Modern Forms


Ecology

 

Class Gastropoda is now likely to be at the peak of its diversity: the 60,000 living species dominating many of the environments they inhabit. Gastropods form an important part of the food chain in both aquatic and terrestrial habitats. Few gastropods are likely to die as the result of old age, since the majority will fall prey to other animals. However, life spans of up to ten years have been recorded in some fresh-water snails.


While many familiar gastropods are terrestrial (such as the land slugs and snails) two thirds of living species are aquatic. Gastropods are present in a vast range of marine habitats; from the deep ocean bottom to the high water mark. Most marine species are benthonic (inhabiting the ocean bottom), although others are pelagic (inhabiting the open sea) or infaunal (burrowing into sediment).


Feeding

 

A great range of feeding habits can also be seen in modern gastropods. Herbivorous, carnivorous and parasitic species are common. In these groups the mouth is adapted to their particular feeding mode. Herbivorous feeders rasp at vegetable matter with the toothed radula. Carnivorous species often possess a long proboscis, which can be everted to capture prey. Parasitic species may use a long proboscis to suck fluids from their hosts. In deposit feeders the proboscis is used to collect detritus from sediment. Ciliary feeders draw sediment on through the inhalent siphon and food particles are consolidated in mucus.


Reproduction

 

In species with external fertilisation, the eggs are shed from the genitals, to the kidneys, ureter, mantle cavity and from the exhalent siphon into water for fertilisation.

 

In species with internal fertilisation a penile organ is present in males. Spermatozoa are transferred from the male penis to the female uterus during copulation and may be stored in the spermatheca before fertilising the ova in the albumen gland. Opisthobranchs and pulmonates are hermaphrodite. Each individual has both male and female genital organs. Most of these groups are protandrous, with a period of male sexual activity followed by a period of female sexual activity. Hermaphrodite gastropods may have separate male and female genital openings and function alternately as male or female, either during separate acts of copulation or during one episode of copulation. Alternatively, the genital openings may be combined and male and female copulation simultaneous.

 

   Illustration 1. Mating in the leopard slug, Limax maximus. Courtship involves mutual biting. This stimulates production of the mucous, which forms the suspension strand. The penile organs of both individuals are everted from the genital opening near the head and the individuals exchange spermatozoa.

Approximately one third of land snails produce mucous covered, calcareous darts, which an individual shoots into it's partner during copulation. The mucous may stimulate sperm storage in the shooter's partner.

 

A few gastropod species (such as Hydrobia jenkinsi) show parthenogenesis involving reproduction without fertilisation. This process produces only female offspring.


Ontogeny

 

In many marine species with external fertilisation, the egg hatches into a larva that is quite different from the adult. In the majority of marine gastropods, larvae go through two developmental stages during which they inhabit the plankton, before they acquire adult morphology.

 

 

 Diagram 1a and 1b. Trochophore (upper) and veliger (lower) larvae. 1a. The early trochophore larva of Patella vulgata. Actual diameter approximately 0.18mm. The larva swims among the plankton using cilia. 1b. The veliger larva of Philbertia gracilis. Actual diameter approximately 2.5mm. This stage develops from the trocophore larva within a few hours (this transition occurs within the egg, before hatching, in some species). Movement of cilia allows the larva to move through the water. The shell begins to develop during this stage. The larva usually settles on the seabed where torsion and metamorphosis to the adult form occur.

Based on, Knight et al., 1960, pp. I137 and I139.

 


Torsion

 

Torsion involves a fundamental change in the orientation of most of the organs so that the visceral mass is twisted through 180º. Torsion has a number of anatomical effects in the adult gastropod: the mantle cavity lies anteriorly and dorsally, the nerve cords between the cerebral and visceral ganglia are crossed, and the gut is twisted into a loop.

 

   Diagram 2. Dorsal view of a generalised gastropod before (upper) and after (lower) torsion. Based on an image from http://www-biol.paisley.ac.uk. During the first anatomical movement, anopedal flexure, the alimentary canal bends so that the anus discharges into the mantel cavity. During the second anatomical movement, torsion, the mantel and juvenile shell rotate anticlockwise (when viewed dorsally) through 180º in relation to the velum or head. The motion is achieved by contraction of a larval retractor muscle attached to the shell interior. In some species torsion occurs in the veliger larva stage. In other species it occurs within the egg.

 

 

In opisthobranchs and pulmonates, which exhibit internal fertilisation, torsion occurs within the egg, sometimes very early in development. Trocophore and veliger larval stages are not visible in these groups and an animal resembling a small adult hatches from the egg. In some groups (such as some opisthobranchs) detorsion has evolved, so that the mantel cavity has moved 90º from the torted position.


Colour and pattern

 

The shells of many gastropods exhibit striking colouration. Shell pigment is secreted by the mantel edge. Pigment may be secreted continuously to produce smooth lines, or rhythmically to produce isolated areas of colour. Pigment secretion may allow remove waste products from within the body. However it is also very likely to be subject to natural selection.

   Photograph 1. Conus marmoreus, Holocene. Actual length 49mm.


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Author: Jennifer Hoyal Cuthill
Last updated: 22.11.05
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