title for amphipod section of A SNAIL'S ODYSSEY
   
  Predators & defenses
   
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Vertebrate predators

 

The present section considers Vertebrate predators, with other sections on DEFENSES: GASTROPOD SHELLS, DEFENSES: CHEMICAL DETERRENTS/WARNING COLORATION, DEFENSES: MIMICRY, and INVERTEBRATE PREDATORS, being dealt with elsewhere.

West-coast researchers are more familiar with species of amphipods that inhabit high intertidal and supratidal regions of the shore, than with the many species that inhabit low intertidal and subtidal regions.  In these high intertidal and supratidal areas the major potential predators of amphipods are birds and beetles, and perhaps mammals such as rodents and raccoons. In California, birds listed as eating beachhoppers Megalorchestia spp. include sanderlings, red phalaropes, water pipits, plovers, willets, tattlers, curlews, and robins.  Studies relating specifically to this subject are hard to find.

Intertidal and subtidal species are common foods of certain whales and many species of fishes, as follows. 

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Gray whales

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Research study 1
 

plan view of Pachena Bay, British Columbia showing distribution of tubicuous amphipods Ampelisca agassiziIn 1984 scientists reported the first instance of gray whales Eschrichtius robustus feeding on infaunal invertebrates south of the Bering Sea.  The incident occurred in Pachena Bay, British Columbia in sandy areas of 10-18m depth where tubiculous amphipods, mostly Ampelisca agassizi, grow in dense mats.  The whales suck up patches of substratum varying from 1-4m in size at a rate of about 6 patches per dive, each dive lasting 3-4min. A single 6m calf-of-the-year is recorded as eating over 100kg of infaunal prey per 12-h day.  Larger-sized whales of 12m are estimated to consume over 500kg per day.  In the 12-m depth zone of greatest abundance, Ampelisca is estimated to represent 60% of the total infaunal biomass. Oliver et al. 1984 Can J Zool 62: 41.

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Research study 2
 

In later studies along the coast of British Columbia scientists from the University of Victoria observe gray whales Eschrichtius robustus foraging along 20m depth contours in areas rich in benthic tube-inhabiting amphipods Ampelisca agassizi and A. careyi.  Other prey probably taken above the sea bottom are mysids and porcelain-crab zoea larvae, and within the sediments, ghost crabs and clams.  The authors do not directly observe the whales feeding on these prey.  Rather, they infer that this is happening during observation of whale feeding-behaviour in areas of known abundances of certain prey and by sampling in areas close to where whales are foraging.  Dunham & Duffus 2002 Mar Mammal Sci 18 (2): 419; see also Dunham & Duffus 2001 Mar Ecol Progr Ser 223: 299.

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Fishes

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Research study 1
 

An investigation in southern California involving poisoning a tidepool with rotenone yields over 200 individual fishes respresenting 22 species.  Stomach analyses of the 4 most abundant species, representing 75% of all individuals collected, indicate a strong preference for small crustaceans, including several species of amphipods, isopods Cirolana harfordi and Idotea spp., several species of decapods, chiefly shrimps Spirontocaris picta and Crangon sp., as well as a few polychaetes, notably Platynereis agassiziMitchell 1953 Am Midl Nat 49: 862.

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Research study 2
 

photograph of caprellid amphipods Caprella laeviuscula courtesy Pearson College, British ColumbiaStudies on the population ecology of skeleton shrimps Caprella laeviuscula and Dentella californica in San Juan Islands, Washington indicate that while various substrata are occupied, a common habitat is hydroids Obelia dichotoma, which themselves often grow on eelgrass Zoster marina.  Primary predators of caprellids in the eelgrass areas are at least 10 species of fishes, including penpoint gunnels and spiny lumpsuckers, and various invertebrates, including sessile jellyfishes Haliclystus auricola and anemones Epiactis proliferaCaine 1979 J Exp Mar Biol Ecol 40: 103. Photograph courtesy Pearson College, PearsonCollege.

 

 

Caprellids Caprella laeviuscula clustered
on a piece of submerged equipment

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Research study 3
  Amphipods are eaten by 12 of 13 common shallow-water fish species found in the Dillon Beach area of California.  In over half of these species amphipods comprise over 50% of the diet.  Grossman 1986 J Zool, Lond, Lond B 1: 317.
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Research study 4
 

A study off the coast of Oregon near the mouth of the Columbia River shows that sub-yearling Chinook salmon Onchorhynchus tshawytscha preferentially eat species of amphipods over a variety of other invertebrate prey.  Schabetsberger et al. 2003 Fish Oceanogr 12 (6): 523.

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Research study 5
 

photograph of shimofuri goby Tridentiger bifasciatusAt least 125 alien species of fishes are estimated to have been introduced into the San Francisco Bay Estuary in the 3 decades up to 1995.  One of these, the shimofuri goby Tridentiger bifasciatus, introduced via ballast water in the early 1980s from Asia, has proliferated and now makes seasonal inroads on populations of native amphipods Corophium spp.  In a somewhat amusing turn-of-events, the California fisheries scientists reporting on the feeding biology of this fish also mention that it favours 2 other bottom-dwelling species, the hydroid Cordylophora caspia and the barnacle Balanus improvisus, themselves introduced alien species and ones not eaten much by other fishes in the estuary.  Matern & Brown 2005 Biol Invas 7: 497.  Photograph courtesy Jeon, Hyung-Bae and seriouslyFish.

NOTE  by numbers, corophiids and other amphipods are eaten almost 10-fold more than all other invertebrate prey combined, especially in spring/summer.  As well, several other species of native and alien fishes eat these amphipods, thus setting the stage for intense interspecific competition for food

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Research study 6
 

Young-of-the-year black rockfishes Sebastes melanops spend time within tidepools in rocky areas of northern California. Researchers from Humboldt State University, Trinidad, California catch and analyse gut contents of 718 of these fishes over a 2yr period from 3 sites. Of these juveniles, only about 10% are found to have empty guts. Fifteen prey categories are identified in the other fishes, with gammarid amphipods being dominant (65% relative importance) and harpacticoid copepods and mysids making up most of the remainder (23% relative importance). Studebaker & Mulligan 2008 J Fish Biol 72: 1393. Photograph courtesy Robert Lee, Bay Area Underwater Explorers BAUE.

photograph of juvenile black rockfish Sebastes melanopsNOTE 5 other rockfish species are included in the study but, as they comprise <1% of the total number of fishes sampled, are not included here

NOTE calculated as %mass eaten times %frequency of occurrence; apparently %N has been removed from this “standard” expression of index of relative importance, making the above values actually modified indices of relative importance

If this image of Sebastodes melanops is life size, then it
would represent the smallest size of black rockfishes sampled
in the present study (actual range sampled: 37-78mm)

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Research study 7
 

A consortium of fisheries researchers from University of Washington and Pacific Biological Station, British Columbia determine that young-of-the-year chinook salmon Oncorhynchus tshawytscha in Puget Sound, Washington rely on gammarid amphipods for about 16% of their yearly sustenance. Collections are made monthly through spring/summer months over 2yr at 3 locations in the Sound (north, central, and south), sampling juveniles of 67-153mm body length. The juveniles change their feeding habits from use of near shore habitats in Apr-Jun to use of offshore habitats in Jul-Sep. Duffy et al. 2010 Trans Am Fisheries Soc 139: 803. Photograph courtesy LETSBEWILD.com.photograph of fry of chinook salmon Oncorhynchus tshawytscha

NOTE the data presentation is visually confusing owing to apparent shading/hatching/squaring inconsistencies. This makes it hard to winkle out summary proportions of different prey items in the diet. Other comparable papers usually provide these kinds of tabulations

NOTE the authors note, rather commendably, that some gut sampling (notably from beach seining) is done non-lethally by gastric lavage

At about 56mm body length this chinook-salmon fry would
be at the small end of the sampling range used in the study

 

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