This paper provides a morphological description of Capitella teleta sp. nov., an opportunistic capitellid that is also commonly used as an experimental polychaete under the provisional designation Capitella sp. I. The species is widely distributed along the east and west coasts of North America and also reported from Japan and the Mediterranean. The species belongs to a group having distinct sexual dimorphism, yet with hermaphrodites occurring under certain conditions. Morphologically, C. teleta has a long, narrow body, with all thoracic segments similar except for sexual modifications on setigers 8–9; the prostomium/peristomium combined are long, narrow and about 2.5 times as long as setiger 1. Capillary setae are present in noto- and neuropodia of setigers 1–7; setigers 8–9 have hooded hooks in noto- and neuropodia of females; genital spines replace notopodial hooks in males. A methyl green staining pattern is limited to some thoracic setigers of females; males lack a distinct staining pattern. The cytochrome oxidase I (COI) sequence is presented. Relationships of C. teleta with the type-species, C. capitata and other known species including siblings identified in laboratory culture are discussed. The syntype of Ancistria acuta Verrill, 1874, the only known species of Capitella described from New England was examined and determined to be incertae sedis. C. teleta is a highly opportunistic species and appears to be the same as the C. capitata identified from southern California as the “polluted zone indicator” by D.J. Reish in the late 1950s. An appendix with over 200 published references to research conducted on C. teleta is included.

Ultrastructural features of the ovotestes, spermatogenesis, and the mature sperm are described for three galeommatid bivalves, Divariscintilla yoyo, Divariscintilla troglodytes, and Scintilla sp., from stomatopod burrows in eastern Florida. All three species yielded similar results except with respect to mature sperm dimensions. The ovotestis contains three types of somatic cells within the testicular portion: flattened myoepithelial cells defining the outer acinal wall; underlying pleomorphic follicle cells containing abundant glycogen deposits; and scattered, amoeboid cells containing lysosomal-like inclusions which are closely associated with developing sperm. Early spermatogenesis is typical of that reported from other bivalves. In contrast, the late stages of spermiogenesis involve the migration and gradual rotation of the acrosomal vesicle, resulting in a mature acrosome tilted about 70° from the long axis of the cell. The mature sperm possesses an elongated, slightly curved nucleus; a subterminal, concave acrosome with a nipple-like central projection; five spherical mitochondria and two centnoles in the middlepiece; and a long flagellum. The rotational asymmetry and the presence of perimitochondrial glycogen deposits in these sperm are unusual in the Bivalvia and may be associated with fertilization specializations and larval brooding common among galeommatoideans.

Two types of oogenesis have evolved in polychaetes: one (extraovarian) in which germ cells are ovulated into the coelomic cavity where they undergo vitellogenesis, and another (intraovarian) in which the germ cells are released from the ovary after or near the completion of vitellogenesis. There are many mechanisms of yolk synthesis and significant variation as to the sources of yolk precursors, their route of entry into developing oocytes, and the oocytic organelles involved in yolk synthesis. The evolution of the coelom and its role in nutrient storage and transfer, as well as the origin of germ cells, are intimately related to the evolution of vitellogenic mechanisms in polychaetes. Comparative studies have revealed a correlation between mechanisms of yolk synthesis and the breeding patterns of polychaetes with the rate and type of vitellogenesis being related to the frequency of breeding. Vitellogenic mechanisms in polychaetes are compared with those of other metazoans in an effort to better understand the evolution of this important process and to identify primitive and derived features of yolk synthesis within the Polychaeta.

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