Porcelain crabs combine suspension-feeding with scavenging.  The 3rd or outermost pair of maxillipeds is heavily bristled and collects particulate food with each pass through the water.  Edible material is scraped off by the innermost 2 pairs of maxillipeds and passed inwards to the mouth.  In southern Puget Sound, Washington porcelain crabs Petrolisthes eriomerus eat mainly diatoms filtered from the water and some green algae cropped from the rocks.  Knudsen 1964 Pac Sci 18: 3.

NOTE  laboratory tests with Petrolisthes cinctipes in Oregon show that various chemicals, including amino acids (e.g., tyrosine, glycine, proline) and sugars (e.g., trehalose, glucose) are phagostimulatory and initiate maxilliped beating and other feeding behaviours.  Hartman & Hartman 1978 Comp Biochem Physiol 56A: 19

Porcelain crab, possibly Petrolisthes eriomerus, is suspension-
feeding with the 3rd-maxilliped filtering basket opened 2X

A noticeable feature of porcelain crabs Petrolisthes spp. is a pubescent tuft of bristles on the inner side of the merus of each chela, clearly visible in the photograph in Research Study 1 above.  Porcelain crabs feed mostly by filtering out suspended matter using the 3rd maxillipeds, and up to the time of the present research study the function of these tufts was unknown.  In California, mixed assemblages of several related species may live together under the same rock, including Petrolisthes cabrilloi, P. gracilis, P. eriomerus, Pachycheles rudis, and possibly others depending on locality.  In the laboratory, using an individual of each of 3 species with a single rock for cover, the author observes P. cabrilloi scraping up food detritus (fish flakes) with its chelae.  The chelae are dragged towards the body and, as they move along, detritus gets tangled in the tufts.  When the tufts arrive close to the mouth, bristles on the 1st and 2nd maxillipeds comb out the tufts, after which the detritus seems to be moved to the mouth (the action is apparently hard to see because the appendages block the view).  The author proposes that in such mixed-species assemblages where competition for suspended food matter may be intense, P. cabrilloi may employ this method as an alternate feeding strategy.  This is an interesting observation and certainly warrents further research. Gabaldon 1979 Crustaceana 36: 110.

Sand crabs: Emerita

An early study by researchers at University of California Los Angeles describes the functional morphology of limbs of sand crabs Emerita analoga in considerable detail, including locomotion, burrowing, gas exchange, and feeding.  The last topic is the one considered here. 

Particulate planktonic food is filtered from the back-swash of waves while the animal is buried.  The flagellar parts of the antennae are extended into the water, and food particles impinge on small branches of the antennae called annuli.  Each annulus has a series of  hairs of different size and stoutness arranged to form a V-shaped trough.   Large, presumably inedible particles impinge on, and are removed by, more stout inner hairs, permitting finer particles to pass onto the more slender and delicate outer rows of hairs,  Each antenna is divided into 3 articulating parts that impart a wide range of movement.  When a flagellum becomes loaded with particles, it is whipped downwards and wiped off by the mouthparts.  This action may occur several times during the passage of a single wave.  When not feeding the antennae are folded up under the protective cover of the 3rd maxillipeds, with the flagella coiled.  The mouthparts comprise the usual decapod complement of 3 pairs of maxillipeds, 2 pairs of maxillae, and a pair of mandibles.  Bristles on certain parts of the 2nd maxillipeds, and to some extent the 3rd maxillipeds, scrape particles from the flagellae and pass them to the maxillae and then to the mouth.  The mandibles are simple in shape and appear not to masticate the planktonic food before it enters the mouth.  Knox & Boolootian 1963 Bull Southern Calif Acad Sciences 62: 45.

NOTE  there are some 170 annuli on each antenna of an adult female, but less in a male, which is smaller in size 

Mole or sand crabs Emerita analoga filter-feed on suspended organic particles in the plankton using their 2nd antennae.  The antennae are long and feathery, and are extended above the sand surface to intercept water washing down the beach as a wave recedes (see Photograph on Left).  The crabs constantly scoot around in the wave-swash area, swimming to find a good location as each wave comes in, then burrowing into the sand and feeding as the waves go out.  Quick burrowing is enabled by the 1st - 4th thoracic appendages working in conjunction with the posterior-most uropods.  The flattened faces of the appendages act as scoops, while the recovery stroke involves a 90^o twisting and is in along the line of least resistance (see drawing upper Left).  Drawing modified from Trueman & Ansell 1969 Oceanogr Mar Biol Ann Rev 7: 315; photograph of antenna from MacGinitie 1938 Am Midl Nat 19: 471.

Fine structure of the filtering
setae on the antennae 20X

Adult and juvenile (or male) Emerita analoga
with 2nd antennae extended for feeding 2X

Included in the food of Emerita analoga are dinoflagellates Alexandrium spp. containing paralytic shellfish poisons (PSP).  The crabs take up and sequester the PSP in sufficient amounts that they can be used as indicators of the toxin’s presence along sandy shores, possibly as counterpoint to assays on mussels along rocky shores.  Bretz et al. 2002 Toxicon 40: 1189.

NOTE  a companion article by researchers at California State University, Monterey Bay shows that feeding by E. analoga on domoic-acid synthesizing diatoms Pseudo-nitzschia spp. also leads to the crab’s sequestrating the toxin in amounts correlative with planktonic abundances of the diatom.  This, combined with the ease of extraction of the toxin from the crabs, suggests that they can be used as a reliable, cost-effective monitoring tool for demoic acid in nearshore waters.  The authors note that the reliability of sand crabs in this way is superior to both mussels Mytilus californianus  and razor clams Siliqua patula.  Ferdin et al. 2002 Toxicon 40: 1259.