VARIATION IN THE DEEP-SEA ISOPOD EURYCOPE IPHTHIMA (ASELLOTA, EURYCOPIDAE): DEPTH RELATED CLINES IN ROSTRAL MORPHOLOGY AND IN POPULATION STRUCTURE

In an especially good sample series of the deep-sea isopod Eurycope iphthima Wilson, substantial variation occurs in the cephalic rostrum and in several aspects of its population structure: the size-frequency distribution, size of maturation of males and females, propor­ tions of female stages, proportions of juvenile stages, and maximum size of adults. Com­ parisons of samples from different depths (2,500 to 4,800 m) and different localities (north­ eastern and central Atlantic Ocean) show the variation to be distinctly depth-related: populations from similar depths are more similar than those from different depths. Because of continuity in other taxonomically important characters, the variation must be clinal and reflects adaptation to local environments, directed genetic variation, or both, The parallel clines in rostral morphology and population structure are most apparent on the continental slopes and rises off Ireland and in the Bay of Biscay. Bimodality in the population structure of the deeper populations is unusual; two hypotheses for its explanation are discussed, although evidence to support either is currently unavailable. Clinal variation in terrestrial organisms is well known (Mayr, 1970; Endler, 1977); it is little known in deep-sea animals. Clines should exist in the deep sea, especially in strong environmental gradients such as those found on the conti­ nental slopes. As depth increases, so does hydrostatic pressure, whereas tem­ perature and the quality and quantity of food decreases. Different levels of the slopes are subject to varying current fields and diurnal tidal rhythms. Most im­ portant, the slope faunas change greatly with depth (e.g., Hartman and Fauchald, 1971; Sanders and Hessler, 1969; Rex, 1979; Grassle et ai., 1979), both signaling this environmental change and becoming part of it. Genetic studies on deep-sea benthic megafauna (Doyle, 1972; Siebenaller, 1978) have demonstrated shifts in allozyme freq~enc. ies between depth-separated pop­ ulations. However, such studies are techn~lly unfeasible on smaller deep-sea invertebrates and must give way to morphological investigations. Individual mor­ phological variation may be important in deep-sea populations (e.g., Schopf, 1976; Hansen, 1967); often there are large differences between localities. For example, Rex (1979) analyzed two populations of Aivania peiagica, a shallow bathyal gas­ tropod found above and below the shelf-slope break off New England, and showed abrupt differences between the popUlations. In contrast, Gardiner (1975) noted "striking vertical morphological similarity" in specimens of the tanaid Neotanais americanus over a depth range of 2,800 to 5,020 m in the same area. Nevertheless, he found variation with depth in two characters. These two studies lack either verification of conspecificity of the populations or enough specimens for a pop­ ulation analysis. In addition to large samples of conspecific populations, the identification of a cline further requires at least three or more popUlation samples, a rare occurrence in the vagaries of deep-sea sampling. This study uses abundant material of the dominant deep-sea eurycopid isopod EuryciJpeiphthima Wilson (1981) (Table 1), to establish the presence of, clinal variation in one morphological character and in various aspects of the population structure of this species.