Loxorhynchus crispatusmoss crab

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). ("Loxorhynchus crispatus-Masking crab", 2013; Hendrickx and Cervantes, 2003)


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. ("Loxorhynchus crispatus-Masking crab", 2013; Garth and Abbott, 1980; Hendrickx and Cervantes, 2003)

  • Range depth
    0 to 183 m
    0.00 to 600.39 ft

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. ("Loxorhynchus crispatus-Masking crab", 2013; Hendrickx and Cervantes, 2003; Hines, 1982a)

  • Sexual Dimorphism
  • male larger
  • Range length
    100 (high) mm
    3.94 (high) in


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. ("Loxorhynchus crispatus-Masking crab", 2013; Garth and Abbott, 1980; Hines, 1982a; Hines, 1986)


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. ("Sheep Crab", 2001; Sal Moyano, et al., 2011)

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. ("Loxorhynchus crispatus-Masking crab", 2013; "Sheep Crab", 2001; Hines, 1982b; Hines, 1982a)

  • Breeding interval
    Females may produce a brood of eggs once every 1-2 months.
  • Breeding season
    Moss crabs may breed year round.
  • Average gestation period
    64 days

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. (Garth and Abbott, 1980)

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


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


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. ("Loxorhynchus crispatus-Masking crab", 2013; Wicksten, 1978; Wicksten, 1979)

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. ("Loxorhynchus crispatus-Masking crab", 2013; Garth and Abbott, 1980)

These crabs have been found to demonstrate a cyclic pattern of population size, peaking in the fall and reaching a low in the spring. (Hines, 1982a)

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. (Chase and Knowlton, 2007)

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. ("Loxorhynchus crispatus-Masking crab", 2013; Hines, 1982a; Stevens and Merilaita, 2011)

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


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. ("Loxorhynchus crispatus-Masking crab", 2013; Stevens and Merilaita, 2011)

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. (Garth and Abbott, 1980; Hines, 1982a)

  • Anti-predator Adaptations
  • 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. ("Loxorhynchus crispatus-Masking crab", 2013; Alvarez, et al., 2001; Jorge, 1985; Stevens and Merilaita, 2011)

Mutualist Species
Commensal/Parasitic Species

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. ("Loxorhynchus crispatus-Masking crab", 2013)

  • Positive Impacts
  • research and education

Economic Importance for Humans: Negative

There are no known adverse effects of this species on humans. ("Loxorhynchus crispatus-Masking crab", 2013)

Conservation Status

This species has no special conservation status. (IUCN, 2013)


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



living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


helps break down and decompose dead plants and/or animals


an animal that mainly eats meat


uses smells or other chemicals to communicate


the nearshore aquatic habitats near a coast, or shoreline.


having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.


an animal that mainly eats decomposed plants and/or animals


particles of organic material from dead and decomposing organisms. Detritus is the result of the activity of decomposers (organisms that decompose organic material).

dominance hierarchies

ranking system or pecking order among members of a long-term social group, where dominance status affects access to resources or mates


animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

female parental care

parental care is carried out by females


union of egg and spermatozoan


An animal that eats mainly plants or parts of plants.

internal fertilization

fertilization takes place within the female's body


offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).


seaweed. Algae that are large and photosynthetic.


A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.


eats mollusks, members of Phylum Mollusca


having the capacity to move from one place to another.


an animal that mainly eats fungus

native range

the area in which the animal is naturally found, the region in which it is endemic.


active during the night


an animal that mainly eats all kinds of things, including plants and animals


reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.


photosynthetic or plant constituent of plankton; mainly unicellular algae. (Compare to zooplankton.)


an animal that mainly eats plankton


the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.


an animal that mainly eats dead animals


remains in the same area


reproduction that includes combining the genetic contribution of two individuals, a male and a female


uses touch to communicate


that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).


movements of a hard surface that are produced by animals as signals to others


uses sight to communicate

year-round breeding

breeding takes place throughout the year


animal constituent of plankton; mainly small crustaceans and fish larvae. (Compare to phytoplankton.)


2013. "Loxorhynchus crispatus-Masking crab" (On-line). Sanctuary Integrated Monitoring Network (SIMoN). Accessed July 19, 2013 at http://www.sanctuarysimon.org/species/loxorhynchus/crispatus/masking-crab.

California Department of Fish and Game. Sheep Crab. 2001098707. University of California: Agriculture and Natural Resources. 2001. Accessed March 25, 2012 at http://www.dfg.ca.gov/marine/status/status2001.asp.

Alvarez, F., E. Campos, J. Høeg, J. O'Brien. 2001. Distribution and prevalence records of two parasitic barnacles (Crustacea: Cirripedia: Rhizocephala) from the west coast of North America. Bulletin of Marine Science, 68/2: 233-241. Accessed July 29, 2013 at http://www.ingentaconnect.com/content/umrsmas/bullmar/2001/00000068/00000002/art00008?crawler=true.

Chase, F., R. Knowlton. 2007. Decapoda (Crustacea). Pp. 286-296 in AccessScience, Vol. 5, 10 Edition. New York: McGraw-Hill Companies. Accessed March 25, 2012 at http://accessscience.com/content/Decapoda-%28Crustacea%29/181900.

Garth, J., D. Abbott. 1980. Intertidal invertebrates of California. Stanford, California: Stanford University Press.

Hendrickx, M., J. Cervantes. 2003. A new species of Loxorhynchus Stimpson (Decapoda, Majoidea, Pisidae) from the Pacific Coast of Mexico. Crustaceana, 76/1: 103-113. Accessed February 15, 2012 at http://www.jstor.org/stable/20105541.

Hines, A. 1982. Allometric constraints and variables of reproductive effort in brachyuran crabs. Marine Biology, 69/3: 309-320. Accessed March 24, 2012 at http://link.springer.com.proxy.lib.umich.edu/content/pdf/10.1007%2FBF00397496.pdf.

Hines, A. 1982. Coexistence in a kelp forest: size, population dynamics, and resource partitioning in a guild of spider crabs (Brachyura, Majidae). Ecological Society of America, 52/2: 179-198. Accessed February 02, 2012 at http://www.jstor.org/stable/1942610.

Hines, A. 1986. Larval patterns in the life histories of Brachyuran crabs (Crustacea, Decapoda, Brachyura). Bulletin of Marine Science, 39/2: 444-446. Accessed March 24, 2012 at http://www.ingentaconnect.com/content/umrsmas/bullmar/1986/00000039/00000002/art00025?crawler=true.

IUCN, 2013. "The IUCN Redlist of Threatened Species. Version 2013.1" (On-line). Accessed July 31, 2013 at http://www.iucnredlist.org.

Jorge, L. 1985. Caries-like infections in Loxorhynchus crispatus (Crustacea, Brachyura, Majidae). Journal of Invertebrate Pathology, 45/2: 247-248. Accessed January 29, 2012 at http://www.sciencedirect.com.proxy.lib.umich.edu/science/article/pii/0022201185900175.

McGaw, I., J. Stillman. 2010. Cardiovascular system of the Majidae (Crustacea: Decapoda). Arthropod Structure and Development, 39/5: 340-349. Accessed February 02, 2012 at http://www.sciencedirect.com.proxy.lib.umich.edu/science/article/pii/S1467803910000265.

Sal Moyano, M., M. Gavio, E. Cuartas. 2011. Copulatory system of the spider crab Libinia spinosa (Crustacea: Brachyura: Majoidea). Journal of the Marine Biological Association of the United Kingdom, 91/8: 1617-1625. Accessed April 02, 2012 at http://journals.cambridge.org.proxy.lib.umich.edu/download.php?file=%2FMBI%2FMBI91_08%2FS0025315411000257a.pdf&code=db77d0c7f29434e7d298f403f4d38dc7.

Stevens, M., S. Merilaita. 2011. Animal Camouflage: Mechanisms and Function. New York: Cambridge University Press.

Wicksten, M. 1978. Attachment of decorating materials in Loxorhynchus crispatus (Brachyura: Majidae). Transactions of the American Microscopical Society, 97/2: 217-220. Accessed January 30, 2012 at http://www.jstor.org/stable/3225595.

Wicksten, M. 1979. Decorating behavior in Loxorhynchus crispatus Stimpson and Loxorhynchus grandis Stimpson (Brachyura, Majidae). Crustaceana (Supplement), 5: 37-46. Accessed February 02, 2012 at http://www.jstor.org/stable/25027481.