The effect of water quality on oviposition in Biomphalaria glabrata (Say, 1818) (Planorbidae), and a description of the stages of the egg-laying process

With the overall goal of developing a method to reliably induce oviposition in the freshwater pulmonate Bioinphalaria glabrata, the effects of water q uality on female reproductive physiology were examined. Groups of snails we re subjected to controlled experimental conditions consisting of a daily re gimen of feeling and water change. After a period of acclimatization, egg m ass (EM) output under these conditions was: relatively stable, and snails l aid a majority (82.5%) of their EM during the initial 4 h following daily w ater change. When this regimen was perturbed by halting water change for 24 h (dirty-water treatment), EM output was significantly inhibited. When wat er change was resumed, EM output returned to previous levels within 4 h pos t-water change (PWC). This dirty-water treatment followed by water change a lso resulted in a significant increase in mean EM size during the 4 h PWC w hen compared to controls. To better describe the events preceding egg-layin g in B. glabrata we then used these experimental manipulations to induce ov iposition 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 (1 982). Stage I was characterized by the presence of ova in the hermaphroditi c duct and carrefour, and fertilized, packaged eggs in the oviduct and muci parous gland. Stage II was characterized by the presence of packaged eggs i n the oothecal gland embedded in a mucous layer, constituting the egg mass to be laid on the substratum. No packaging events were occurring in the car refour/albumen gland region during this stage. When snails were dissected i mmediately after oviposition (Stage III), unpackaged ova were observed in t he hermaphroditic duct, carrefour, and oviduct. The mean time it took for s nails to reach Stage III was 120 +/- 49 min (SD), and this value wa statist ically different from the mean time to Stages I and II, showing that our in duction protocol results in a temporal progression through the egg-laying p rocess. Gonadal oocyte density (oocytes/mm(2) of ovotestis) was quantified as a function of these stages of the reproductive cycle, and was found to b e significantly lower during Stage II (fully formed egg mass in oothecal gl and) than all other stages examined. Taken together, these results show tha t 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 regu lation of female reproductive activity in B. glabrata are discussed.