Quick facts for kidsGastropod
Gastropods, or univalves, are the largest and most successful class of molluscs. 60,000–75,000 known living species belong to it. Most of are marine, but many live in freshwater or on land. Their fossil record goes back to the later Cambrian.
Slugs and snails, abalones, limpets, cowries, conches, top shells, whelks, and sea slugs are all gastropods. The gastropods are in origin sea-floor predators, though they did evolve into many other habitats. Many lines living today evolved in the Mesozoic era, taking advantage of the huge supply of food on the sea floor.
Snails are distinguished by an anatomical process known as torsion, where the visceral mass of the animal rotates 180° to one side during development, such that the anus is situated more or less above the head. This process is unrelated to the coiling of the shell, which is a separate phenomenon. Torsion is present in all gastropods, but the opisthobranch gastropods are secondarily de-torted to various degrees.
Torsion occurs in two stages. The first, mechanistic stage, is muscular, and the second is mutagenetic. The effects of torsion are primarily physiological - the organism develops an asymmetrical growth, with the majority occurring on the left side. This leads to the loss of right-paired appendages (e.g., ctenidia (comb-like respiratory apparatus), gonads, nephridia, etc.). Furthermore, the anus becomes redirected to the same space as the head. This is speculated to have some evolutionary function, as prior to torsion, when retracting into the shell, first the posterior end would get pulled in, and then the anterior. Now, the front can be retracted more easily, perhaps suggesting a defensive purpose.
However, this "rotation hypothesis" is being challenged by the "asymmetry hypothesis" in which the gastropod mantle cavity originated from one side only of a bilateral set of mantle cavities.
Gastropods typically have a well-defined head with two or four sensory tentacles with eyes, and a ventral foot, which gives them their name (Greek gaster, stomach, and pous, foot). The foremost division of the foot is called the propodium. Its function is to push away sediment as the snail crawls. The larval shell of a gastropod is called a protoconch.
The principal characteristic of the Gastropoda is the asymmetry of their principal organs. The essential feature of this asymmetry is that the anus generally lies to one side of the median plane.; The ctenidium (gill-combs), the osphradium (olfactory organs), the hypobranchial gland (or pallial mucous gland), and the auricle of the heart are single or at least are more developed on one side of the body than the other ; Furthermore, there is only one genital orifice, which lies on the same side of the body as the anus.
Most shelled gastropods have a one piece shell, typically coiled or spiraled, at least in the larval stage. This coiled shell usually opens on the right-hand side (as viewed with the shell apex pointing upward). Numerous species have an operculum, which in many species acts as a trapdoor to close the shell. This is usually made of a horn-like material, but in some molluscs it is calcareous. In the land slugs, the shell is reduced or absent, and the body is streamlined.
Some sea slugs are very brightly colored. This serves either as a warning, when they are poisonous or contain stinging cells, or to camouflage them on the brightly colored hydroids, sponges and seaweeds on which many of the species are found.
Lateral outgrowths on the body of nudibranchs are called cerata. These contain an outpocketing of digestive gland called the diverticula.
Sensory organs and nervous system
In terrestrial gastropods (land snails and slugs), the olfactory organs, located on the tips of the four tentacles, are the most important sensory organ. The chemosensory organs of opisthobranch marine gastropods are called rhinophores.
The majority of gastropods have simple visual organs, eye spots either at the tip or base of the tentacles. However, "eyes" in gastropods range from simple ocelli that only distinguish light and dark, to more complex pit eyes, and even to lens eyes. In land snails and slugs, vision is not the most important sense, because they are mainly nocturnal animals.
The nervous system of gastropods includes the peripheral nervous system and the central nervous system. The central nervous system consist of ganglia connected by nerve cells. It includes paired ganglia: the cerebral ganglia, pedal ganglia, osphradial ganglia, pleural ganglia, parietal ganglia and the visceral ganglia. There are sometimes also buccal ganglia.
Many marine gastropods are burrowers, and have a siphon that extends out from the mantle edge. Sometimes the shell has a siphonal canal to accommodate this structure. A siphon enables the animal to draw water into their mantle cavity and over the gill. They use the siphon primarily to "taste" the water to detect prey from a distance. Gastropods with siphons tend to be either predators or scavengers.
Almost all marine gastropods breathe with a gill, but many freshwater species, and the majority of terrestrial species, have a pallial lung. The respiratory protein in almost all gastropods is hemocyanin, but one freshwater pulmonate family, the Planorbidae, have hemoglobin as the respiratory protein.
In one large group of sea slugs, the gills are arranged as a rosette of feathery plumes on their backs, which gives rise to their other name, nudibranchs. Some nudibranchs have smooth or warty backs with no visible gill mechanism, such that respiration may likely take place directly through the skin.
The primary organs of excretion in gastropods are nephridia, which produce either ammonia or uric acid as a waste product. The nephridium also plays an important role in maintaining water balance in freshwater and terrestrial species. Additional organs of excretion, at least in some species, include pericardial glands in the body cavity, and digestive glands opening into the stomach.
The main aspects of the life cycle of gastropods include:
- Egg laying and the eggs of gastropods
- The Embryonic development of gastropods
- The larvae or larval stadium: some gastropods may be trochophore and/or veliger
- Estivation and hibernation (each of these are present in some gastropods only)
- The growth of gastropods
- Courtship and mating in gastropods: fertilization is internal or external according to the species. External fertilization is common in marine gastropods.
The diet of gastropods differs according to the group considered. Marine gastropods include some that are herbivores, detritus feeders, predatory carnivores, scavengers, parasites, and also a few ciliary feeders, in which the radula is reduced or absent. Land-dwelling species can chew up leaves, bark, fruit and decomposing animals while marine species can scrape algae off the rocks on the sea floor. Certain species such as the Archaeogastropda] maintain horizontal rows of slender marginal teeth. In some species that have evolved into endoparasites, such as the eulimid Thyonicola doglieli, many of the standard gastropod features are strongly reduced or absent.
A few sea slugs are herbivores and some are carnivores. The carnivorous habit is due to specialisation. Many gastropods have distinct dietary preferences and regularly occur in close association with their food species.
Some predatory carnivorous gastropods include, for example: Cone shells, Testacella, Daudebardia, Ghost slug and others.
The taxonomy of the Gastropoda is under constant revision: two major revisions have been published in the last twenty years. There will certainly be other revisions using the data from DNA sequencing. At present, the taxonomy of the Gastropoda may differ from author to author.
According to modern cladism, the taxonomy of the Gastropoda should be written in terms of strictly monophyletic groups. That means only one lineage of gastropods in each group. It will be difficult to do this, and still have a practical taxonomy for working biologists. Classifying animals in practice means using morphology (what they look like). But there are differences between the older groupings got from morphology, and those based on genome sequences. Convergent evolution, which has occurred often in Gastropods, may account for this.
This was the traditional classification into four subclasses:
- Prosobranchia: (gills in front of the heart).
- Opisthobranchia: (gills to the right and behind the heart).
- Gymnomorpha: (no shell)
- Pulmonata: (with lungs instead of gills)
The first gastropods were exclusively marine. The first of the group appeared in the Upper Cambrian (Chippewaella, Strepsodiscus). By the Ordovician period the gastropods were a varied group present in a few aquatic habitats. Commonly, fossil gastropods from the rocks of the early Palaeozoic era are too poorly preserved for accurate identification. Still, the Silurian genus Poleumita contains fifteen identified species. Fossil gastropods are less common during the Palaeozoic era than bivalves.
Most of the gastropods of that era belong to primitive groups. A few of these groups still survive today. By the Carboniferous period many of the shapes seen in living gastropods can be matched in the fossil record. Despite these similarities in appearance, the majority of these older forms are not directly related to living forms. It was during the Mesozoic era that the ancestors of many of the living gastropods evolved. One of the earliest known terrestrial (land-dwelling) gastropods is Maturipupa which is found in the Coal Measures of the Carboniferous period in Europe. In the case of the common fossil Bellerophon, from Carboniferous limestones in Europe, it is not known whether it is a gasropod or not.
In rocks of the Mesozoic era gastropods are more common as fossils and their shell is often well preserved. Their fossils occur in beds of both freshwater and marine environments. The Purbeck Marble of the Jurassic period and the Sussex Marble of the early Cretaceous, which both occur in southern England, are limestones containing the tightly packed remains of the pond snail Viviparus.
Rocks of the Cainozoic era have very large numbers of gastropod fossils in them. Many of these fossils are closely related to modern living forms. The diversity of the gastropods increased markedly at the beginning of this era, along with that of the bivalves.
Gastropods are one of the groups that record the changes in fauna caused by the advance and retreat of the Ice Sheets during the Pleistocene epoch.
- Reconstructions of fossil gastropods at www.emilydamstra.com
Images for kids
The Devonian gastropod Platyceras from Wisconsin.
Gastropoda Facts for Kids. Kiddle Encyclopedia.