title for amphipod section of A SNAIL'S ODYSSEY
  Food, feeding, & growth

The following account on foods and feeding is divided “family-wise” into studies on the Talitridae, Corophiidae, and Caprellidae.  Talitrids are mainly herbivorous, feeding on seaweeds and other plant matter.  High-intertidal species on sand beaches live in temporary, loosely fashioned burrows in the sand, and consume algae washed ashore by waves.  Corophiids inhabit burrows on sand/mudflats and generally deposit-feed.  Caprellids feed by browsing, filter-feeding, predation, scavenging, and scraping.

graph showing trophic relationships of 5 amphipod families in San Diego Bay, California as determined by stable-isotope analysisThere are 2 basic ways to determine what an animal eats: you can watch it, or you can check its gut/feces contents and go from there. The former is difficult to do for amphipods for several reasons, the foremost being their small body and mouth sizes; while the latter is confounded by the degraded quality of the food after it passes through the gut. Although by no means giving completely equivalent results, stable-isotope analysis is another way to determine feeding history and trophic diversity of marine consumers. In the present research study, this method is used by members of San Diego State University and Scripps Institution of Oceanography to determine feeding habits of representatives of 5 families of amphipods inhabiting eelgrass ecosystems in San Diego Bay, California. Results (see graph) show pretty much what others have found from direct observation, that genus Protohyale (F. Hyalidae) eats eelgrass almost exclusively, Ampithoe (Amphithoidae) and Erichthonius (Ischyroceridae) eat mostly eelgrass and epiphytic algae, respectively, Hartmanodes (Oedicerotidae) appears to be partially carnivorous (note high N value), and Caprella (Caprellidae) enjoys a more balanced algae/eelgrass diet (see graph). The isotope-ratio method gives results that generally support the notion that amphipods on an ecosystem level are functionally redundant and have enough overlap in diets that they can continue to be considered as occupying a single trophic guild. Farlin et al. 2010 Mar Ecol Progr Ser 420: 277.

NOTE the technique involves comparing isotope ratios of carbon (13C/12C) and nitrogen (15N/14N) in both primary producers and consumers, with the carbon data being used to trace primary production, and the nitrogen to infer trophic level. The premise in the method is basically “you are what you eat”. Another example of its use for marine invertebrates can be found at ANEMONE/SYMBIOSIS

NOTE the genera identified here represent only the numerically dominant genus within each of the 5 families. In fact, all amphipods collected are sorted to family, then together are dried, ground to powder, and analysed along with similarly dried and homogenised samples of living eelgrass, detrital eelgrass, and epiphytic algae growing on the eelgrass

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  Studies on talitrids are considered here, while ones on COROPHIIDS & AMPITHOIDS and CAPRELLIDS are dealt with elsewhere.
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Research study 0

graph comparing number of podomeres in the 2nd antennae of male and female talitrids Orchestia traskianaA researcher at the Marine Science Institute, University of California, Santa Barbarba compares growth in male and female talitrids Orchestia traskiana from the standpoint of increase in number of segments in the flagellum of the 2nd antenna over time.  If this were found to be consistent for both sexes, it would provide a means to assess moult status in field populations from one-time collections, something not yet able to be done.  Results from both laboratory and field caging experiments confirm that number of podomeres can be used to assess moult status in this species and, moreover,  that their frequency of addition differs between males and females (see graph).  At emergence from the brood pouch all individuals have 3 podomeres in their 2nd antennae (see graph).  Prior to sexual differentiation at about 12 podomeres, an individual has added one podomere per moult, but this decreases on subsequent moults to less than one, with females adding significantly fewer podomeres per moult than males thereafter.  Note that, drawing of head of a talitrid amphipod showing location of antennal podomeresas expected, moulting frequency decreases in both sexes with increasing age.  Page 1979 Crustaceana 37 (3): 247.

NOTE  these are termed podomeres, a word referring generally to a segment in any limb of a crustacean (see drawing on Right).  In the flagellar portions of the antennae, these segments are undifferentiated and are referred to as podomere 1, podomere 2, and so on


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Research study 1

histograms comparing amounts of fresh versus wrack (dried) seaweeds eaten by 2 species of talitrid amphipods Traskorchestia traskiana and Megalorchestia californianaLaboratory studies on feeding preferences of 2 supralittoral species Traskorchestia traskiana and Megalorchestia californiana at the Bamfield Marine Sciences Centre, British Columbia disclose that both significantly prefer wrack seaweeds over fresh seaweeds (see histograms).  Wrack is characterised by toughness, reduced nitrogen content, and high mineral content, and could reasonably be thought to have reduced palatability in comparison with fresh seaweed.  However, drying and decaying seaweeds are colonised by bacteria and fungi that may themselves be more digestible than fresh seaweed tissue.  Moreover, wrack has greatly reduced water content in comparison with fresh tissue; hence, has several times more organic content per “bite” than fresh seaweed.  It is also possible that defensive secondary metabolites in the seaweeds degrade during drying, as is known for terrestrial plants so, overall, the amphipods may indeed be getting a superior diet when consuming wrack.  Pennings et al. 2000 Can J Zool 78: 1918.

NOTE  seaweeds cast up on the shore by waves and aged; often dried to crispy or leathery texture by the sun

NOTE  chemicals in a plant that play no known role in the plant’s metabolism; rather, are thought to be involved as deterrents against consumers.  Secondary metabolites are absent in green seaweeds and, in brown and red seaweeds, are present mainly as phenolics and in various forms of terpenes, respectively

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  Photographs of consumption of drift algae by west-coast beachhoppers Megalorchestia spp.: Photograph on Left courtesy Laura Richards, DFO, British Columbia.
photograph of amphipods Megalorchestia californiana on a piece of kelp courtesy Laura Richards, DFO, British Columbia
Mixed ages and sexes of amphipods Megalorchesia californiana attack a piece of kelp 1.3X
barest remnants of a bull kelp plant Nereocystis luetkeana after being consumed by amphipods
Only a shadow remains of a bull kelp Nereocystis luetkeana after its consumption by amphipods
photograph of fronds of bull kelp Nereocystis luetkeana preferentially consumed by beach amphipods
Preferential consumption of the frond ends of a bull kelp Nereocystis luetkeana by beachhopper amphipods
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