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Introduction

Example of a gastropod.

Gastropods are defined as snails and slugs, belonging to a larger group called Molluscs. Gastropods have a very unique reproductive system, varying significantly from one taxonomic group to another. They can be separated into three categories: marine, freshwater, and land. Reproducing in marine or freshwater environments makes getting sperm to egg much easier for gastropods, while on land it is much more difficult to get sperm to egg. The majority of gastropods have internal fertilization, but there are some prosobranch species that have external fertilization. Gastropods are capable of being either male or female, or hermaphrodites, and this makes their reproduction system unique amongst many other invertebrates. Hermaphroditic gastropods possess both the egg and sperm gametes which gives them the opportunity to self-fertilize.

Reproductive System of Land Gastropods

Separate Sexes

Land snails mating.

Members of the snail family Pulmonata, which includes carboniferous land sails and some freshwater snails of the order Basommatophora, are protandrous hermaphrodites, meaning they are born male and later in life become female. In this family of snails, the male phase ends in December, followed by an egg maturation phase, and ends with oviposition, the act of laying eggs during May of the following year. Phylogenetic evidence for this is present based on the overall condition of the gonads especially in the degree of development of the genital ducts.

Hermaphrodites

The structure of the reproductive system is strictly hermaphroditic. From the gonads, a hermaphrodite duct, a duct which is designed to transport both sperm and eggs, leads to a portion of the reproductive tract where the duct splits into a strictly male and strictly female portion.

The female portion includes a fertilization pouch and posterior and anterior mucus glands, which open up into a pallial cavity which leads to a small muscular vagina. The male portion of the reproductive tract includes both a short posterior vas deferens and a longer anterior vas deferens. The posterior vas deferens is followed by the prostate, and the anterior vas deferens flows through the haemocoele, an enlarged blastula filled with blood, of the head and opens into a muscular penis which is engulfed in a small portion of skin called the prepuce sac.

Reproductive System of Marine Gastropods

Saltwater Environments

Separate Sexes and Hermaphrodites

The reproductive system of marine gastropods such as those from the class Opisthobranchia and order Archaeogastropoda from the class Prosobranchia, is a continuous cycle of alternating male and female reproductive role prevalence. Immediately after spawning in late summer, the predominance of the female reproductive functions are terminated and gametogenesis initiates immediately, with the start of the predominance of the male reproductive role. Gametes remain in the gonads throughout the winter and early spring. The female reproductive role takes over again in May with fertilization of the gametes to form zygotes. The cycle comes full circle in late summer once again, with spawning.

Freshwater Environments

Separate Sexes and Hermaphrodites

Marine snails mating.

Species in the freshwater gastropod family such as the Caenogastropoda from the class Prosobranchia, are largely self-fertilizing; however after many generations of selfing, a physiological barrier halts sperm generation in that organism, and only allows for the introduction of foreign sperm. Gametes form in the ovotesties, an organ which produces both ova and sperm, and pass down into the hermaphroditic duct to the albumen gland, the junction of where the common duct splits to either vas deferens or oviduct, where they are stored until they are needed for either mating or self-fertilization. It is believed that this junction acts as a regulatory mechanism via contracting muscles, to help direct sperm or eggs into the correct ducts.

Mating

The sperm passes into the male duct, or vas deferens, where is receives secretory additions in the form of mucus from the prostate. After getting modified, the sperm passes into the penis. During mating season, the glandular cells in the penis sheath and prepuce swell to facilitate eversion of the penis. The sperm gets pushed through the penis, where they are introduced into the tail end of its copulatory partner. Within the partner snail, after fertilization from the foreign sperm, the eggs pass into the albumen gland where they are coated in mucus which forms the egg capsule.

Selfing

Eggs are released immediately before oviposition. Unlike in land gastropod species where fertilization occurs in fertilization pockets, fertilization in freshwater species happens at the lower end of the hermaphroditic duct, near the junction. Sperm is deposited into the bursa copulatrix which opens up into the vagina. The ova then enter the albumen gland to get a nutrient dense mucus coating which serves to form the egg capsule.

Genital Structures

Gastropod anatomy.

Males

The prostate is found in the mantle tubule and penis are both connected to the gonoduct; which are connected to the testis (produce sperm). During sex, the sperm travels along the mantle tube in which seminal fluid fills the mantle tube and exits the body via the penis and enters the females gonopore.

Additional reproductive structures include:

• Vas deferens: it is found attached to the penis and the prostate and aids in the movement of sperm from the prostate to the penis
• Dart sac: helps males grasp onto females during sex. Not found in all gastropods

Females

Females have a gonopore that is connected to a seminal receptacle. The gonopore acts as an opening where eggs are deposited through. The opening leads into the mantle tubule, in which eggs flow from the oviduct and ovary. The mantle tubule produces three things, yolk; carries most of the nutrients needed to develop a healthy offspring, egg capsule formation, and sperm reception and storage; where fertilization occurs. After fertilization, eggs travel to the albumin glands to fill the yolk with protein, and lastly, the egg travels through the capsule glands, which coat the egg in a protective jelly.

Additional reproductive structures include:

• Uterus: stores the fertilized eggs until the eggs are ready to be laid
• Vagina: location where the eggs pass through to exit the gonopore

Hermaphrodites

Comprised of both male and female reproductive parts. The male and female system act as separate units until the egg and sperm are ready to fuse together. The fusion of egg and sperm takes place in the uterus of the female system. Hermaphrodites also have a hermaphroditic duct, which helps change the sex of the gastropod during certain times of the year.