A study of Thermosbaena mirabilis (Malacostraca, Peracarida) and its reproduction

Abstract

ABSTRACT Knowledge of the anatomy of Thermosbaena is extended as follows: (i) The topography of the head and anterior thoracic region, and the orientation and relationships of the mouthparts and anterior appendages are described, (ii) A branchial chamber is located underneath each lateral carapace flap, (iii) ‘Skin’ glands are demonstrated to occur at the bases of the paragnaths, maxillules, and maxillipedes; in the labrum and in the mandibular palps; in the groove leading from the food basin to the paragnath bases; and in the basal segments of the endopodites of peraeopods II-V. The function of these glands is obscure; histochemical tests indicate that they do not secrete mucus, (iv) The posterior aorta is traced from the heart to the level of the first abdominal segment, where it bifurcates into 2 lateral vessels which diverge to each side of the intestine, (v) Four conditions of the female reproductive system are distinguished, namely, the immature, mature, copulatory, and brood-pouch conditions; temporary seminal receptacles are formed by dilatation of the oviducts in the copulatory condition. An inhalant respiratory current is produced by oscillation of the maxillipede epipodites in the branchial chambers. The exhalant respiratory current emerges via carapace lappets on either side of the head. Thermosbaena feeds on particles of detritus suspended in a mucilaginous matrix that is probably derived from the mucilaginous sheaths of the Cyanophyceae in the diet. A main food-stream appears to be driven forwards between the peraeopods, through a channel formed by the maxillipede bases to the food basin, and from thence along a ventral groove and between the paragnath bases to the oral cavity. It is possible that the main food-stream is augmented by detritus brought by the inhalant respiratory current, and by lateral feeding currents drawn in from the sides. Following internal fertilization, early segmentation of the embryo occurs within the ovaries. The blastomeres are first formed within the body of the yolk and subsequently rise to the surface, the segmentation thus resembling that of Hemimysis and Mesopodopsis. The embryos are transferred to the dorsal brood-pouch shortly after segmentation begins, probably by expulsion from the posteriorly directed vaginae while the female is upside down, followed by suction into the brood-pouch with the inhalant respiratory current. The embryos emerge from the brood-pouch as miniature adults with the full complement of peraeopods. There appears to be no transitory 7th abdominal ganglion. Thermosbaena is probably paedomorphic in certain respects and less primitive than Monodella. There are insufficient grounds for establishing a new malacostracan division Pancarida, as advocated by Slewing (1958), to receive the Thermosbaenacea.