Oregonia gracilis

From marinelife1011

Oregonia gracilis, commonly known as the graceful decorator crab, is a "true crab". It often decorates itself with objects from its surroundings in order to blend in with its natural environment. This form of crypsis is used to avoid predation. Because of this crypsis, it can be difficult to spot in its habitat.

Contents 1 Description 1.1 Phylogeny 1.2 Defining Characteristics 1.3 Similar Species 2 Natural History 2.1 Range and Habitat 2.2 Reproduction 2.3 Feeding 2.4 Predation 2.5 Decorating 3 External Links and Resources 4 References

Description

Phylogeny

Oregonia gracilis belong to the phylum Arthropoda, subphylum Crustacea, and are further grouped into the class Malacostraca. As with all Arthropoda they grow through shedding their exoskeletons and as with Crustacea, experience a distinct larval stage. ^[1]

Like all Malacostraca, O. gracilis have three body segments: head, thorax, and abdomen. The head and thorax are fused into a cephalothorax, as in most crustaceans, and are covered by a carapace (continuous exoskeleton). They have large, stalked, compound eyes and 2 sets of antennae. The cephalothorax also bears three pairs of small appendages for feeding, the mandibles and 1st and 2nd maxillae. ^[1]

They belong to the order Decapoda, meaning they have 5 paired pereopods (thoracic walking legs) and the first 3 pairs generally assist in feeding and are called maxillipeds. Like most crabs, the first pair of pereopods have pincher-claws and are called chelipeds, which are usually for feeding or defense; O. gracilis also use these claws to decorate themselves. The abdominal segments each bear a pair of appendages called pleopods that function to generate respiratory currents and brood offspring.^[1]

Defining Characteristics

This species has a triangular-shaped carapace that is usually tan in color and matches bryozoans, sponges, and hydroids usually found on its body and legs. ^[2] With its small body and long, thin legs, it is the most spidery, in appearance, of all the intertidal Majid (spider) crabs on the North Pacific coast.^{[2] [3]} The carapace can reach lengths of 5 to 7.5 cm.^{[2] [3] [4]}

Its carapace is covered with setae, long hairs, and tubercles. It has two rostral spines that are near-parallel and a sharp, curved spine directly posterior to each eye that protrudes out (postorbital spine). The 2nd set of antennae are longer than the rostral spines and the tips of the chelae are orange. ^[5]

In mature males, the chelipeds are longer than the pereopods. In mature females, the chelipeds are shorter than the pereopods. ^[5]

Similar Species

• Pugettia richii – Cryptic Kelp Crab

This species has a carapace covered with larger, more distinct spines and chelae are tipped with white not orange.^[4]

• Oregonia bifurca – Split-nose Decorator Crab

This species is found at depths below 500 m.^[4]

• Scyra acutifrons – Sharped Nose Crab

This species has shorter, flatter rostral horns and shorter, less spider-like legs.^[2]

Natural History

Range and Habitat

Generally, this species can be found from northern Alaska, along the Bering Strait, to central California as well as the Pacific coast of Japan. It tends to live on the rocky bottoms and mixed bottoms of the low intertidal up to roughly 400 m depth. ^{[3] [4]}

Reproduction

For most of its life cycle, O. gracilis lives solitarily, except during breeding season. While courtship behaviors have been reported, it is believed that chemical cues play the most significant role in mate acquisition.^[6]

The first and second abdominal appendages of males are used to transefer spermatophores (sperm packets) to females. Normal sexual reproduction, fertilizing an egg with sperm, occurs and females produce eggs that they carry attached to their abdominal segments. Abdominal setae produce a substance that binds the eggs to the setae.^{[7] [6]}

While brooding, embryos experience an unseen naupliar life stage and then hatch as zoea, a more advanced larval stage with compound eyes and a spiny carapace. These zoea larvae drift upwards and become part of the pelagic plankton in the water column.^[1] During the zoea life stage, O. gracilis uses its maxillipeds to swim, making it relatively agile compared to other zooplankton. ^[8] After a period of growth and development, zoea larvae metamorphose into a megalopa stage which looks very similar to the adult form except that the abdomen is not yet under the thorax.^[1] The megalopa life stage is the final larval stage before entering the benthic, adult stage. ^[8]

Feeding

Using its chelipeds, O. gracilis cuts and grasps small pieces of food that are pushed passed the maxillepeds and inward to the mandibles. While the mandibles hold the food, the maxillae rip the food into smaller pieces. Finally, once the food pieces are small enough, they are passed through the mouth for digestion. Chelipeds of O. gracilis are not built to crush shells or strong exoskeletons, but are used to pluck soft, small marine organisms and to scavenge the sea floor.^[6]

This species relies heavily on drifting kelp and algae as a main food source and employs a waiting strategy to maintain crypsis and avoid predation.^{[9] [6]}

As zoea and megalopa, they are mostly carnivorous and most likely feed on smaller zooplankton.^[10]

Predation

Octopus, sea otters, and multiple species of fishes feed on adult O. gracilis.^[9]

Individuals commonly waft their legs and body through the water while remaining attached to a substrate with their last set of pereopods. This activity is thought to be cryptic, resembling seaweeds and other sea life moving in the waves. It is used to avoid predation. ^[11]

As zoea and megalopa, O. gracilis join the complicated trophic food web of the epipelagic and are most likely consumed by larger zooplankton and nekton.^[10]

Decorating

Oregonia gracilis decorates by cutting small pieces of material with its chelipeds and fraying and softening the material with its mouth parts. Once the material is soft, it rubs the material onto its carapace until the material adheres to the setae on the carapace.^[5] Individuals will use virtually any material available to decorate themselves to the environment. In captivity, individuals will change their decorations to better match the environmental colors offered.^[4]

A study from 2009 found males to decorate significantly less than females and for males to decrease decorating with aging more than females. It is thought that individuals decorate less as they age because they are no longer as susceptible to predation with their increased size. As individuals age, they also have less setae present on their carapaces. Individuals were also found to decorate the anterior part of the carapace, near the rostum, most heavily. Anecdotally, individuals tend to decorate the rostrum first. This may be due to the valuable and relatively vulnerable appendages near this area including the stalked eyes and antennae.^[12]

External Links and Resources

Video of O. gracilis wafting as an additional form of crypsis. video by S. Rosewall

Images of O. gracilis

Basic crustacean information

Basic crustacean larvae information

References

1. ↑ ^{1.0 1.1 1.2 1.3 1.4} Pechenik, J.A. (2010). Biology of the Invertebrates: 6th Edition. Boston: McGraw Hill.
2. ↑ ^{2.0 2.1 2.2 2.3} Kozloff, E.N. (1993). Seashore Life of the Northern Pacific Coast: an illustrated guide to Northern California, Oregon, Washington, and British Columbia. Seattle: Univ. of WA Press.
3. ↑ ^{3.0 3.1 3.2} Gotshall, D.W. (2005). Guide to Marine Invertebrates: Alaska to Baja California 2nd Edition (revised). Santa Barbara: Shoreline Press.
4. ↑ ^{4.0 4.1 4.2 4.3 4.4} Lamb, A. and B.P. Hanby. (2005). Marine Life of the Pacific Northwest: A photographic encyclopedia of invertebrates, seaweeds, and selected fishes. British Columbia: Harbour Publishing.
5. ↑ ^{5.0 5.1 5.2} Rathbun, M.J. (1925). The Spider Crabs of America.Bulletin 129 of the United States National Museum.
6. ↑ ^{6.0 6.1 6.2 6.3} Cattle Point: Where Fashion Is the Best Defense. Friday Harbor Labs, University of Washington. http://depts.washington.edu/fhl/zoo432/cattlepoint/432pagemill/432dec.html
7. ↑ Crustacean. Encyclopedia Britannica. http://www.britannica.com/EBchecked/topic/144848/crustacean/33799/Natural-history#
8. ↑ ^{8.0 8.1} Plankton. Friday Harbor Labs, University of Washington. Phttp://depts.washington.edu/fhl/zoo432/plankton/plarthropoda/plarthropoda.html
9. ↑ ^{9.0 9.1} Daly, B. and B. Konar. (2009). Temporal trends of two spider crabs (Brachyura, Majoidea) in nearshore kelp habitats in Alaska, USA. Crustaceana. 6: 659-669
10. ↑ ^{10.0 10.1} Castro, P. and M.E. Huber. (2010). Marine Biology: 8th Edition. New York: McGraw Hill.
11. ↑ Berke, S.K. and S.A. Woodin. (2009). Behavioral and morphological aspects of decorating in Oregonia gracilis (Brachyura:Majoidea). Invertebrate Zoology. 128: 172-181.
12. ↑ Hultgren, K.M. and J.J. Stachowicz. (2009). Evolution of decoration in Majoid crabs: a comparative phylogenetic analysis of the role of body size and alternative defense strategies. The American Naturalist. 173: 566-578.