Assumed Amphi-Atlantic distribution of Oxyurella tenuicaudis (Cladocera, Chydoridae) denied by a new species from North America

Abstract

Up to now the taxon Oxyurella tenuicaudis, described from Norway, has been claimed to occur in North America as well. However, our close study and comparison of populations from the two continents show them to be different in many major morphological details, involving principally the female and male postabdomens, labrum, headpore configuration, and the external sculpturing of the shell. The new species O. brevicaudis occurs from the Maritimes to Manitoba in southern Canada, southward throughout the region east of the Plains to Guatemala and possibly Panama. Two enigmatic outlier populations are known from the Northwest Territories and the State of Washington. A greater frequency of occurrence in the southeastern states compared with southern Canada, together with its extension of range into the Tropics, suggests that this taxon prefers the lower north temperate latitudes. Although widely distributed, its frequency of occurrence is very low.

Just as interesting as the characters that separate these two species are those that unite them into the cohesive genus Oxyurella. Although the few other species in the genus have not been studied in sufficient detail, fragmentary information suggests that they also conform to the pattern of commonality emerging from this study. These potential generic characters include: 1) only one seta on the outer distal lobe of trunklimb I and two on the inner distal lobe; 2) 4 median headpores not connected to one another by any channel or raised chitinous ridge, although the light microscope shows irregular dark thickenings around each pore and occasional connections between the three anterior pores; 3) antennules of the male being very similar to those of the female except for the 2-segmented male seta and the addition of one more aesthetascs at the tip, for a total of 10 compared with the typical number of 12 in chydorid males; 4) a significant reduction in size of the seta arising from the middle endopodite segment of the male antenna as compared with the female; 5) use of the entire shell as an ephippium, without any external modification but with an internal foam-like mass secreted (?) into the brood pouch around the egg.

Other populations outside western Europe claimed to be O. tenuicaudis need to be studied closely to evaluate their presumed conspecificity. Very likely at least some will be found to be different. As aids in this process of evaluation, besides specimens from Gjennestadvannet placed in various museums, including the Zoological Museum in Oslo, a lectotype has been selected and designated herein from among the few specimens mounted and studied by G. O. Sars.