Clitellates

The clitellates consist of both the oligochaetes and the hirudineans. While oligochaetes are often thought of as being terrestrial, many species occur in both freshwater and marine sediments and while few studies have been carried out they are known to be common in marine sediments on the GBR. Species tend to be small, <1 cm in length, and thread-like (Fig. 22.3K), and often bright red in colour due to the blood pigment haemoglobin. They play an important role in meiofaunal communities in the breakdown of particu-late matter and they themselves are predated upon by many animals.

BOX 22.1 MASS SPAWNING OF POLYCHAETE SPECIES

In the late 1980's when mass coral spawning was discovered, often polychaetes were also seen swimming in the water column and they too were spawning. Such mass spawning has been known and exploited by the Samoans for centuries. They could predict the night on which these sedentary eunicids, the 'Palolo' worms that live deep within the coral substrate, would swim up into the water column where their posterior ends would split open to release their gametes, triggered by the particular phase of the moon. The Samoans would collect these swimming sacs full of protein-rich eggs and sperm and dry them to produce a nutritious flour that they would make into biscuits or else trade with highlanders. On the GBR a suite of species spawn at particular phases of the moon during the summer months including species of nereidids, amphinomids, syllids and phyl-lodocids (Fig. 22.2L). In these species the entire worm develops swimming chaetae and large eyes, and leaves the substratrum and swarms in the water column where the males secrete a pherome that stimulates the females to spawn. By the end of the night the beaches are littered with the remains of their bodies. All the gametes are released into the water column, providing fish and other predators with an amazing feast. Almost certainly eunicids spawn on the GBR like in Samoa but to date this has not been documented. In all these cases, while water temperature and light levels are critical for co-ordinating spawning, other factors must be involved months before to ensure that the germinal epithelium of these worms proliferate the gametes and that there is sufficient time for the eggs and sperm to develop to maturity within the coelom. Again, laboratory studies have revealed that all these processes are controlled by an endocrine system that responds to external cues such as temperature and light.

BOX 22.2 ASEXUAL REPRODUCTION OF POLYCHAETES

Some species of polychaete can undertake asexual reproduction as well as sexual reproduction. Dodecaceria (family Cirratulidae) is common in dead coral substratum as it can bore into such habitats. An adult worm is capable of splitting into individual segments and each one develops a new head and tail to become a new individual. Another family that exhibits asexual reproduction is the syllids. In some cases the worm develops a series of stolons, resembling railway carriages, with the most posterior one the next one to be released as a new individual (Fig 22.3J); in other cases the stolons are proliferated off at angles to the adult worm.

Studies on the diversity of marine oligochaetes on the GBR are few and have tended to concentrate on soft sediment habitats close to research stations where facilities exist to carefully sort the sediment under a dissecting microscope, or elutriate the sediments under running seawater to allow the meiofauna to float off and be collected for study.

Oligochaetes are segmented worms without para-podia, but have segmental bundles of simple hook-like or capillary chaetae; often only a few are present, or they are sometimes absent. The prostomium is pointed, lacking any sensory appendages. They are hermaphrodites, and worms copulate to exchange gametes; fertilised gametes are laid in cocoons that are secreted by the clitellum. The cocoons are poorly developed, containing both the male and female gonopores and are deposited in the sediment. Miniature worms hatch out from the cocoons after variable periods of time. Some species undergo asexual reproduction by transverse division of the parent worm into two or more individuals. Marine oligochaetes feed on fine detritus, algae and other micro-organisms using their eversible pharynx. The diversity of the oligochaete fauna is a good indication of the 'health' of the sediment.

Marine leeches (Hirudinea) are present on the GBR. While some are free living (Fig. 22.4A, B) others rely upon an intermittent supply of blood from marine vertebrates, including turtles and fish. Some appear to be fairly host specific, whereas others are not. They use their anterior sucker to attach onto the host and inject an anticoagulant that allows the leech to suck out the blood without it clotting. A blood feeding leech will be two to three times as large after feeding than it was before. Leeches are typically dorsoventrally flattened and tapered at each end, with segments at both extremities modified to form suckers. Leeches are segmented, although external segmentation is obscured, and cha-etae are absent. A clitellum is present and always formed by segments 9, 10 and 11, although these are only conspicuous during reproductive periods. Branchiae are present in the Pisciolidae as lateral leaflike or branching outgrowths. In species without branchiae respiration occurs through the skin. Leeches are hermaphrodites and animals mate and exchange sperm that is then stored and used as required to fertilise the eggs, which are laid in cocoons from which miniature adults hatch out after some weeks. Relatively little is known about marine leeches but some act as vectors for protozoan parasites.

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