Honeycombing of the Carapace in the Chydorid Cladocera: the Elusive Male of Chydorus Faviformis

Abstract

Unlike the other honeycombed taxa in the genus Chydorus, the males of North American C. faviformis completely lose their honeycombed exterior on molting to instar III, at which time they closely resemble mature males of a nonhoneycombed species of Chydorus that can co-occur. The honeycomb pattern in these chydorids results from thin sclerotized extensions of the shell and head, whereas the honeycomblike pattern sometimes present in Pleuroxus trigonellus and a few other species results from the coalescing of columns of secreted material, which can be loosened easily (as by weak detergents) and sloughed off in ragged pieces. The honeycombed taxa in Chydorus are so different from one another in certain conservative aspects of morphology, particularly of the males, that it is reasonably certain honeycombing has arisen more than once during evolution. Males of all chydorid Cladocera, so far as known, have two immature instars before they become functionally reproductive in instar III. Moreover, unlike the daphniids, where the males after reaching maturity continue to molt and grow at more or less regular intervals (e.g., up to 12 instars in Scapholeberis mucronata: Rammner, 1928, 1929), most chydorid males quite certainly have only a single reproductive instar. Smirnov (1971: 120, 1974: 135) makes the general statement, without supporting evidence, that chydorid males continue to molt but do not grow after reaching maturity. My contrary opinion that males have only a single mature instar is based on never finding exuviae of mature males in populations that are strongly gamogenetic, even though exuviae of instar II males do occur, and in the lack of evidence that mature males are producing an exoskeleton for a next instar. The new exoskeleton in formalin-preserved specimens tends to pull away from the old, and from the distinctness of teeth, spines, setae, and setules in the new exoskeleton, especially of the postabdomen and claw, one can make a subjective judgment as to where developmentally the specimen is in its intermolt period (e.g., Frey, 1973). Such exoskeletons in process of formation can be observed in females and in both instars of immature males but not in instar III males. The occasional exoskeletons of mature males that have died from some nondisruptive cause cannot be construed to represent a fourth instar. The uncoupling of reproduction from molting and growth in mature chydorid males may be unique among the Cladocera, and certainly its validity and significance merits close scrutiny. Since, as stated above, most chydorid males have only a single reproductive instar, and also because there is generally very little or even no increase in size between instars II and III, whereas the females continue to molt and grow throughout their lives, functional males of chydorids are typically smaller than the smallest reproductive females, sometimes, as in Eurycercus, markedly so. The stepwise ontogenetic changes in morphology of chydorid males are such that each of these three instars can be recognized' positively (examples documented in Frey, 1975, 1980, in press a, in press b; Chengalath and Hann, 1981). Progressive changes occur in the structure of trunk limb I, including the development of the copulatory hook and accessory structures, and also in the structure of the antennule, postabdomen, postabdominal claw, and labrum. Other morpho469