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


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

Research study 1

photograph of a kelp crab Pugettia producta on a gravel beach

In southern Puget Sound, Washington kelp crabs Pugettia producta eat mainly brown algae such as Nereocystis luetkeana, Fucus sp., and Sargassum spp., and some red algae and barnacles. Knudsen 1964 Pac Sci 18: 3.

Kelp crab Pugettia producta placed momentarily on a gravel beach for photography 0.5X

Research study 3

A study at the Marine Science Institute, University of California, Santa Barbara on chemosensory location of food in kelp crabs Pugettia producta shows that first response to an odour (e.g., a cracked sea mussel Mytilus californianus upstream of a test individual) is probing by legs and chelae. If this behaviour fails to find food, the crab crawls about in searching mode. This hierarchical response is maintained unchanged for up to 14d of starvation. The threshold levels of mussel or seaweed odour to initiate searching are identical for starved and satiated animals. Based on unchanging flicking rate before and after presentation of mussel or amino-acid stimuli (rates remain at about 1.5 flicks . sec-1), antennule flicking does not appear to be associated with chemical recognition of a potential food source. Zimmer-Faust & Case 1982 J Exp Mar Biol Ecol 57: 237.

NOTE kelp crabs are primarily herbivores, but will respond to chemical scents from both animal (mussels, clams) and plants (kelps). Field tests employ cracked mussels while lab tests use a variety of amino acids (types used are based on the amino-acid composition of clams) at different concentrations. The researchers include dilute fluorescein dye to enable visual monitoring of the movement of test substances

Kelp crab Pugettia producta (this is a teaching specimen, preserved in a special mixture of alcohol and glycerine to keep it supple) 0.7X

Research study 3.1

Gut analyses of 12 shore crabs Pachygrapsus crassipes freshly collected from beaches near the Scripps Institution of Oceanography, California reveal that 15 species of seaweeds comprise the major part of the diet (about 96% of the total volume, visually estimated), with the addition of a few unidentified polychaete worms and amphipods (about 4%). Of the 15 algal species eaten, 10 reds comprise 73% of the total volume, 4 greens 26%, and 1 brown 1%. Four algal species make up about 76% of the total gut volume. These are the articulated red corallines Corallina sp. (23%) and Lithothrix aspergillum (15%), the fleshy red Gigartina canaliculata (16%), and the filamentous green Enteromorpha sp. (22%). The large amounts of nutritionally poor calcareous algae apparently consumed suggests that the crabs either like their crunchiness, like the nutritional contribution of calcium carbonate or, as suggested by the authors, are perhaps simply over-represented in the data by their poor digestibility and ease of identification in the gut contents. Barry & Ehret 1993 Envir Biol Fish 37: 75.

NOTE the authors also conduct large choice-type experiments using tubs each containing a single crab with 3g portions (fresh mass) of 20-55 algal species and left for 24h but, as the actual protocol used is unclear and amounts eaten of each alga much too large for what is apparently described, results for these experiments are not included here. Also provided are data on algal availabilities in the habitat, but electivity indices are calculated only for algae combined into “functional groups” and not for species

Research study 4

photograph of fruits of surf grass Phyllospadix torreyi eaten by kelp crabs Pugettia producta in the lab Holbrook et al. 2000 Mar Biol 136: 739

Field and laboratory studies at Santa Barbara, California on survival of seeds of surf grass Phyllospadix torreyi suggest that kelp crabs Pugettia producta, along with omnivorous crabs such as Pachygrapsus crassipes, may be a major cause of loss. Up to 50% of seeds produced are possibly consumed by these and other herbivores. Holbrook et al. 2000 Mar Biol 136: 739.

Research study 5

histogram showing amounts of live mass of brown algae Macrocystis and Undaria eaten by kelp crabs Pugettia producta

Kelp crabs Pugettia producta are opportunistic feeders, but favour a diet of kelps, especially Macrocystis pyrifera. In 2002 its feeding activities in Santa Barbara Harbor effectively destroyed a complete cohort of invasive settling stages of brown algae Undaria pinnatifida. This alga is native to western Pacific shores including southeastern Russia, Japan, China, and Korea. Since the 1970’s however, the alga has spread to temperate coastal regions of Europe, South America, and New Zealand and apparently is the only kelp known to be invasive. Its presence at sites along the west coast of North America dates from 2000 and appears to be associated with lower than normal water temperatures. In fact, during periods of higher seawater temperatures (>15oC) in Santa Barbara Harbor its gametophytic stages lie dormant. About 2mo after periods of cooler temperatures these stages release spores that settle and grow into juvenile plants, which are then consumed by resident kelp crabs Pugettia producta. In laboratory feeding-preference experiments kelp crabs consume Undaria as readily as they consume Macrocystis (see accompanying histogram), suggesting that the native herbivore is capable of effectively controlling the spread of the invasive alga, at least within the confines of the Harbor. Thornber et al. 2004 Mar Ecol Progr Ser 268: 69. Photo courtesy FAO.

Research study 6

photographs of feather-boa kelp Egregia menziesii  grazed and ungrazed by kelp crabs Pugettia productaphotograph of kelp crab Pugettia producta on plastic seaweedAt sites around the Bodega Marine Laboratory, California kelp crabs Pugettia producta favour feather-boa kelps Egregia menziesii as food. While hanging in the plants the crabs selectively feed on the small bladelets lining the central supporting stalk or rachis (see photographs on Right). In removing these parts with high relative surface area the crab affects the kelp in two ways, the first being in loss of photosynthesising surface area; the second, in loss of surface area of nitrate-absorbing tissue. Field assays show that where crab densities are low such losses may not be important, but in areas of high crab density the graph showing grazing effect by kelp crabs Pugettia producta on nitrate uptake by feather-boa kelp Egregia menziesiilosses may be more critical. Grazing losses to P. producta are therefore doubly jeopardous to Egregia, involving loss of biomass and also interfering with the plant’s physiological mechanism to regenerate the lost tissue. Light for photosynthesis is never in short supply, but nitrogen, as an essential component for protein manufacture, is often a primary limiting nutrient for growth. Comparison in the laboratory of nitrate uptake by Egregia shows that plants with bladelets removed from the stalks to mimic grazing by kelp crabs have 4-fold less nitrate uptake than control plants with bladelets intact (see graph lower Left). The authors note that theirs is the first demonstration of a herbivore’s effect on nutrient uptake by a seaweed. Bracken & Stachowicz 2007 Mar Ecol Progr Ser 330: 75.

Research study 7

While kelp crabs Pugettia producta in southern California prefer to eat giant kelps Macrocystis pyrifera, ones in Puget Sound, Washington are mostly denied this opportunity and subsist mainly on bull kelps Nereocystis luetkeana, the dominant canopy-forming kelp in the area. Fragments photograph of kelp crab Pugettia producta hanging out in a kelpof Macrocystis spp., and sometiimes whole plants, do wash into the quiet-water habitat of the Gulf Islands from outer-shore stands, however, so the crabs may not be totally unfamiliar with this food source. Which kelp is preferred, though? Short-duration laboratory feeding-preference tests at Friday Harbor Laboratories show that kelp crabs overwhelmingly prefer the local kelp Nereocystis over Macrocystis. A few modest chemical differences exist between the 2 foods, but the authors instead think the preference for bull kelps likely owes to ingestive conditioning, although without actually saying so. One guesses that some long-term feeding tests might now be in order, comparing growth and reproductive output on the 2 diets. Dobkowski et al. 2017 Mar Biol 164: 210. Photograph courtesy of J. Nestler & Dave Cowles, Washington https://inverts.wallawalla.edu

NOTE other feeding observations on kelp crabs mostly show that their preferred foods are whichever kelp species abound in an area, especially those that provide them with a canopy to hang out in. These include bull kelp, giant kelp, feather-boa kelp Egregia menziesii, and stalked kelp Pterygophora californica, to name a few

Kelp crab Pugettia producta crawling on a subtidal kelp 1X

Research study 8

histrogram showing rates of consumption of kelp crabs Pugettia producta on bull kelp Nereocystis luetkeana and snails Lacuna victa in food-choice experimentsIn a related paper the senior author of RS7 above adds considerably to the Pugettia producta story, including comparisons of feeding rates in laboratory and field enclosures, and the addition of a second kelp-consumer P. gracilis. Results from experiments at Friday Harbor Laboratories, Washington show that P. producta in cafeteria-type experiments consumes more bull kelp Nereocystis luetkeana than 7 species of common other macroalgae (including 4 other kelps) and, contrary to expectation based upon studies on other crustacean herbivores, prefers fresh kelp over aged1 kelp (6:1 ratio of mass consumed; see histogram). Equally surprising is a readiness of P. producta to consume live snails Lacuna vincta2. Feeding rates of P. producta and P. gracilis on bull kelp in ad libitum-type experiments are found after testing to be allometric to body size for the former (slope b = 1.37), but not for the latter3 (b = 0.77). The author comments that this may indicate greater relative impact of older, larger individuals on bull-kelp survival, but does not speculate on reasons either for the allometry or why only one species exhibits it. One wonders if sex may play a role, especially if females are larger than males. Mature females crabs in reproductive season (date of the study is not mentioned) may consume disproportionately more algae than mature male counterparts and juvenile stages. Conversely, could it be an age-related loss of digestive/absorptive functions, requiring more food to be consumed by larger individuals? Dobkowski 2017 PeerJ DOI 10.7717/peerj.3372.

graph showing rates of consumption of kelp crabs Pugettia producta and P. gracilis on bull kelp Nereocystis luetkeana

NOTE1 “aged” in this study means simply separating fronds from living, attached specimens and keeping them in seawater for 1wk - not as in other studies where “aged” usually refers to beach-stranded and sun-dried seaweeds

NOTE2 this species is common on bull kelp N. luetkeana and feeds on its tissues by radular rasping

NOTE3 in fact, the author states that P. gracilis is exhibiting negative allometry, but this is not so. A slope value b of 0.77 is actually isometric to the slope of 0.75 based on predicted metabolic performance (the so-called 3/4 rule)