title for amphipod section of A SNAIL'S ODYSSEY
   
  Habitat ecology
 

There are hundreds of species of amphipods on the west coast of North America.  Most research other than that on taxonomy/distribution has been done on a few high intertidal and supratidal/semiterrestrial species and these studies will be featured in the ODYSSEY.  These species are classified within the Family Talitridae. How terrestrial are amphipods?  There are at least 120 species of fully terrestrial talitrids, known commonly as "landhoppers", inhabiting forests from floor to canopy, soils, ferns, caves, grasslands, and mountains, but they occur in tropical and subtropical areas in the southern hemisphere1 and in southern Japan3.  Reliance on water in terrestrial species is reduced to that available in dewfall and in food.  The evolutionary success of amphipods in colonising land among crustaceans is second only to oniscidean3 isopods.  Just as in oniscideans, much of this colonising success owes to features thought to have been present in ancestral intertidal species.  Friend & Richardson 1986 Ann Rev Entomol 31: 25.

NOTE1  there are no indigenous species of terrestrial talitrids in North photograph of aspidistra plantAmerica. A few spotty occurrences owe to introductions. 

NOTE2  one report notes that an amphipod Platorchestia japonica is a chief pollinator of aspidistra plants Aspidistra elatior at altitudes of 300-620m on small islands in the south of Japan.  Evidence of the amphipod’s presence is the presence of fecal pellets containing aspidistra pollen husks on and around the aspidistra flowers.  Apparently, as the amphipod feeds on the pollen, bits cling to its body and are carried along as the amphipod hops from plant to plant.  Kato 1995 Nature 377: 293.

NOTE3  more information on morphological and physiological adaptations of land-colonising crustaceans can be found elsewhere in the ODYSSEY: LEARN ABOUT ISOPODS: EVOLUTION TO LAND

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Research study 1
 

schematic drawing showing movements of sandhoppers Megalorchestia californiana and M. corniculata on a sand beach in relation to changing daily tidesThere are 2 common Megalorchestia species in California that similarly inhabit sandy beaches, occupy burrows near the strand line, and eat the same kinds of seaweed foods. However, studies at Hopkin Marine Station, Pacific Grove show that their distributions rarely overlap owing to preferences for different types of beaches.  For example, Megalorchestia corniculata prefers sheltered, steeper beaches with coarser, poorly sorted sands, while M. californiana prefers long, exposed flat beaches made up of fine, well-sorted sands.  Where the species do coexist, other features of habitat preference and behaviour may minimise interspecific competition.  For example, the burrows of M. californiana are located somewhat higher on the beach than those of M. corniculata, thus tending to partition the habitat with respect to living space and food availability. The accompanying schematic shows how the sandhoppers tend to follow the daily changes in tide levels when constructing their burrows. Bowers 1964 Ecology 45: 677.

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Research study 2
 

photograph of amphipod Megalorchestia californiana courtesy Dave Cowles, Walla Wall University, WashingtonTwo of the largest west-coast talitrid species Megalorchestia californiana and M. corniculata inhabit sand beaches throughout California, with the former’s range extending into southern British Columbia.  Both species inhabit burrows at the top of the beach and forage at night on seaweeds cast up by the tide.  The burrows are temporary and their location within the supralittoral zone fluctuates with the tide (see Research Study 1 above).  Both species are nocturnally active. Juvenile M. corniculata emerge from their burrows somewhat earlier than do adults, at sunset as compared with about an hour later for the adults.  Craig 1973 Mar Biol 23: 101. Photo courtesy Dave Cowles, Walla Walla University, Washington wallawalla.edu.



Megalorchestia californiana is sometimes called the
“long-horned” beach hopper, while M. corniculata is known
as the “short-horned” beach hopper.  Both species have
orange-, reddish-, or salmon-coloured 2nd antennae

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Research study 3
 

photograph of caprellid amphipods living on a tunicate Styela montereyensisSkeleton shrimps, or caprellid amphipods, like to hang out on living substrata.  Studies around Dane Point and San Simeon in southern California show that Caprella californica prefers to inhabit bryozoans Bugula neritina over algae, possibly because the amphipod appears to be cryptically coloured against this substratum.  In comparison, C. equilibra seems to have no particular preference for substratum occupied.  Overall, caprellids can be found on hydroids, bryozoans, algae, sponges, and other living substrata, with seasonal variation depending upon species.  Keith 1971 Pac Sci 25: 387. 

NOTE a study in San Francisco Bay shows surprisingly that over 90% of the epifauna inhabiting the tangled rhizomes of eelgrass Zostera marina are non-native species (mainly caprellid and gammarid amphipods). Carr et al. 2011 Mar Ecol 32: 88

 

 

 

The caprellids inhabiting the surface of this tunicate
Styela montereyensis appear not to be overly "concerned"
with being cryptic. Note the presence of several juveniles
amongst the larger individuals

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Research study 4
 

Studies on caprellid amphipods in Friday Harbor Laboratories, Washington reveal at least 10 species inhabiting intertidal and subtidal areas.  Most species associate with various species of hydroids, notably Obelia dichotoma, O. longissima, and Aglaophenia sp.  Other species, for example, Caprella californica and C. laeviuscula, inhabit the blades of intertidal eelgrass Zostera marina, where they scrape off algal growth or suspension-feed.  Those species coexisting on a single hydroid substratum, such as the hydroid Obelia longissima, tend to feed at different heights or utilise different-sized particles if suspension-feeding, or to use different feeding modes altogether, thereby minimising competition. Caine 1977 Mar Biol 42: 331.

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Research study 5
 

photograph of a several caprellid amphipods Caprella laeviuscula courtesy Pearson College, British ColumbiaCaprellid amphipods grazing on epibiotic algae on eelgrass blades Zostera can have substantial effects on the biomass and turnover of the algae.  Laboratory studies at Friday Harbor Laboratories, Washington show that when caprellids Caprella laeviuscula are excluded from the eelgrass blades, the algal biomass increases by over 400% over a 3-wk period.  By removing this potentially light-blocking growth, the amphipods may actually allow Zostera to grow in areas where it would otherwise be excluded.  However, removal of another dominant caprellid, this time the predatory Deutella californica, from hydroids Obelia dichotoma, does not significantly affect community structure.  In this case, the author suggests that the structure of the epibiotic community on the hydroids depends more on the seasonality of occurrence of O. dicotoma than on other interactions.  Caine 1980 Mar Biol 56: 327. Photograph courtesy Pearson College, Victoria PearsonCollege.

NOTE  C. laeviuscula is a large species, (9-18mm body length, and is competitively dominant over other caprellids.  On close approach by another individual, C. laeviuscula rapidly flexes its body downwards, thus causing the broad side of the second gnathopod to strike the newcomer.  After repeated bumpings the recessive individual generally moves away

Amphipods Caprella laeviuscula clustered on a submerged
bolt-head. Note the diversity of sizes in the group

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Research study 6
 

photograph of a cluster of amphipods Traskorchestia traskiana on a piece of alga courtesy Mary Jo Adams and BEACH WATCHERSResearch on competitive interactions within amphipod guilds is difficult enough when there is just 1 or 2 species, but when there are up to 6 or more species, some free-living and some tubicolous, and these are growing intermixed with copepods and gastropods as on fronds and stipes of the brown alga Pelvetia fastigiata at Bird Islands, La Jolla, California, the task become near impossible. One researcher has capably attempted to do this, but the study is much too detailed to try to attempt to include it here. Gunnill 1982 Mar Biol 69: 103; Gunnill 1983 Mar Biol 73: 115; and Gunnill 1984 Mar Biol 82: 277. Photograph courtesy Mary Jo Adams and BEACH WATCHERS, Washington State University, Everett.

 

 

 

This photograph of talitrid amphipods Traskorchestia
traskiana
feeding on a piece of alga give an idea of
the enormous densities of some amphipod assemblages 1X

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Research study 7
 

drawing of gammarid amphipod with perionites and gill locations indicatedWest-coast beachhoppers Megalorchestia californiana, M. columbiana, M. corniculata, and Traskorchestia traskiana host at least 2 types of invertebrate symbionts.  These are a rhabditid nematode and a uropodid mite, Thinoseius brevisternalis. Both live in the protected region under the dorsal perionites. Infestations may be considerable.  For example, an individual M. californiana at Bamfield, British Columbia may host as many as 1200 mites and 2100 nematodes.  While the nematodes appear to be feeding on the host (but possibly also on bacteria living on the host) and thus may be parasitic, the relationship of the mites is not as clear.  There appears to be no pathogenicity associated with their presence, nor has feeding on the host’s tissues been observed. In fact, the author suggests that the mites may actually feed on the nematodes. The author considers the mites to be commensal, in part, owing to the fact that they are found in dried seaweeds on the beach without hosts, so it may be that their association with amphipods if facultative.  The mites may use the beach hoppers for transport between temporarily available patches of beach wrack, where they may feed on nematodes associated with the wrack.  Rigby 1996 J Nat Hist 30: 1329.

NOTE documentation of this is provided for 2 populations at Bamfield, British Columbia and Santa Barbara, California, but it is likely that populations inbetween these also carry the parasites  

NOTE  of all the relationships described between organisms, commensalism (benefit to one partner, no harm to the other) seems to be one of the least certain.  Even if the mites are not feeding on the host, the load of carrying up to 1200 of them will exact a cost from the amphipod; however, if the mites do eat the nematodes, then perhaps mutualism would be a better descriptor.  The complexity of the hopper’s relationships is increased by the possibility that the nematodes may eat bacteria on the host’s exoskeleton, and so on, and so on, ad infinitum...

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Research study 8
 

graph showing numbers of mites on the gills of individual amphipods Trasorchestia traskianaAdditional tudies in Santa Barbara, California provide more detail on the juvenile mites present on the gills of Traskorchestia traskiana.  Ninety percent of mites are on the 6 pairs of gills, with Gill 5 bearing twice as many parasites as on any other gill.  Gills 2 and 6 are actually larger than 5, but for some reason have lower incidences of attached mites.  Number of attached mites varies in direct proportion to host size (see graph).  Why attach to the gills?  The gills are protected from abrasion by the walking legs and coxal plates, which keep the ventrum raised above the substratum surface; also, the gills have high humidity.  Rigby 1996 J Nat Hist 30: 1617. 

NOTE an unidentified species in Family Uropodidae

NOTE  although the author fits a straight line to the data, because it is surface area of the gills that presumably governs how many mites can be accommodated, then the scaling relationship would be predicted to be allometric, not isometric. The ordinate axis of the graph, labelled "mean intensity", is not equal to actual numbers of mites; rather, to another measure relating to numbers but not defined by the author

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Research study 9
 

graph showing colonisation of kelp wrack by beach hoppers Megalorchestia californianaThe principal foods of talitrid amphipods on the west coast of Vancouver Island, British Columbia are beach-cast macrophytes in the form of wrack.  Wrack patches are used also for shelter from drying and predators.  How amphipods locate and colonise wrack patches is the subject of a study at the Bamfield Marine Sciences Centre.  The researchers set out experimental wrack patches both on the sand surface and buried within, and assess colonisation dynamics over periods varying from minutes to days.  Location of patches by Traskorchestia traskiana appears to be primarily by olfaction.  Depending upon tidal height of the experimental wrack patches, dense colonisation occurs generally within 1h or less.  Colonisation densities directly relate to tidal height, but with marked species differences.  Thus, while individuals ofT. traskiana colonise patches in increasing density with tidal height, sand hoppers Megalorchestia californiana do the reverse (see graph).  The reason for the different behaviour is not known, but is discussed by the authors along with several photograph of amphipods Traskorchestia traskiana courtesy Mary Jo Adams, BEACH WATCHERS, Washington State Universityinteresting ideas for future research . Pelletier et al. 2011 Estuaries & Coasts 34: 863. Photograph of T. traskiana courtesy Mary Jo Adams, BEACH WATCHERS, Washington State University, Everett.

NOTE  principal macrophyte species on 3 study beaches in the Barkley Sound area are kelps Macrocystis integrifolia and Nereocystis luetkeana, seagrass Phyllospadix fouleri, and brown algae Fucus gardneri and Leathesia difformis

NOTE  the graph is a bit difficult to interpret, and possibly should have been rendered with the axes reversed for greater clarity

Amphipods Traskorchestia traskiana hunting for
food 2X. See also Research Study 6 above

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