Gastropods, also called gastropoda, make up a huge and diverse taxonomic class that includes thousands of different species of slugs and snails of various sizes and shapes. The total number of species comprised in the class range between 60,000 and 80,000, with the behavior, the feeding and reproductive habits the member species adopt being so diversified that the group comes second only to the class of insects in terms of variety. Given the diversity and complexity of the class, it is very difficult to make generalizations, but there are a few features shared by all members.
Gastropoda can be found all over the world. Many species live in freshwater environments, but the vast majority of them, more than two-thirds of all the member species live in marine habitats. Wherever you go from the Antarctic to the Arctic pole, from tropical regions to colder areas, you will find gastropoda everywhere.
Gastropoda vary largely varied in terms of size – the largest marine snail ever recorded is Australia’s giant whelk, or Syrinx aruanus, whose shell length can reach 70 cm, while the smallest gastropod species do not weigh more than a few ounces.
All members of the class have a shell to cover their soft and sensitive bodies. However, the shape, size, coloring and thickness of the shell are largely diversified – some species that live in habitats where they have access to plenty of calcium carbonate grow thick and resistant shells, while others that live in environments where there is no or very little calcium carbonate have almost completely translucent shells that do not offer too much protection. Slugs are distinguished from snails by the total absence of the shell.
Other physical features shared by most gastropoda are the relatively large and well-developed head, the existence of 2-4 tentacles on the head ending in eyes and the strong and flexible ventral foot. They are also characterized by an asymmetric distribution of the main organs.
Gastropoda are not only a physically diverse group. They are also hardy and incredibly adaptable creatures, each of the species having developed feeding habits that allow them survive in their chosen habitat. Most marine species usually feed on the algae and small particles of nutrients floating in the water.
Most gastropoda are sexually dimorphic, meaning that they have separate sexes, but there are some species that are hermaphrodites. Most aquatic species proliferate by the females releasing their gametes and the males releasing their sperm into the water, with fertilization taking place externally. The individuals belonging to hermaphroditic species have both male and female reproductive organs, but they need to mate to become fertile, case in which both members of the couple will lay eggs. The stage that follows egg laying is the embryo phase; then comes the development into larvae, and then life as adult gastropods, the length of which varies between 5 and 20 years.