Shore crabs Hemigrapsus nudus and H. oregonensis are omnivores, eating algae, diatoms, live prey, and scavenged dead prey.  Knudsen 1964 Pac Sci 18: 3.

Gut analyses of Hemigrapsus nudus at Friday Harbor Laboratories, Washington reveal mostly algal components, although the author notes the uncertainty of trying to identify macerated and digested food components in a shore crab.  Of 30 guts examined, 10 are found to be empty and 3 contain contents too digested to identify.  Contents of the remaining 17 include green algae (69% of all contents), brown algae (14%), red algae (15%), and animal tissue (2%).  Only traces of diatoms are found.  Food-preference tests show a favouring for the green alga Ulva sp. over several red and brown species, supporting the results of the gut-content analyses. Birch 1979 Crustaceana 36: 186.

Gut and fecal analyses of Loxorhynchus crispatus at Hopkins Marine Station, Pacific Grove, California reveal that field animals eat red algae, sponges, small crustaceans, and bryozoans.  In laboratory feeding experiments, where test subjects are provided with various living and dead foods, the crabs eat a variety of algae (but not browns), sponges, worms, limpets, sea stars, sand dollars, fish bits, nudibranchs, and sea urchins.  The last are crushed if small enough to fit within the jaws of the chelae or, if too large, may be stabbed by the crab through the soft perioral membrane.  The crab then crushes the Aristotle’s lantern and scoops out the soft tissues of the urchin.  Wicksten 1977 Cal Fish Game 63: 122.

Loxorhynchus crispatus, also known as the masking crab 0.5X

Food of juvenile King crabs Paralithodes camtschaticus in Alaska consists mainly of the contents of sediments, including diatoms, small crustaceans, foraminiferans, and detritus.  Feder et al. 1980 Crustaceana 39: 315. See also Research Study 6 below.

That the juvenile stages of Dungeness crabs Cancer magister are generally omnivorous is confirmed in a study by researchers at University of Washingon, Seattle and Shannon Point Marine Center, Washington.  The researchers collect megalopae from the seawater surface in Puget Sound and raise them after metamorphosis from 1st to 3rd juvenile instars on 4 different diets: 1) mussel flesh, 2) mixed mussel flesh and filamentous diatoms, and 3) “raw” and 4) “clean” diatoms.  Results after 40-45d of culture show up to 25% longer intermoult periods on the 2 diatom treatments than on the mussel or mixed diets, but no significant difference in moult-size increments between the treatments from either 1st-2nd or 2nd-3rd instar stage.  As shown in the histograms the presence of animal matter along with the diatoms has no significant effect on growth of the juveniles, indicating that the crabs in this treatment are obtaining their nutrition primarily from the plant component.  Jensen & Asplen 1998 J Exp Mar Biol Ecol 226: 175.  Photograph courtesy The Watershed Project, Richmond, California.

NOTE  the filamentous diatoms Melosira sp. and Grammatophora sp. when collected from the outflow raceway of the seawater system are infested with nematodes, small crustaceans, and other plant and animal components.  Two diatom diets are created: raw, which is this material “as is” and clean, which has all extraneous material removed, leaving just the 2 intermixed species of diatoms

A study at the Bamfield Marine Sciences Centre, British Columbia shows that shore crabsHemigrapsus nudus are commonly found in supratidal beach wrack immediately after night-time high tides.  During day-time and ebb tides the crabs are either submerged or hidden beneath intertidal rocks.  The night-time sojourns are spent feeding both on the wrack algae and on talitrid amphipods, the latter themselves having emerged from daytime sand burial to feed on the wrack algae.  In feeding trials, H. nudus readily feeds on amphipods and prefers them over another common prey, littorinid snails Littorina sitkana (see histogram).  The former are more energy-rich and require less handling time by the crabs.  In other laboratory experiments the researchers find that the crabs prefer wrack seaweed over fresh seaweed, and also eat a few barnacles Balanus sp.  The authors conclude that purple shore crabs are omnivores, feeding on both fresh and decaying algae, as well as on animal prey.  Lewis et al. 2007 Estuaries & Coasts 30 (3): 451.

NOTE  dried and decaying seaweeds cast up on the strand line of the shore.  For reasons of both drying and decaying, wrack generally has more nutrients and energy per mouthful than fresh seaweeds

NOTE  most common are Megalorchestia pugettensis and Traskorchestia traskiana

Laboratory studies of habitat choice of post-recruitment red king-crabs Paralithodes camtschaticus at the University of Alaska facilities in Juneau and at the Alaska Fisheries laboratory in Newport, Oregon show several age-related preferences.   Crabs of 3-7mo in age prefer “structural” substrata such as bryozoans, hydroids, and macroalgae on which they can feed. Habitat mimics (unspecified synthetic and natural fibers) are more preferable if they are fouled with bacteria and microalgae than if they are clean.  Both young (3-4mo) and old (5-7mo) crabs prefer to eat animal matter over macro- or microalgae. At a later age when their exoskeletons become stronger, the crabs tend to frequent more open habitats. Overall, the authors conclude that optimal nursery habitats for early juveniles will be “highly complex” ones that provide shelter, refuge from predators, and food.   Pirtle & Stoner 2010 J Exper Mar Biol Ecol 393: 130.

Juvenile king crab Paralithodes
camtschaticus of about 3mo age 15X