FUNCTIONAL MORPHOLOGY AND BEHAVIOR OF VARGULA HILGENDORFII (OSTRACODA: MYODOCOPIDA) FROM JAPAN, AND DISCUSSION OF ITS CRUSTACEAN ECTOPARASITES: PRELIMINARY RESULTS FROM VIDEO RECORDINGS

Abstract

ABSTRACT Newly investigated (video recordings) aspects of the biology of myodocopid ostracodes are presented, exemplified by Vargula hilgendorfii (Muller, 1890) (Cypridinidae) from shallow marine environments of the Pacific coast of central Japan. Like other myodocopid species attracted to bait (Cohen, 1982; Cohen and Morin, 1986), Vargula hilgendorfii is infaunal by day and becomes active by night, either on the substratum (e.g., feeding activities on fish carcasses) or in the water column (e.g., vertical migrations) with associated blue luminescence (Irie, 1953). All individuals from the first juvenile (A-5) to the adult stage (both sexes) bore a yellow luminescent organ (upper lip region) and were able to emit light in laboratory conditions. A synchronized stroking movement of the second pair of antennae (A2) was responsible for various locomotory activities such as swimming and digging the sediment (bioclastic sand). The first antennae (A1) have a probable tactile function in the recognition of the substratum and also in copulatory activities (suckerlike features of males). The furcae facilitated the penetration into the sediment and assisted rotations in a vertical plane, on or within the substratum. Males with a stronger muscular protopodite and a smaller body were faster swimmers than females. Specimens almost invariably reacted to light by shifting and adjusting their moving direction to reach the least illuminated area of the experimental container and/or by digging the sediment. Diggings were superficial, rarely exceeding a few mm below the sediment-water interface; they are interpreted as a response (camouflaged shelter) against predation, most probably controlled and initiated by visual stimuli. The seventh limbs are complex transformed appendages used by the brooding female to stir her free embryos within the marsupium; the limbs were more commonly used by both sexes from the fourth juvenile stage (A-2) to brush any region of the domiciliar concavity (appendages, lateral eyes, vestment, body) and also part of the external margins of the valves. The morphology of the seventh limb (e.g., articulated stem, bell-shaped setae) and its peculiar oscillating movement, both contributed to eject foreign particles from the domicilium. The ancestry of the seventh limbs in myodocopid ostracodes can be traced to, at least, the Lower Triassic (Weitschat, 1983a). Smaller crustacean ectoparasites frequently inhabit Vargula hilgendorfii. Onisocryptus ovalis (Shiino, 1942) (Isopoda, Epicaridea) preferentially infects nonovigerous females or females with eggs in ovaries and clutches the body of its host near the heart. There is no evidence that this parasite feeds by tearing into the ostracode's integument. The cryptic habit of O. ovalis may reduce predatorial risks.