands, producing seminal fluid (males) and nutritional support for fertilized eggs (female), are located in the gonopods of males and the pleopods of females. They may also aid in molting. Sheep crab ( Loxorhynchus grandis ), another closely related species, comes to shoreline areas to mate. Males compete with each other to find female partners. After a male is successful, he uses his back legs to hook a female to him, back to back, creating an obstetrical pair.

• Mating System
• polygynandrous (promiscuous) polygynandrous (promiscuous)

Mating occurs throughout the year, rather than seasonally. Females reproduce for 1-2 month periods with short resting periods in between, producing an average of 3.5 broods per year. Though information on brood size for moss crabs is not available, closely related sheep crabs lay between 125,000 and 500,000 eggs per brood, and moss crabs may produce broods of similar sizes. Recently laid eggs may be found on a female's pleopods in an undifferentiated, gelatinous mass. Within a day, however, each egg develops a membrane and a thin filament, which attaches it to the pleopod setae.

• Key Reproductive Features
• iteroparous iteroparous
• year-round breeding year-round breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• fertilization fertilization
  • internal internal
• oviparous oviparous

Female moss crabs carry fertilized eggs on their pleopodal setae for a short period of time. Once eggs have hatched, there is no further investment from either parent.

• Parental Investment
• female parental care female parental care
• pre-fertilization
  • protecting
    • female
• pre-hatching/birth
  • protecting
    • female

Lifespan/Longevity

There is currently no data available on the average or maximum lifespan for this species.

Behavior

Moss crabs are known for decorating their shells by attaching other organisms such as algae, sponges, and bryozoans to their shells, helping them to blend in with their environments. They do not use an adhesive material to attach decorations, instead using their hooked setae to secure materials and other organisms; in one experiment, crabs whose setae were removed were unable to decorate until their next molt. Decorating materials may be chewed before attaching, probably to soften them. Molting only occurs during juvenile stages; Moss crabs do not molt again after reaching sexual maturity. After molting, they may remove decorations from their old shells and re-attach them to their new ones. Larger crabs decorate much less than smaller ones, and younger crabs have longer setae and generally decorate more, aiding in camouflage and predator avoidance.

As is characteristic of brachyuran crabs, moss crabs can shed a limb if it gets stuck or is attacked by a predator. The crab can still effectively walk after the leg is released. Eventually a new leg grows to replace the missing limb. Males with larger claws tend to be dominant, which is particularly noticeable during mating.

These crabs have been found to demonstrate a cyclic pattern of population size, peaking in the fall and reaching a low in the spring.

• Key Behaviors
• nocturnal nocturnal
• motile motile
• sedentary sedentary
• dominance hierarchies dominance hierarchies

Home Range

Moss crabs are not known to occupy or defend particular home ranges or territories.

Communication and Perception

Although data specific to moss crab perception is not currently available, research on decapods sheds some light on traits common to the order. They are known to have compound eyes, tactile setae and olfactory filaments.

• Communication Channels
• visual visual
• tactile tactile
• chemical chemical

• Perception Channels
• visual visual
• tactile tactile
• vibrations vibrations
• chemical chemical

Food Habits

Moss crabs are generalist omnivores and scavengers, feeding on many kinds of invertebrates, both living and dead (including worms, molluscs, and other crabs), algae, and giant kelp. These crabs may also eat small amounts of their decorations to sustain themselves, particularly under adverse conditions.

• Primary Diet
• carnivore carnivore
  • eats non-insect arthropods
  • molluscivore molluscivore
  • vermivore
  • eats other marine invertebrates
  • scavenger scavenger
• herbivore herbivore
  • algivore
• omnivore omnivore
• planktivore planktivore
• mycophage mycophage
• detritivore detritivore

• Animal Foods
• mollusks
• aquatic or marine worms
• aquatic crustaceans
• other marine invertebrates
• zooplankton zooplankton

• Plant Foods
• algae
• macroalgae macroalgae
• phytoplankton phytoplankton

• Other Foods
• fungus
• detritus detritus
• microbes

Predation

The decorated shells of moss crabs act as camouflage, which is important because these crabs move very slowly. Moss crabs favor bryozoans as decorative materials, as some species secrete chemicals that may deter predators.

Due to their relatively large size, moss crabs have fewer predators than other crab species. They are, however, still a food source for a variety of fishes, marine mammals (more rarely), and invertebrates.

• Anti-predator Adaptations
• cryptic cryptic

Ecosystem Roles

Moss crabs form mutualistic relationships with the organisms they use to decorate their shells; the crabs gain protection in the form of camouflage, while the organisms used for decorating benefit from the water currents created by the crabs' movements. As scavengers, moss crabs also help to remove detritus from their environments. They may also serve as hosts to bacterial infections, barnacles, and other parasites.

• Ecosystem Impact
• biodegradation biodegradation

Economic Importance for Humans: Positive

There are no known economic benefits of moss crabs to humans, beyond their display in aquaria and use in various research projects.

• Positive Impacts
• research and education

Economic Importance for Humans: Negative

There are no known adverse effects of this species on humans.

Conservation Status

This species has no special conservation status.

Additional Links

Encyclopedia of Life

Contributors

Alyssa Sanders (author), University of Michigan-Ann Arbor, Alison Gould (editor), University of Michigan-Ann Arbor, Jeremy Wright (editor), University of Michigan-Ann Arbor.

References

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