Spermatozoan ultrastructure and spermatogenesis in aplousobranch ascidians,with some phylogenetic considerations

Spermatozoa and some stages of spermatogenesis were studied for four species of aplousobranch ascidians. Spermatozoa of Clavelina lepadiformis (Müller) (family Clavelinidae) are plesiomorphous in that they have apical acrosomal vesicles and a moderately elongated, cylindrical nucleus. The elongated mitochondrion is twisted ea. 11/2 times around the nucleus. In their ultrastructural morphology, C. lepadiformis sperm conform in some respects to the least-derived ascidian sperm, those of the phlebobranch ascidian Ciona intestinalis L.; however, the sperm of Clavelina lepadiformis have two apomorphies not shared with those of Ciona sp.: (1) the mitochondrion of Clavelina lepadiformis is long and spiralled along the entire nucleus rather than being comparatively compact and not at all helical; (2) the mitochondrial cristae are elongated parallel to the long axis of the nucleus, whereas in Ciona sp. sperm the cristae are unmodified. In Distaplia sp., Aplidium sp. and Synoicum pulmonaria (Ellis and Solander) the spermatozoa are more derived and consist of a proximal cylindrical and a distal corkscrew-like part. The mitochondrion in Distaplia sp. and Aplidium sp. contains electron-dense material and extends in a long thread around the nucleus. In S. pulmonaria the mitochondrion surrounds the anterior part of the nucleus in mature spermatozoa, and an elongated, dense structure displaying fine striation is enclosed in the mitochondrion in late spermatids. The sperm ultrastructural morphology observed in this study is consistent with the majority view that clavelinids are closest to the ancestral ascidian but is also consistent with other conclusions, particularly that the Cionidae are closest to the stem ascidian.     

Zoogeography of Northwest Atlantic opisthobranch molluscs

Currently available information on the distribution of species of the opisthobranch orders Cephalaspidea, Nudibranchia and Sacoglossa (Mollusca: Gastropoda) in the northwest Atlantic Ocean is presented. Except for Sacoglossa, the fauna is markedly amphiatlantic, but the temperate West Atlantic component increases southward and dominates south of New Jersey. Species' diversity of nudibranchs and cephalaspids on the continental shelf decreases between northern New England and Cape Hatteras in association with a southward decline in the proportion of amphiatlantic species. This is not evident for the cephalaspids of the continental slope, a higher percentage of which are amphiatlantic. The entire fauna is subdivided into 5 groups: arctic, boreo-subarctic, boreal, West Atlantic temperate, and West Atlantic tropical. The latter does not extend north of Cape Hatteras, but the remaining groups broadly overlap in the northwest Atlantic. The southward limits of amphiatlantic species are presented and related to sea surface temperatures. The thermal characteristics of a critical zone between Cape Cod and Connecticut are discussed, and a maximum temperature of 15 °C is suggested for successful reproduction of the boreo-subarctic component, and 25 °C for the boreal component. The distribution of boreal species on both sides of the North Atlantic is discussed, and a suggestion is made that the southward speread of these species in North America is limited by extreme summer warming south of New Jersey, despite favorable winter temperatures, which extend much farther south.Contribution No. 66 of the Marine Research Laboratory, University of Connecticut, USA.