An embryological study of Physa integra (Haldeman)

The primary objective of this investigation was to study the embryology of Physa and to photograph the stages of egg development, from the uncleaved egg to hatching. A secondary objective was to expose groups of eggs to solutions of different cupric ion concentrations, and to observe any effects on the eggs, or subsequent embryology. The amount of information on snail embryology is decidedly limited in detail, particularly for any one species. Material available on egg cleavage and developmental stages is often general for several species, and tends to be diagrammatic. Spiral cleavage is the rule in all molluscs with the exception of cephalopods. It was a part of this study to present the general characteristics of spiral cleavage, however the material presented is not necessarily the exact pattern for Physa integra. Only one reference was found that included a brief account of spiral cleavage for the Physidae.In this study one species of snail was reared in a five-gallon aquarium, under laboratory conditions, with temperature and other factors as constant as possible. Eggs were photographed at various intervals, in an attempt to show a developmental sequence up to the point of hatching. It should be noted that the sequence of photographs presented here was not taken from the same egg throughout. The egg and/or egg case tends to move slightly, so that a given view of the egg cannot be maintained; the egg case contains some type of fluid which may allow some shifting of the egg itself. Larval forms are also active due to ciliary movement.It was the intention of the writer to give an account of the approximate time required for egg development, in addition to showing as clearly as possible the various stages of development. Family Physidae has representative species on nearly every continent; species are found in various bodies of fresh water, such as ponds, lakes, and streams. Physa integra is generally abundant in the Great Lakes states (2).Fertilization is internal, and eggs are deposited in jelly-like clusters, each of which may contain from 30 to 200 eggs. Clusters are attached to underwater vegetation, rocks, or other available surfaces. Developmental time depends largely on temperature; a few degrees warmer than usual may reduce hatching time by a day or more. Physa will hatch in 6 to 8 days at 25 to 27 degrees C.Reproductive size is attained in three to four months, and under laboratory conditions eggs are produced daily. Approximately one third of a snail population are active egg producers on a given day. In this investigation, from 200 to 500 snails were maintained at all times.It may be noted that Physa integra, including all freshwater pulmonates, is hermaphroditic. Reciprocal copulation with exchange of sperm is the general rule. Pulmonates may sometimes practice self-fertilization, accomplished by the animal's own sperm in the hermaphroditic duct. This is evidenced by individual snails isolated since birth and still laying viable eggs. Protandry, not known in pulmonates, is common in opisthobranchs. No cases of parthenogenesis or self-copulation are known in pulmonates (6).