Learn About Crabs & Relatives: Feeding, foods, & digestion

Omnivory & Scavenging

Decapod crustaceans have a variety of feeding modes including OMNIVORY & SCAVENGING, considered in this section, and CARNIVORY, HERBIVORY, and SUSPENSION-FEEDING considered in other sections.

Research study 0

drawing of lined shore crab Pachygrapsus crassipesAn early report on diet of lined shore crabs Pachygrapsus crassipes on the mid-California coast lists the following in order of decreasing importance: 1) live algae, 2) organic detritus, and 3) live animals. Much of the foraging for algae is done at night, especially high on rocks in the splash zone when the crabs busy themselves scraping off filamentous algae with their chelipeds. The least significant but most interesting foodstuff is live animal matter, which includes black turban snails, winkles, other crabs (especially newly moulted ones). Included in this last are Hemigrapsus spp. and conspecific P. crassipes. Cannibalism is apparently not uncommon, especially involving freshly moulted juveniles. Analysis of stomach contents of 50 adult specimens discloses the following dietary constituents in decreasing order of volume: algae, diatoms, and fragments of striated muscle fragments, the last from several partially eaten sardines found nearby. Had the sardines not been adventitiously placed there by fisherfolk, the author doubts that any animal matter would have been found in the stomachs. The author neatly categorises P. crassipes as essentially an herbivore (ordinarily a grazing one and less commonly a plant scavenger), and also a facultative carnivore (chiefly scavenging and occasionally preying). Hiatt 1948 Pacific Sci 2:

Research study 1

photograph of crab Hemigrapsus nudus in seaweedphotograph of crab Hemigrapsus oregonensis

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

Research study 2

A common laboratory behaviour of the omnivorous shore crab Pachygrapsus crassipes when sensing the presence of food, such as a dead squid, is to scrape their claws on the bottom of the tank, lift them to their mouths, then search about for the source of the food. A researcher at the University of California, Berkeley addresses the issue of what chemical substance induces the feeding response by offering crabs small pieces of filter paper containing dried solutions of 3 amino acids, L-leucine, L-isoleucine, and L-valine. Of the 3 tested, only L-isoleucine elicits photograph of crab Pachygrapsus crassipes courtesy US Environmental Protection Agency100% positive response, but only if the crabs are not satiated. Tests on other crustaceans Pagurus hemphillii and Hemigrapsus oregonensis show that the effective doseage ranges around 1-2 x 10-6 M. Tests of 22 other amino acids elicit no feeding responses in P. crassipes. However, if L-isoleucine is added to the other 22 amino acids and tested, 100% of the crabs show a response. Kay 1971 Experientia 15 (1): 103. Photograph courtesy U.S. Environmental Protection Agency.

NOTE these are chosen because of their 1) wide distribution in nature, 2) low molecular masses, and 3) relatively high solubility in aqueous solutions

Crab Pachygrapsus crassipes appears to be eating something in a tidepool 0.7X

Research study 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.

Research study 4

photograph of a masking crab Loxorhynchus crispatus

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

Research study 5

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.

Research study 6

histograms comparing growth of 1st-instar Dungeness crabs Cancer magister on different dietsphotograph of a 3rd-instar Dungeness crab Cancer magister courtesy of The Watershed Project, Richmond, California  That the juvenile stages of Dungeness crabs Metacarcinus 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

Research study 7

photograph of seaweed wrack on a beach near Bamfield, B.C.A study at the Bamfield Marine Sciences Centre, British Columbia shows that shore crabsHemigrapsus nudus are commonly found in supratidal beach wrack immediately after histogram comparing prey consumption by shore crabs Hemigrapsus nudusnight-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

Research study 8

photograph of post-settlement king crab Paralithodes camtschaticus courtesy Pirtle & 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

Research study 9

Large aggregations of hermit crabs are usually thought to be associated with availability of rock- and shell-habitat, and with reproductive events, but field experiments by researchers at the University of California, Berkeley show that the availability of carrion-type food may be equally important to these scavengers, usually considered to be microphagous detritivores. In fact, comparisons with other marine scavengers such as snails, crabs, and goby fishes in intertidal areas near to Bodega Marine Laboratory show that Pagurus spp. arrive at sites where snails and mussels have been experimentally smashed significantly faster than the other taxa. Not only do the hermit crabs arrive faster than the others, but more accumulate, leading to their competitive dominance at the provisioning sites. Laidre & Greggor 2015 Mar Biol 162: 969.

NOTE species are Pagurus hirsutiusculus, P. samuelis, and P. granosimanus; see LEARN ABOUT: CRABS for photographs