Foods & feeding
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  Herbivory
  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.
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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

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Research study 2
 

photograph of Pachygrapsus crassipes courtesy Jackie Soanes, Bodega Marine Laboratory, California
Studies on successional events in an algal-dominated boulder field at Ellwood Beach near Santa Barbara, California show that the green algae Ulva spp. is dominant during the early recolonisation period following natural clearance by storm waves.  In fact, its presence inhibits colonisation by perennial red algae such as Gelidium sp. and Chondra-canthus (Gigartina) spp. 

This effect is clearly shown on experimental concrete blocks by manually removing early settling Ulva spp.  Within a week or two Chondracanthus spp. begin to recruit as sporelings (see graph on Right).  Within 2-3yr these algae come to dominate the area mainly through vegetative propagation.  The relevance to this part of the ODYSSEY is that if shore crabs Pachygrapsus crassipes (a juvenile is shown in the photo) are present, they selectively graze the Ulva spp. and accelerate succession to a community dominated by long-lived red algae.  In removing the inhibitory effect of Ulva, Pachygrapsus therefore acts as a facilitator of succession in the community. Sousa 1979 Ecol Monogr 49: 227. Photo of Pachygrapsus crassipes courtesy Jackie Soanes, Bodega Marine Laboratory, California.

NOTE in laboratory preference tests  Pachygrapsus crassipes  exhibits strong preference for Ulva spp. over 8 other species of brown and red algae

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Research study 3
 

photograph of a preservedkelp crab Pugettia producta
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

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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.

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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.

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