This group of unsegmented worms includes the free living turbellarians and three classes of parasites. The endoparasitic cestodes (tapeworms, which are secondarily segmented) are common in many of the marine vertebrates, especially elasmobranchs; the trematodes (flukes) (Fig. 22.3L) occur in all vertebrate classes, and monogeneans are ectoparasites found on the gills of fish. Recent studies have revealed that all the para-
Figures 22.4 A, Hirudinae, Trachelobdella sp. (photo: R. Steene); B, Hirudinae, Stibarobdella macrothela that feeds exclusively on elasmobranchs (photo: R. Steene); C, Myzostome, Hypomyzostoma dodecephalcis on its host Zygometra elegans (a crinoid) from Lizard I. (photo: G. Rouse); D, Myzostoma sp. (may be new) on its host Amphimetra tessellata (a crinoid) from Lizard I. (photo: G. Rouse); E, marine flatworm gliding over the coral (photo: R. Steene); F, flatworm Pseudobiceros hancocki appears to be almost swimming over the reef (photo: R. Steene); G, Sipunculida, an unidentified species of Aspidosiphon (family Aspidosiphonidae), removed from its burrow deep within the coral substrate (photo: K. Atkinson); H, Sipunculida entombed in the coral (photo: P. Hutchings); I, Echuira extracted from its burrow, probably a species of Bonellia (photo: Australian Museum collection); J, Echuira, bifid tentacle of Bonellia spread out over the sediment at the base of a coral bommie at night (photo: R. Steene); K, line drawings of nematode (artwork: K. Attwood); L, nemertean, Baseodiscus hemprichii (photo: R. Steene).
sitic groups (cestodes and the trematodes) are really a subgroup, the Neodermata, of one of the subgroups, Rhabdocoela, of the old Class Turbellaria. So 'Turbel-laria' as a classificatory group is no longer recognised, but for convenience, and in many publications the term is still widely used to distinguish between free living flatworms and their parasitic relatives.
Cestodes are at their most abundant in sharks and rays; on the GBR only rare individuals of these groups are without tapeworms. All tapeworms have complex multihost life cycles. All commence with an egg in the faeces of the final host. First intermediates are usually small crustaceans (although this has never been demonstrated on the GBR). Second intermediate hosts are a range of invertebrates, especially teleost fishes, that are often heavily infected with tapeworm metacestodes.
It has been estimated that there may be 20 000 to 25 000 species of parasitic trematodes worldwide and it has been suggested that the fish of the GBR may sustain 2270 digenetic trematodes, with about 10% of these known to science. Digenetic trematodes parasitise the gut or its outgrowth and they are common primarily in teleost fishes, as well as large marine reptiles and mammals. Green turtles can host 12 digenetic species and 16 species have been recorded from the dugong. The life cycle of digenetic trematodes is complex, involving two or three, and sometimes more, hosts and the first intermediate host is almost always a mollusc. To date only one complete life cycle of a trematode on the GBR is known: that of Paucivitellosus fragilis; the final hosts are blennies and mullet occurring on the Heron Island reef.
Many years of fish sampling around Heron Island suggests that the 1000 fish species may be infected with as many as 2000 monogenean species that are usually external parasites in the gill chambers or on the skin. Unlike digeneans their life cycle only involves a single host (Fig. 22.3L). It has been suggested that under normal circumstances the host lives in some sort of harmony with its parasites, however, under conditions of stress (as occurs in aquaculture), declining water quality with increased levels of fertiliser or pesticides can disrupt this balance with the health of the host species declining.
Free living flatworms are divided up into a number of groups, but they are all bilaterally symmetrical and unsegmented, with a body in which the organs are embedded in a solid cellular matrix of parenchymatous tissue rather than lying in a body cavity, the coelom, as occurs in the annelids. The gut is sac-like, unless it has been lost, and their nervous system has an anterior 'brain' and lateral nerve cords. The body is made up of three layers: the ectoderm, from which the epidermis and nervous tissue develops; endoderm, from which the gut arises, and mesoderm, from which the muscles and other organs arise. These soft-bodied worms regulate their body fluids by a complex series of channels in which specialised cells, the protonephridia, beat and propel fluids to the exterior. Flatworms, especially those living on coral reefs, are often brightly coloured (Fig. 22.4E, F) and are sometimes confused with nudi-branch molluscs. They can be easily separated, however, as nudibranchs have a muscular foot, anterior rhinophores and posterior frilly gills, all of which are absent in flatworms.
Flatworms have anterior pseudotentacles and anterior pigment spots that are light sensitive, and a mouth that is not terminal and which may be quite small, leading to a muscular pharynx that enables these worms to suck up their prey as they glide over the substrate. Some animals glide on a sheet of mucus by the beating of the ciliary epidermis. Other species swim using well developed dorso-ventral muscles that allow the worm to send waves along the body to swim up into the water column. All flatworms can rapidly regenerate lost body tissues.
Flatworms occur on a variety of reefal habitats, and some live in close association with other invertebrates, especially species of echinoderms and soft corals. They are carnivores and may either injest their prey whole or just remove bits at a time. They evert their pharynx and secrete enzymes that begin to digest the prey tissue and the partially digested prey is sucked up into the gut. Target organisms include individual zooids of corals, bivalve molluscs, and colonial ascidians. As they lack an anus, any undigested particles must be ejected via the mouth. Nutrients diffuse into the body from the gut as there is no circulatory system.
Many of the species of flatworms on the reef are highly conspicuous and, as already mentioned, some can swim up into the water column by undulating their body margins, and yet they are almost universally ignored by predatory fish as their body tissue often contain toxins. The bright colours may also warn potential predators that they are poisonous to eat. Others mimic other poisonous animals so that while they are not themselves poisonous the animal that they are mimicking is.
While flatworms are hermaphrodites they appear not to self fertilise. When two mature individuals meet, one slowly glides over the other and raises the front half of the body. A penis armed with a hypodermiclike stylet is pushed into the body wall of the other worm and the sperm are injected into the animal, they then move through the body to find an egg in the oviduct where fertilisation occurs. In some other species the worms copulate tail to tail and the sperm are delivered into the female part of the reproductive system, being released into the oviduct as required. Fertilised eggs are placed in a capsule that is attached to the substratum and after a while miniature adults are produced, which just creep away from the capsule that then disintegrates.
Flatworms are abundant and diverse on the GBR, and range in size from very small (less than 1 mm) to several centimetres in length. Colour patterns are useful in species identification, although these are often lost when the material is preserved. Identification from good quality coloured photos is often possible, although many species still await formal description.
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