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

Active defenses of amphipods include batting with the gnathopods, burrowing, nocturnal activity, and jumping, while passive defenses include possible mimicry and chemical deterrents, the last associated with warning or aposematic coloration.

This section considers mimicry of amphipods, while INVERTEBRATE PREDATORS, VERTEBRATE PREDATORS, and WARNING COLORATION are dealt with elsewhere.

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  Mimicry
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Research study 1
 

photograph of camouflaged amphipod crawling in a hydroid Corymorpha sp.Several amphipod species live on or close to other invertebrate species and have colours and colour patterns that appear to mimic those of their hosts.  A good example is a yet to be identified amphipod, photographed in Barkley Sound, British Columbia that crawls about in the hydroid Corymorpha sp. (see photograph). This may be an example of camouflaging or crypsis, where the amphipod matches its background and gains protection by being less visible to visual predators such as fishes, or of Batesian mimicry, where an otherwise palatable mimic, the amphipod, gains protection from predators by being identified by a predator as the toxic model, the hydroid.

 

The amphipod, visible here in one of the polyps of
Corymorpha
sp. at the 6 o'clock position, does not
cause the hydroid's nematocysts to discharge 2X

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Research study 2
 

photograph of columbellid gastropod Alia carinata courtesy Linda Schroeder,Pacific Northwest Shell Club, Seattle, Washington photographs of a snail Alia carinata and a putative mimicking amphipod Pleustes platipaBatesian mimicry requires that the model be unpalatable or otherwise inedible, and conspicuously coloured, and the mimic be coloured similarly and be palatable.  However, there are several examples described in the literature where an amphipod’s colours and patterns match those of various snails, with the presumption that the shells of the snails make them inedible or they are in some way distasteful. 

Examples include the resemblance of pleustid amphipods to certain species of littorinid Lacuna spp. and columbellid Alia snails.  Both types of organisms live in the same areas of eelgrass and algae.  In what may be the first description of mimicry published on a west-coast marine invertebrate, observations in Point Loma, California suggest that the amphipod Thorlaksonius platypus (Pleustes platypa) is a Batesian mimic of the snail Alia (Mitrella) carinata (see photo on Right).  Not only are the colours of the 2 species similar, but the amphipod tucks its appendages and other parts tidily up into itself to create a convincingly snail-like appearance. In the Point Loma area, both species are found in association with the kelp Macrocystis pyrifera.  The presumption by the author is that the snail is protected from being eaten by fishes by its shell or by distasteful chemicals, and the amphipod enjoys the same protection.  Crane 1969 Veliger 12: 200. Photograph of Alia carinata on upper left courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washington PNWSC.

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Research study 3
 

photograph showing a colorful west-coast amphipod and a Calliostoma sp. snail crawling on a colonial tunicateA decade after the publication of Research Study 2 above, a similar relationship is reported at San Luis Obispo, California between another pleustid amphipod Thorlaksonius (Pleustes) depressus, the putative mimic, and the columbellid snail Alia (Mitrella) carinata, the putative model.  The 2 not only resemble one another, but they exhibit similar locomotory behaviour.  Once again, the authors presume Batesian mimicry but, just as with the earlier study, further work would be needed to establish this.  The first question that comes to mind is: which is the model and which is the mimic?  Even worldwide there are few convincing documentations of Batesian mimicry in marine organisms but, in the case of snail and amphipod, it should be possible to devise experiments to test such an hypothesis. Carter & Behrens 1980 Veliger 22: 376.

 

 

Who's who? Some colorful but unidentified west-coast
amphipods and a gastropod Calliostoma ligatum crawl on a
colonial tunicate. The snail is palatable to fishes. Are the
amphipods chemically protected? If so, are we to believe the snail is
mimicking the amphipods? Perhaps the amphipods are palatable and
are mimicking the snail? Or is it possible that the similarity of
colours and patterns on the amphipods and snail is just coincidental?

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Research study 4
 

photograph of shell patterns in 4 species of lacunid snails Lacuna in San Juan Islands, Washingtonphotographs showing possible Batesian mimicry between a snail Lacuna sp. (the model) and an amphipod Stenopleustes sp. (the mimic)Indeed, such experiments have been done and the results are convincing.  Experiments at Friday Harbor Laboratories, Washington test the hypothesis that the amphipod Stenopleustes1 sp. is a Batesian mimic of snails Lacuna2, several species or types of which are shown in the photograph on upper Left. The snails and amphipods both live on eelgrass where they crawl and walk, respectively, along eelgrass blades, and consume epiphytic diatoms.  The species are similar in size, and have similar conspicuous3 colour patterns (see photo upper Right). The snail is solid brown or dark purple with white bands, while the amphipod is buff to chocolate-brown with white bands. The two are both about 5mm in length and exhibit similar locomotory behaviour. 

photograph of a snail Lacuna sp. crawling along a piece of eelgrassphotograph of an amphipod Stenopleustes sp. crawling along a piece of eelgrassBecause of its ditaxic4 muscular mode of crawling, when moving along an eelgrass blade Lacuna rocks from side to side (see photograph lower Left). Similarly, when walking along the same blade, Stenopleustes dips and rocks its body (see photograph lower Right). In accordance with one of the assumptions of Batesian mimicry, the putative mimic is much less common than the model, estimated from net-sweeps of eelgrass beds to be only 1-4% as common. 

Observations by the author indicates that predatory fishes in the same habitat, mostly cottids and pholids, rarely eat Lacuna or Stenopleustes when they are on eelgrass blades, but will readily eat the amphipod if it starts to swim, which it does usually only if disturbed.  In laboratory feeding experiments with the 2 commonest predatory fishes in the habitat, tidepool sculpins Oligocottus maculosus and crescent gunnels Pholis laeta, when Stenopleustes is paired with a non-mimetic, co-occurring gammarid amphipod (not identified, but one that commonly swims), only 9% of the Stenopleustes are eaten compared with 63% of the gammarid species.

The author is not certain why Lacuna is not eaten from its visually obvious perch on the eelgrass blades, but suggests that it may owe to the property of the protective shell or to chemical distastefulness. If it is eaten, the snail often passes through the fish’s digestive tract alive. Is it possible that Stenopleustes is itself distasteful to fishes or otherwise unattractive as food to the fishes?  This question always arises in cases of purported Batesian mimicry and is often difficult to answer.  However, the fact that fishes will prey on the amphipod when it betrays itself by swimming suggests that it is, indeed, palatable.  Field experiments would be necessary to demonstrate positively the existence of the mimicry.  The author concludes by stating that by resembling a conspicuous and inedible snail, Stenopleustes enjoys a protective advantage from being eaten by fishes in its eelgrass habitat.   Field 1974 Pac Sci 28: 439.

NOTE1  pleustids are gammarid amphipods.  Although mostly bottom-dwelling, they swim occasionally.  The Family Pleustidae is taxonomically large and in a state of almost continual revision. The genus Stenopleustes, referred to by the author, appears not to be valid, so it is not clear what the research species actually was, but certainly a pleustid

NOTE2  lacunids represent a difficult complex of species, with known hybridisations.  Half of the species used in the study are polychromatic

NOTE3  possession of a conspicuous (warning?) colour pattern is requisite to being a Batesian model

NOTE4  locomotory waves move on both sides of the foot, but asynchronously

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Research study 5
 

photograph of possible Batesian mimicry between an amphipod Podocerus cristatus and an aeolid nudibranch Flabellina iodinea in central california, courtesy Todd Huspeni and Mike Behrens, California possible A possible example of Batesian mimicry is represented in the striking resemblance of the amphipod Podocerus cristatus to the aeolid nudibranch Flabellina trilineata.  The colour form of the putative amphipod mimic shown in the photograph on Left is only known from southern Oregon, where it can often be found under the same rocky overhangs and cobble rock as Flabellina.  No experimental work has been done on the 2 species, possibly because the amphipod is extremely rare.  Goddard 2000 SEA SLUG FORUM. Photos courtesy Jeff Goddard, UC Santa Barbara.

photograph showing possible mimicry between an aeolid nudibranch Flabellina trilineata and an amphipod Podocerus cristatus in southern Oregon, courtesy Jeff Goddard, UC Santa BarbaraEqually interesting is the observation that in central California the model may be replaced by the bright purple and orange-coloured aeolid Flabellina iodinea (see photograph on Right).  In the area around Santa Barbara, the same amphipod species Podocerus cristatus adopts a markedly different colour pattern from that exhibited in southern Oregon.  Again, no experimental work has been done on the 2 species. Gosliner & Behrens 1990 p. 127 In Adaptive Coloration in Invertebrates (Wicksten, compiler) Texas A&M Univ Press; see also Goddard 2000 SEA SLUG FORUM.  Photograph of Flabellina courtesy Todd Huspeni, California and photograph of Podocerus cristatus courtesy Mike Behrens, California.

Aeolid nudibranch Flabellina iodinea
(2.5X) and its putative mimic, the
amphipod Podocerus cristatus (8X)



Aeolid nudibranch Flabellina trilineata (3X) and its
putative mimic, the amphipod Podocerus cristatus (5X)

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