Comparative biology of oogenesis in nemertean worms

Abstract

In order to supplement previous analyses of oogenesis in nemertean worms, this study uses light and electron microscopy to compare the ovaries and oocytes in 16 species of nemerteans that represent various taxa within the phylum, Nemertean ovaries comprise serially repeated sacs with an ovarian wall that characteristically includes myofilament-containing cells interspersed among the germinal epithelium. Each oocyte can attach to the germinal epithelium by a vegetally situated stalk and resides in the ovarian lumen without being surrounded by follicle cells. In the ovary, oocytes arrest at prophase I of meiosis and contain a hypertrophied nucleus ('germinal vesicle') that often possesses multiple nucleoli. Intraovarian growth apparently involves an autosynthetic mode of yolk formation in most nemerteans and generates oocytes that measure ~60 Jim to 1 mm. When fully developed, oocytes can be discharged through a short gonoduct and are either spawned freely or deposited within egg cases. In most species, oocytes released from the ovary possess extracellular coats and resume maturation by undergoing germinal vesicle breakdown (GVBD). Such post-GVBD specimens also form a punctate endoplasmic reticulum that may facilitate fertilization and development.

First record on the development of the larva of the basally branching nemertean species Carinina ochracea (Palaeonemertea)

“…The sperm head is of a compact, almost spherical shape and measures 1.76 (±0.14) µm in length and 1.61 (±0.14) µm in width (published erroneously as Tubulanus linearis in von Döhren et al [41]). The compact shape of the sperm heads along with the thin vitelline membrane of the oocytes support the assumption that in C. ochracea fertilization is external [35]. The occurrence of both large and smaller oocytes within the same gonad over a period of several months as well as the observed motility of sperm during the same period of time indicates that not all gametes may be shed in one single spawning event.…”

“…Carinina ochracea is a dioecious species with males and females being of roughly the same size [37].The egg size of approximately 125 µm is well within the range of egg sizes reported for anoplan nemerteans that develop from pelagic larvae [10,36]. As in the majority of nemertean species, mature eggs in the gonad are arrested in the prophase of the first meiotic division indicated by the germinal vesicle [34,36].…”

Calcium signals and oocyte maturation in marine invertebrates

“…Nemerteans are usually dioecious, and gravid females typically possess numerous ovaries with relatively small (~70-200 mm) and translucent oocytes that lack follicle cells (Stricker et al, 2001). Whether nemertean oocytes in the field begin GVBD before or after being spawned has yet Nishiyama et al (2010) and Chiba (2011), which contain further details regarding various subtypes within these patterns.…”

Endoplasmic reticulum reorganizations and Ca2+ signaling in maturing and fertilized oocytes of marine protostome worms: the roles of MAPKs and MPF

“…Hence, the False Bay Cerebratulus are simply referred to as Cerebratulus sp. Such worms were ~10-50 cm long, 0.5 to 1.5 cm wide, olive-tan to chocolate-brown in color, and their sperm had an unmodified, 5 µm long head (Stricker and Folsom, 1998).…”