Fishes and birds are likely to be the most important predators of intertidal amphipods, and birds of semiterrestrial forms.  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.

The following accounts deal with gray whales and fishes.

In 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.

In a later study 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.

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 agassizi .  Mitchell 1953 Am Midl Nat 49: 862.

Studies 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 prolifera.  Caine 1979 J Exp Mar Biol Ecol 40: 103. Photograph courtesy Pearson College, PearsonCollege.

Caprellids Caprella laeviuscula clustered
on a piece of submerged equipment

Photis conchicola is an amphipod that uses empty gastropod shells as a shelter, perhaps as a defense against predatory fishes.  The amphipod constructs a soft tube with a thread-like material intermixed with detrital and other material within an empty shell, and attaches the shell to algae.  The attachment device consists of a bundle of the same threads comprising the tube, fixed to the inside of the shell and extending to the alga.  Shells used are the same species of gastropods present on the algae to which the domicile-shells are attached.  Densities of P. conchicola of up to about 800 ^. m^-2 have been recorded in intertidal areas of central and southern California.  Some shells are inhabited by one sex separately, while other shells host several of each sex.  The drawing shows a male on the Left, gripping a portion of the alga with its large gnathopod, and a female on the Right, both in their domiciles.  Note the attachment thread circled in purple)that suspends the female's shell from the alga.  The shells can be re-attached, but the author is uncertain whether translocation to new habitats is common.  The shells likely function to reduce desiccation in intertidal habitats and provide protection from predators such as fishes.  The author suggests that an amphipod in its shell domicile may appear as an unpalatable, cryptic, or disruptively coloured item to a potential visual predator.  Carter 1982 J Crust Biol 2: 328.

NOTE  in their manner of inhabiting and perhaps carrying a shell-habitat around with them, Photis’ behaviour is similar to that of hermit crabs

A recent paper published on an Atlantic species of bottom-dwelling amphipod Crassicorophium bonelli may be germane to the study on Photis presented in Research Study 3 above.  The reason is that C. bonelli uses a filamentous silk substance to bind together sand grains to form its tube (see illustrations on Left), reminiscent of the threads used by Photis to attach and line its shell domicile.  The unique discovery of silk production in an amphipod is reported by researchers at the University of Oxford, using specimens collected in Scotland.  The filamentous silk consists of mucopolysaccharides and proteins, is chemically similar to barnacle glue, and is produced in a similarly functioning gland/chamber system to that of barnacles. Its discovery leads the authors to some interesting speculation on past phylogenetic relationships between the two crustacean groups. 

The secretory glands are located in the large segments of walking legs 3 and 4. Other products are received from protein-secreting rosette glands. Ducts carry the secretions to the tips of the legs where they are released as silk (see drawing on Right).  The silk's properties include adhesion, durability, and seawater resistance.  Like barnacle glue, it cures underwater and, like spider silk, it is fibrous and “extrusion-spun”. Also, just as spiders pull silk from their spigots using their legs, the amphipod attaches a bit of silk to the substratum, then pulls its limbs away. Whether the shell-inhabiting P. conchicola produces its threads in a similar way is of course not known, and will need to be researched.  Kronenberger et al. 2011 Naturwissenschafte 98: XXX.  Photograh and drawings courtesy the authors.
  
NOTE  the species is actually “trans-polar”, being found from the north Atlantic to Chile.  It is thought likely to inhabit the northeastern Pacific region, although is not yet formally recorded as being there