J. Moll. Stud. (2000), 66, 83-94
© The Malacological Society of London
2000
THE EFFECT OF WATER QUALITY ON OVIPOSITION IN BIOMPHALARIA GLABRATA (SAY, 1818) (PLANORBIDAE), AND A DESCRIPTION OF THE STAGES OF THE EGG-LAYING PROCESS
Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin, 2015 Linden Drive West, Madison, WI. 53706, U.S.A.
Author to whom correspondence should be addressed.
With the overall goal of developing a method to reliably induce oviposition in the freshwater pulmonate Biomphalaria glabrata, the effects of water quality on female reproductive physiology were examined. Groups of snails were subjected to controlled experimental conditions consisting of a daily regimen of feeding and water change. After a period of acclimatiz-ation, egg mass (EM) output under these conditions was relatively stable, and snails laid a majority (82.5%) of their EM during the initial 4 h following daily water change. When this regimen was perturbed by halting water change for 24 h (dirty-water treatment), EM output was significantly inhibited. When water change was resumed, EM output returned to previous levels within 4 h post-water change (PWC). This dirty-water treatment followed by water change also resulted in a significant increase in mean EM size during the 4 h PWC when compared to controls. To better describe the events preceding egg-laying in B. glabrata, we then used these experimental manipulations to induce oviposition in groups of snails, and dissected them during the 4 h following water change. Observations of the reproductive tracts of stimulated snails allowed us to divide the egg-laying process, from ovulation to oviposition, into discrete stages, after de Jong-Brink, Koop, Roos & Bergamin-Sassen (1982). Stage I was characterized by the presence of ova in the hermaphroditic duct and carrefour, and fertilized, packaged eggs in the oviduct and muciparous gland. Stage II was characterized by the presence of packaged eggs in the othecal gland embedded in a mucous layer, constituting the egg mass to be laid on the substratum. No packaging events were occurring in the carrefour/albumen gland region during this stage. When snails were dissected immediately after oviposition (Stage III), unpackaged ova were observed in the hermaphroditic duct, carrefour, and oviduct. The mean time it took for snails to reach Stage III was 120 6 49 min (SD), and this value was statistically different from the mean time to Stages I and II, showing that our induction protocol results in a temporal progression through the egg- laying process. Gonadal oocyte density (oocytes/mm2 of ovotestis) was quantified as a function of these stages of the reproductive cycle, and was found to be significantly lower during Stage II (fully formed egg mass in othecal gland) than all other stages examined. Taken together, these results show that female reproductive activity can be experimentally controlled through the manipulation of water quality, and that such a protocol is a valuable tool for addressing specific questions regarding the reproductive physiology of B. glabrata. The implications of these results as they pertain to the regulation of female reproductive activity in B. glabrata are discussed.
(Received 15 January 1999; accepted 17 May 1999)