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

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

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.

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 (>15^oC) 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.