In two sympatric species of siphonariid limpets

A Centenary Research Grant Report by Purba Pal and Alan N Hodgson, Department of Zoology and Entomology, Rhodes University, Grahamstown, 6140, S. Africa

Siphonariid limpets are intertidal pulmonates belonging to a primitive family of Basommatophora. Although they have a wide distribution they are particularly abundant in the Southern Hemisphere where they display the richest species diversity in warmer waters (Hodgson, 1999). These limpets are thought to have marine ancestry although some authors (e.g., Borland, 1950; Yonge, 1952) believe their origin to be terrestrial. Three life history strategies have been described for siphonariid limpets; direct, planktonic and intermediate development (Chambers and McQuaid, 1994). Eckelbarger (1994) stressed the necessity of undertaking life history studies to help understand the selective forces that may have shaped the evolutionary history of an organism. To understand the differences between life history patterns one must also look at the part reproduction plays. Reproductive traits are not only limited by ancestry but also by the structure of ovary and associated mechanisms of yolk synthesis (Eckelbarger, 1994). Although there is a great deal of information on the biology of siphonariid limpets (see Hodgson, 1999, for review) knowledge on their reproductive biology is incomplete. For example whilst spermatogenesis has been described (Hodgson et al. 1991) there are no descriptions on oogenesis in these pulmonate limpets.

The aim of this study was to examine and compare oogenesis and vitellogenesis in a species with planktonic development, with one where development is direct. Because planktonic developers produce a large number of small eggs and direct developers fewer larger eggs, it was hypothesized that the mechanism of vitellogenesis would be different i.e., would reflect this life history trait. Siphonaria serrata and S. capensis which have different reproductive modes are both common on South African shores especially in the Eastern Cape where they occupy the same habitat. S. serrata is a direct developer, laying a small number of large eggs (capsule size 483 x 348 um), which spend 3-4 weeks on shore before they hatch into small juveniles. S. capensis, a planktonic developer, produces a large number of small eggs (capsule size 200 x 150 Ïm), which take 4-5 days to hatch into planktonic veliger larvae. In both species, each egg is enclosed in a capsule, which is embedded in a gelatinous matrix (in the form of either an egg ribbon or collar) attached to the rocky substrata.

The gonad (hermaphrodite gland or ovotestis) was sampled seasonally (summer-February, autumn- May, winter- July/August, spring- September/October) and processed for transmission electron microscope following a standard protocol. Embedding was done in an Araldite/Taab resin mixture (Cross, 1989) via propylene oxide and ultra thin sections were stained in uranyl acetate and lead citrate and viewed with a transmission electron microscope (Jeol 1210).

As in most gastropods, the ovotestis in Siphonaria is composed of numerous acini or sac like structures in which both sperm and eggs form. Oocytes develop next to the acinar wall with stages of spermatogenesis separated from oocytes by a layer of Sertoli cells. In both species oogenesis is intraovarian (when oocytes develop inside the ovary till they are ready to spawn) and follicular i.e., during the early stages oocytes are surrounded by few follicle cells but as oocytes mature, the follicle cells move away from the maturing oocytes. Before the onset of yolk synthesis, oocytes have a large, spherical nucleus (about 7.6 x 7.5 Ïm) often with two nucleoli, and few organelles in the cytoplasm. During early stages of vitellogenesis the oocytes display a marked increase in rough endoplasmic reticulum (RER) and the number of mitochondria and Golgi bodies. The Golgi bodies produce small vesicles which later fuse to give rise to nascent yolk granules (Fig. 2). The presence of a few endocytotic-coated pits along the oolemma of the mid- to late vitellogenic oocytes in S. serrata (Fig. 1) suggests incorporation of extraoocytic yolk precursors. This was not observed in S. capensis. In S. capensis, the oocytes develop one type of membrane bound yolk granule only (about 2.4 x 2.9 um) with a dense granular core (Fig.3). It is formed autosynthetically. In S. serrata two types of membrane bound yolk granules were found. Type I yolk granules (about 3.1 x 3.8 um), develop as highly electron-dense granules but mature into structures with an electron-dense crystalline core. On the other hand, heterosynthetically produced Type II yolk granules (about 2.8 x 3.5 um) have a dense granular core with an electron-lucent cortex. Later stage oocytes in both species had lipid droplets and glycogen granules in the ooplasm. In both species autosynthesis involves RER and Golgi bodies. During early vitellogenesis the follicle cells, which were surrounding oocytes, had a single large nucleus, abundant RER, Golgi bodies and a few lysosomes (Fig. 4). The abundance of proteosynthetic organelles in these cells suggests that follicle cells may play a role in providing nutrition to the developing oocytes during vitellogenesis.

In conclusion, in S. capensis yolk is produced mainly autosynthetically whereas S. serrata uses both autosynthetic and heterosynthetic modes of yolk synthesis. Mixed synthesis of yolk may be necessary in S. serrata to produce larger eggs with a greater endogenous energy supply. This in turn is needed for the time these eggs spend on the shore.

P.P would like to thank Malacological Society, London for making this study possible by supporting this work financially.

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