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

There are only a few studies on the physiology of west-coast intertidal and subtidal amphipods. Most attention has been paid to the physiological challenges facing amphipod crustaceans during their evolutionary incursion onto land, especially relating to water loss and temperature effects. With respect to the former, here is a list of adaptations found in supratidal- and land-inhabiting amphipods to resist desiccation:

1.  absorption of water from the gut to produce “dry” feces.
2.  direct uptake of water from saturated air and moist surfaces.
3.  excretion of gaseous ammonia (NH3).
4.  possible presence of an epicuticular wax layer.
5.  curling up to enclose the gills in a protective chamber formed by the coxal plates and abdomen.
6.  ability to rehydrate when placed in contact with a moist surface.
7.  reduction of relative gill area.
8.  conduction of urine along special cuticular ducts that pass close to the gills before leading to the outside.

Additionally, behaviours such as nocturnalism, consumption of moist algae and detritus, aggregation, and hygrokinesis also minimise water loss.  Some species have evolved high sensitivity to small differences in saturation deficit, such as to differences in RH as low as 3%in air pockets in algal deposits on the shore.  This leads to aggregation in pockets of high humidity, where evaporative water loss is minimised.  Reviewed in Koch 1989 Crustaceana 56: 162.                                

NOTE  most aquatic marine invertebrates, including gammarid amphipods, rely on production of ionic ammonium (NH4+) for excretion of toxic nitrogenous material.  This material is highly soluble, but requires much water for its excretion.  Ammonia gas readily diffuses through the cuticle into the air

NOTE  lit. “moist movement” G., referring to a tendency to move towards moist areas

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Research study 1
 

Gas exchange in amphipods takes place over the surface of gills that hang into the ventral area beneath the thorax (see drawing on Right).  The gills are simple, flat sacs through which hemolymph is pumped.  In the sandhopper Megalorchestia californiana, as in other amphipod species, there graph comparing total gill areas in amphipods showing increasing terrestrialnessare 5 pairs of gills. Some additional exchange of gases may occur in these semiterrestrial sandhoppers on the inner, moist) faces of the thoracic or coxal sideplates.  The gills in intertidal beachhoppers and supratidal sandhoppers are smaller in relative size than in aquatic gammarid relatives, illustrating a principle seen in other land-colonising crustaceans such as crabs and isopods, that of a decreasing relative surface area of gills with increasing terrestrialisation.  In the graph, which represents a selection of European and other world species, regressions for 5 aquatic gammarids drawing showing gill locations & relative sizes in an amphipodare shown in blue, 4 intertidal beach-hopping talitrids in beige, and 2 supratidal land hopping talitrids in brown. The reduction in gill size is thought to represent an adaptive strategy to reduce water loss in air.  No comparable studies appear to have been done on west-coast species.  Moore & Taylor 1984: J Exp Mar Biol Ecol 74: 179; graph from Spicer & Taylor 1986 J Nat Hist 20: 935; Spicer & McMahon 1994 Can J Zool 72: 1155.

NOTE  the gills are attached on the innermost sides of the 2nd-6th walking legs.  Usually the 2nd and 6th are larger than the other gills.  Surface area of a single gill is determiined from a photograph, then doubled to get total surface area for the gill pair

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Research study 2
 

graph showing temperatures at different depths in wrack seaweed representing habitats for ampipods Traskorchestia traskianaThroughout Traskorchestia traskiana’s distributional range from the Aleutian Islands to Baja California, preferred habitat is amongst seaweed wrack cast up on the shore.  Within the wrack, air temperatures are buffered. In the example habitats in deposits of wrack on beaches at Cherry Point, Washington shown here, temperatures on the wrack surface in full sun are 5-16oC greater than temperatures within air pockets at 5-20cm depth in the wrack.  If forced to leave the wrack photograph of talitrid amphipods Traskorchestia traskiana courtesy Mary Jo Adams & BEACHWATCHERSbecause of tidal innundation, the amphipods migrate up the shore.  During daytime journeys, losses may be high to seagulls and other predatory birds.  The author suggests that beach orientation is important during these migrations, but perhaps just moving ahead of an incoming tide would provide for correct navigation up the beach, even at night.  Koch 1989 Crustaceana 57: 295. Photograph courtesy Mary Jo Adams and BEACH WATCHERS.

NOTE  this topic is considered in detail elsewhere in this section of the ODYSSEY: CELESTIAL NAVIGATION


Beachhoppers Traskorchestia traskiana forage
for edible particles amongst the sand grains 4X

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Research study 3
 

graph showing relationship of hemolymph osmotic concentration with external medium concentration in amhipods Traskorchestia traskianaIf innundated by the incoming tide, talitrid amphipods Traskorchestia traskiana can survive quite well for periods of up to 2wk, as long as the seawater salinity is within 5-40ppt.  Optimum survival is at 30ppt, which is also near the limit of osmotic regulation in the species (see graph).  Good hyperosmotic regulation allows at least temporary survival in face of freshwater innundation as from rainfall, and may permit limited incursions into estuaries.  The graph also shows some ability for hyposmotic regulation, which may be useful when salty air pockets within the wrack become innundated with seawater and the salinity of the incoming seawater is increased.  The author notes that, although completelyl aerial in habit, T. traskiana can do little to minimise contact with water of variable salinity if trapped within the wrack.  Koch 1991 Crustaceana 61: 21.

NOTE  “parts per thousand”, equivalent to grams of salt . liter-1.  Full-strength oceanic seawater is about 35 ppt, equivalent to about 1000 mOsmol on the Y-axis of the graph

NOTE  ability to maintain its internal hemolymph osmotic concentration above that of the external seawater.  Thus, Traskorchestia can hyperosmoregulate in hyposmotic conditions

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