Traskorchestia traskiana (Stimpson, 1857)
Traskorchestia traskiana (formerly known as Orchestia traskiana) is a beach-dwelling amphipod found along the Pacific coast of North America (Bousfield 1982). Amphipods are small crustaceans, the vast majority of which are marine. Traskorchestia traskiana, however, is among the many amphipod species that have adapted to life outside the sea. These are sometimes known as "beach fleas" as a consequence of their bouncy agility, but they are not fleas and they do not bite.
A variety of studies indicate that behavioral rather than physiological or morphological mechanisms account for a large part of the success of amphipods in colonizing semiterrestrial and terrestrial habitats, where the risk of life-threatening desiccation (drying out) is very high. Among these behavioral mechanisms are finding and remaining in moist microhabitats, consumption of moist seaweed as a means of replacing evaporative water losses, forming large aggregations on moist substrates, and nocturnal activity (Koch 1989 and references therein).
Traskorchestia traskiana reaches a maximum length of about 2 cm. It is usually dark gray with drably colored antennae. It is typically found at the borders of bays, estuaries, and salt marshes, although it does also occur on somewhat protected beaches of the open coast. In contrast, the large Megalorchestia californiana (formerly known as Orchestoidea californiana: Kozloff 1993; Carlton 2007) reaches 2.5 cm (excluding antennae). The body is grayish white or ivory (younger individuals have dark patches down the back), but the long antennae are a striking pinkish orange or red. It is found only on exposed beaches, usually in the vicinity of decaying seaweed. It stays hidden away in burrows or under mats of decomposing seaweed during the day, emerging at night to scavenge closer to the water's edge, using the moon to find its way back to the upper part of the beach before daybreak. (Kozloff 1993; Sheldon 1999)
Adult individuals of T. traskiana are euryhaline (i.e., tolerant of a wide range of salinities), capable of tolerating salinities of 2.5 to 50 parts per thousand (ppt) for at least a week at 10 C, with a salinity optimum near 30 ppt. In fresh water and in salinities greater than 60 ppt, survival is severely limited. Similar results have been reported for other marine beach hoppers, although estuarine populations exhibit higher tolerances of low salinities and have lower salinity optima than populations inhabiting marine supralittoral (i.e., just above the high tide line) environments. (Koch 1991 and references therein). The salt content within the wrack (washed up seaweed) microhabitat has been shown to depend mainly on such factors as salt wash-out by rain or surface runoff, salt crystallization caused by evaporation, and salt liberation during wrack decomposition, rather than on the salinity of the local sea water. One study reported salinities of 0.3 to 51 ppt NaCl in the surface layers of Swedish wrack banks and the author suggested that organisms with a low salt tolerance would not be able to live permanently in these habitats. Wrack inhabitants may be able to avoid prolonged exposures to salinity extremes by retreating into the deepest wrack layers, where the interstitial salinity is often less extreme and more constant compared to layers near the wrack bank surface. (Koch 1991 and references therein). Traskorchestia traskiana copes with such fluctuating external salinities in part by hyper- and hyporegulating the osmotic concentration of its haemolymph ("blood"), an adaptation that has also been demonstrated in other marine and estuarine supralittoral talitrids. The ability to keep the haemolymph hypo- and hyperosmotic to the external medium is generally associated with habitats and behaviors that are likely to expose animals to widely fluctuating salinities. Moreover, hyporegulation has been regarded as a possible pre-adaptation to terrestrial life since desiccation, like exposure to hypersaline surroundings, can lead to lethal concentration of body fluids in the absence of osmotic control. The ability to hyporegulate the haemolymph may, therefore, increase an animal's tolerance of evaporative water loss during desiccation stress. Acclimatization of T. traskiana to elevated habitat salinities during the summer may partly account for an observed increase in its tolerance of hypersaline experimental media between spring and fall. Seasonal shifts in salinity tolerance have been reported for other coastal amphipod populations and may be related to temperature-salinity interactions and to seasonal changes in habitat salinity and population structure (e.g., with respect to sex, maturity, and reproductive condition). Molt stage, sex, and life stage factors such as age and reproductive condition have been shown to influence the survival of amphipods at different salinities. (Koch 1991 and references therein)
In the Pacific Northwest of the United States, Traskorchestia traskiana is abundant on protected and semi-protected beaches, living mainly within and beneath washed up seaweed ("wrack"). This amphipod is one of the major wrack detritivores (consumers of decomposing organic matter) and can reach densities of 40 to 70 animals per gram of dry wrack. (Koch 1989; Koch 1990)
Traskorchestia traskiana is normally strictly nocturnal, remaining hidden within or beneath wrack during the day and coming to the surface only at night. Similar diurnal patterns of surface activity have been observed in other semiterrestrial amphipods. (Koch 1989 and references therein).
Occasional tidal inundation of the microhabitat of Traskorchestia traskiana results in a highly oriented upshore migration during which the animals search for other suitable microhabitats. Tidal inundations, and the landward migrations engendered by them, may be among the driving forces leading to the colonization of terrestrial leaf litter habitats by supralittoral beach hoppers in regions where such habitats border the supralittoral zone. (Koch 1989)
Traskorchestia traskiana is found in the supralittoral zone (i.e., region just above the high tide line) of rocky to muddy beaches and in salt marshes from the Aleutians and western Alaska to northern Baja California. Some terrestrial leaf litter populations of the species have been found near the shore around Santa Barbara, California. (Bousfield 1982; Koch 1991)
During mating, amphipod males transfer sperm to the females, where they fertilize the eggs (which may number from a few to hundreds) that have been placed in a brood pouch (marsupium). Unlike most crustaceans, amphipods belong to a group of crustaceans that do not have a larval stage. The eggs hatch in nine to 30 days as miniature adults, with the newly hatched young remaining in the brood pouch for several days. (Headstrom 1979)
Koch (1990) studied the population biology of Traskorchestia traskiana in the Pacific Northwest, U.S.A. The overwintering population consisted primarily of non-reproductive adults. Breeding began in February, during which a small percentage of large overwintering females bore eggs. The majority of the overwintering females were ovigerous (carrying eggs) by May. New young were added to the population continuously from April to mid- or late October and juveniles dominated the population between May and late August. The majority of overwintering females stopped breeding in July, during which a small proportion of young adult females born in early spring became ovigerous. These females bred throughout August and September. Field observations in 1978 revealed the presence of ovigerous females in mid-October. In 1977, females stopped producing eggs between late September and late October and recruitment of new young ceased after late October. Thus, the breeding season of T. traskiana in the northwestern United States appears to extend from about mid-February to mid-October. Females dominated the adult population throughout most of the year, particularly during the primary breeding peak in spring. Female-dominated populations have been reported for numerous species of amphipods. Mating in T. traskiana involves some degree of selective coupling based on body length. The smallest males (10.3 to 0.7 mm) were always paired with the smallest females (7.2 to 7.7 mm), whereas the larger males (11.0 to13.2 mm) were paired with both small and large females (7.2 to 8.8 mm). Size-selective coupling has been reported for other gammaridean amphipods and appears to be required by the mechanical features of the clasping and carrying process during mating. In semiterrestrial amphipods, it also ensures that the male is free to walk or to jump with the female curled beneath. (Koch 1990 and references therein)
Evolution and Systematics
Systematics and Taxonomy
Bousfield (1982) revised the genus level taxonomy for amphipods in the family Talitridae in the northeastern Pacific region. As a consequence of this revision, the former genus Orchestia was split into several genera, changing the name Orchestia traskiana to Traskorchestia traskiana.