Polychaetes

Polychaetes are the most diverse and abundant of the worm groups and originally were referred to as a class within the Phylum Annelida, together with the earthworms (Class Oligochaeta) and the leeches (Class Hirudinea), and this classification will be found in many textbooks. Recent studies (both morphological and molecular) on the annelids have failed to show that they are a monophyletic group, and while the earthworms and leeches do form a single clade (the clitellates) their relationship to the polychaetes is still being debated. So, today the term 'polychaetes' is widely used rather than the term 'Annelida' and this group includes all the traditional polychaete families as well as the leeches, earthworms, and the siboglinids, sometimes known as 'beard worms'. This latter group until recently was considered either as one or two phyla and included the Vestiminifera, which are restricted to cold and hot vents, and the Pogonophora, or thin worms found in sediments. But recent studies have shown that these worms are closely related to each other and represent a family within the polychaetes. To date, no examples of siboglinids have been found in the GBR, but some may occur in deeper waters off the outer barrier reef. While it is widely accepted that the leeches, earthworms, siboglinids and probably the echiuroids and polychaetes are all closely related, their relationships are still being debated and numerous molecular and morphological studies have failed to completely resolve these issues.

Polychaetes consist of two presegmental regions, the prostomium and peristomium, a segmented trunk and postsegmental pygidium (Fig. 22.1A). The body wall consists of circular and longitudinal muscle layers enclosing a body coelom. Usually they have a well defined head with sensory and/or feeding appendages, followed by numerous body segments that may

PARAPODIUM

CHAETIGER

PARAPODIUM

PYGIDIUM ANUS

ANAL CIRRUS

F—NOTOPODIUM

NEUROPODIUM

PALP

ANTENNA

PROSTOMIUM

PERISTOMAL CIRRUS

PERISTOMIUM

SEGMENT (achaetigerous)

CHAETIGER

PARAPODIUM CHAETA

PYGIDIUM ANUS

ANAL CIRRUS

F—NOTOPODIUM

NEUROPODIUM

VENTRAL NERVE

Figure 22.1 A, Stylised diagram showing major morphological characters of a generalised polychaete. B, Cross-section of the body of a stylised polychaete, showing structure of the parapodia and location of the gut and ventral nerve cord. (After Fauchald, 1977.)

be differentiated into thoracic and abdominal regions. Typically each segment has a pair of parapodia with chaetae (bristles) (Fig. 22.1B). The diversity of lifestyles exhibited by polychaetes is often reflected in their morphology. For example, species that burrow through sediments (like the capitellids) tend to have few, if any, sensory or feeding appendages and reduced parapo-dia; in contrast nereidids that crawl actively over the substratum have well developed sensory appendages and parapodia (Fig. 22.2A).

Traditionally, polychaetes consist of about 72 families and these are grouped into a series of clades; such a term is preferred to assigning them an actual taxonomic rank (such as order) as the relationships among clades, and in some cases even their exact composition, are still being debated. The problem arises because polychaetes lack a good fossil record as they are typically soft-bodied worms, but they are an old group with some fossils known from the Ediacaran period (around 580-545 Mya) being identified as polychaetes. So, since that time, presumably many polychaetes have evolved and became extinct, so today's polychaetes are descendants of a very ancient lineage. Many morphological characters, while closely resembling each other, may not necessarily indicate that species are closely related (i.e. are homologous structures), rather they may have evolved over long periods of time in response to the similar environments in which the species live. So, while similar terms have been used across the families to describe particular traits, they may not represent homologous structures, making coding of characters for any phylo-genetic analysis difficult. In addition, many species have lost characters, which also makes coding difficult.

In the 'Additional reading' at the end of the chapter references are given that will allow people interested in this area of research to find out the latest developments. Not only are the relationships within the polychaetes still being actively debated, but also the relationships of these segmented worms to other invertebrate groups. For many years these segmented worms were regarded as being closely related to the arthropods, based upon the presence of segmentation in both groups, but recently it has been shown that they are more closely related to molluscs. Polychaetes remain one of the last large groups of invertebrates for which a widely accepted classification is still not available.

Planta Animales Marinos

Figure 22.2 Polychaetes: A, Family Nereididae, Perinereis helleri with its pharynx everted showing paragnaths, an important diagnostic character for this family. Size: up to 4-5 cm in length. (Photo: H. Nguyen.) B, Family Amphinomidae, Chloeia flava moving over the sediment. The chaetae easily break off and become embedded in fingers if the animal is picked up. Size: up to 100 mm in length. (Photo: R. Steene.) C, Family Amphinomidae, Eurythoe complanata, 'Fire worm'. These worms are commonly found under boulders intertidally and shallow subtidally. The dark red bushy branchiae are present adjacent to the parapodia all along body. Size: up to 60-80 mm in length. (Photo: K. Atkinson.) D, Family Amphinomidae, Pherecardia sp. crawling over the substrate in search of food. It uses its eversible muscular pharynx to feed on sponges, anemones, hydroids etc. (Photo: R. Steene.) E, Family Terebellidae, Reteterebella queenslandia deeply embedded within the coral substratum with highly extensile feeding tentacles spread out over the surface of the reef collecting food particles and moving them along the tentacles to the mouth. (Photo: P. Hutchings.) F, Family Serpulidae, Spirobranchus corniculatus-complex illustrating the diversity of colours on a single live colony of Porites. This species settles on a damaged polyp, secretes a calcareous tube and encourages the coral to grow around the worm. (Photo: R. Steene.) (Continued)

Figure 22.2 Polychaetes: A, Family Nereididae, Perinereis helleri with its pharynx everted showing paragnaths, an important diagnostic character for this family. Size: up to 4-5 cm in length. (Photo: H. Nguyen.) B, Family Amphinomidae, Chloeia flava moving over the sediment. The chaetae easily break off and become embedded in fingers if the animal is picked up. Size: up to 100 mm in length. (Photo: R. Steene.) C, Family Amphinomidae, Eurythoe complanata, 'Fire worm'. These worms are commonly found under boulders intertidally and shallow subtidally. The dark red bushy branchiae are present adjacent to the parapodia all along body. Size: up to 60-80 mm in length. (Photo: K. Atkinson.) D, Family Amphinomidae, Pherecardia sp. crawling over the substrate in search of food. It uses its eversible muscular pharynx to feed on sponges, anemones, hydroids etc. (Photo: R. Steene.) E, Family Terebellidae, Reteterebella queenslandia deeply embedded within the coral substratum with highly extensile feeding tentacles spread out over the surface of the reef collecting food particles and moving them along the tentacles to the mouth. (Photo: P. Hutchings.) F, Family Serpulidae, Spirobranchus corniculatus-complex illustrating the diversity of colours on a single live colony of Porites. This species settles on a damaged polyp, secretes a calcareous tube and encourages the coral to grow around the worm. (Photo: R. Steene.) (Continued)

Kelp WormsFan Worms Strain Water Through Their

Figure 22.2 (Continued) G, Sabellidae, a colourful fan worm. Each radiole has numerous fine filaments along its length that are used to strain the water passing through the crown. Particles pass down the axis of each radiole to the mouth, where they are eaten, used for tube construction or rejected. The crown is 50-100 mm in diameter. (Photo: R. Steene.) H, Family Sabellidae, Sabellastarte sp. This worm is fully extended from its muddy tube. The tentacular crown 50-100 mm in diameter is used in filter feeding and for respiration. (Photo: R. Steene.) I, Family Eunicidae, Marphysa sp. These are commonly known as blood worms and are common in seagrass and muddy habitats adjacent to reefs. The animal is up to 200 mm in length. It has branched branchiae adjacent to the parapodia. (Photo: K. Atkinson.) J, Family Eunicidae, Eunice aphroditois emerges at night from its burrow to feed, keeping its jaws open for passing prey such as crustaceans and drift algae that are then grabbed and drawn down into the buccal cavity. They are sensitive to vibration and can rapidly withdraw into their burrows. They are found on shallow coral reefs and may reach 1-2 m in length. (Photo: R. Steene.) K, Family Phyllodocidae. Phyllodocid found in coral sediments, with expanded dorsal cirri all along its body. Size: 4-5 cm in length. (Photo: K. Atkinson.) L, Family Phyllodocidae, Phyllodoce sp. Mass spawning of these worms coincides with coral spawning. These worms normally live under rubble and swim up into the water column to spawn at around 2100 hrs. (Photo: P. Hutchings.)

Figure 22.2 (Continued) G, Sabellidae, a colourful fan worm. Each radiole has numerous fine filaments along its length that are used to strain the water passing through the crown. Particles pass down the axis of each radiole to the mouth, where they are eaten, used for tube construction or rejected. The crown is 50-100 mm in diameter. (Photo: R. Steene.) H, Family Sabellidae, Sabellastarte sp. This worm is fully extended from its muddy tube. The tentacular crown 50-100 mm in diameter is used in filter feeding and for respiration. (Photo: R. Steene.) I, Family Eunicidae, Marphysa sp. These are commonly known as blood worms and are common in seagrass and muddy habitats adjacent to reefs. The animal is up to 200 mm in length. It has branched branchiae adjacent to the parapodia. (Photo: K. Atkinson.) J, Family Eunicidae, Eunice aphroditois emerges at night from its burrow to feed, keeping its jaws open for passing prey such as crustaceans and drift algae that are then grabbed and drawn down into the buccal cavity. They are sensitive to vibration and can rapidly withdraw into their burrows. They are found on shallow coral reefs and may reach 1-2 m in length. (Photo: R. Steene.) K, Family Phyllodocidae. Phyllodocid found in coral sediments, with expanded dorsal cirri all along its body. Size: 4-5 cm in length. (Photo: K. Atkinson.) L, Family Phyllodocidae, Phyllodoce sp. Mass spawning of these worms coincides with coral spawning. These worms normally live under rubble and swim up into the water column to spawn at around 2100 hrs. (Photo: P. Hutchings.)

Polychaetes occur throughout the world in all habitats from the supralittoral to the deepest parts of the ocean. They are predominantly marine or estuarine but a few species occur in moist terrestrial environments. Most are free living although some are commensal or parasitic. Currently over 13 000 species have been described worldwide and many more remain to be described. Many benthic marine communities are dominated by polychaetes not only in terms of the number of species but also by the number of individuals present. In estuarine environments with fluctuating salinity levels the diversity of polychaetes is typically low, although individuals may be abundant. They range from species a few millimetres or less in length with few segments to those many centimetres in length and hundreds of segments.

While polychaete families and many of the genera are known to occur worldwide or have very wide distributions, it is at the species level that one finds restricted distributions. While no comprehensive survey of the polychaetes of the GBR has been undertaken, they are diverse and abundant both in inter-reefal sediments and as borers or nestlers in dead coral substrate. Other families, such as the serpulids, always live in calcareous tubes that are firmly attached to hard substrates or to other organisms such as algae, seagrasses (Fig. 22.3A), molluscs or crab carapaces. Some adult pelagic polychaetes, as well as the larval stages of many species, are also found in the reefal plankton. In addition, species are attached to floating debris, as commensals on the undersurface of holothurians and as fouling organisms on buoys and hulls of ships (Fig. 22.3B, C). Few polychaetes have common names and scientific names must be used.

It has been suggested that there are over 500 species of polychaetes on the GBR. This is almost certainly a serious underestimate, although to date no comprehensive study has been undertaken. Reefal polychaetes are important at all levels of the ecosystem. Many of those living in the substratum are important in terms of bioturbation—breaking down the organic matter in the sediment as it passes through their bodies—and others shred plant material, making it more available to other organisms. They are abundant in seagrass beds and mangrove areas where large concentrations of organic matter accumulate from shed leaves. Such soft-bodied worms on intertidal reel flats are an important food source for wading birds at low tide and for fish and crustaceans as they move over such flats at high tide. Other polychaete species found on the reef are active carnivores, such as the large fire worms commonly found underneath boulders or rubble at the base of reefs. Fire worms belong to the Amphinomidae (Fig. 22.2B-D), and they should be handled with care as their long chaetae easily break off and lodge in your fingers. Apart from being an irritant there is a poison gland at the base of the chaetae and some people are quite allergic to this poison—hence their name. This family is well represented in tropical waters, and floating pieces of pumice from a distant volcanic eruption are often covered with them, together with goose barnacles. They range from a few millimetres to several centimetres in length, with robust, typically square-shaped bodies, and most are active carnivores although others are omnivores or opportunistic feeders.

Long white tentacles can often be seen spread out over the reef; these belong to a species commonly referred to as the 'spaghetti worm' (Reteterebella queens-landia), a member of the Terebellidae. When touched, the tentacles rapidly contract back into their tubes (Fig. 22.2E). These tubes are soft and flimsy, made of fine sediment cemented together with mucus, and are found either under boulders on the reef flat, or in deeper water, the tubes are deeply embedded in boulders well below the surface of the reef. It seems likely that these feeding tentacles contain noxious compounds as they are avoided by fish. This and other terebellid species are all surface deposit feeders. They spread their tentacles ('U' shaped in cross-section) out over the surface of the substratum. Longitudinal rows of cilia along the tentacles create currents that move fine sediments into the centre of the 'U', where it is trapped in mucus and formed into small bundles. These bundles are moved along the length of the tentacle to the mouth that is surrounded by a series of lips that sort these particles, the finest being swallowed, medium sized particles used for tube construction and maintenance, and the large particles are thrown out. The animal is not digesting the actual sediment but rather the algae and bacteria and organic matter that are attached to the surface of the particles. Another common species of tere-bellid is Eupolymnia korangia (Fig. 22.3D) that lives at the base of small bommies. The striped buccal tentacles are often seen spreading out over the sand and can rapidly retract back into the sandy tube in which this animal lives wedged between the coral.

Conspicuous 'Christmas tree' (Spirobranchus corniculatus-complex) worms occurring in a diverse range of colours (Fig. 22.2F) are common in live coral, especially the massive corals such as species of Porites. The tree-shaped branchial crown, when held upright in the water column, is able to effectively strain the water and small particles of food are trapped in mucus along the ciliated filaments; these particles are transported along the axis of the filaments down to the mouth at the base of the crown where they are eaten. The branchial crown has well developed eyes and they rapidly withdraw their crowns in response to shadows and vibrations, closing the calcareous tube with a modified branchial filament that forms a plug (or operculum). This reduces the opportunity for fish to eat the crown, but if it is eaten (and examination of fish guts suggests that this does occur) it can rapidly be regenerated. While these worms, which belong to the family Serpulidae, on first inspection appear to have bored into the live coral, they have not; rather they have stimulated the coral to grow around them. This happens when their pelagic larvae, which have spent several weeks in the plankton, settle on a damaged polyp on the living coral (to avoid being eaten), and rapidly secrete a thin calcareous tube. This stimulates the coral to grow around the tube and gradually the worm tube becomes firmly embedded in the coral. While commonly referred to as S. giganteus on the GBR this species is actually restricted to the Caribbean. The Australian species belong to the Spirobranchus corniculatus-complex and some undescribed species occur. The genus has been widely reported from all coral reef areas of the world. However, the colour of the branchial crown, which can vary from yellow, blue, red and purple, is not a useful character to distinguish species, and the structure of the opening tube as well as chaetal characteristics are important. Obviously, once the worm has become embedded in the coral it is entombed for life. This species-complex is restricted to living coral colonies, so presumably if the coral dies then the worms also die. Why this occurs is unknown, but a possible explanation is that the local water currents created by the coral when it feeds also are beneficial to the filter-feeding polychaete by continually renewing the water mass above them and its associated food particles. Many other species of serpulids are common on the reef and most have an operculum, the structure and ornamentation of which is critical for species and generic identification. Common on algal fronds and on blades of seagrasses are small calcareous tubes that are circular and comma-shaped; these all belong to the spirorbids (Fig. 22.3A), a group that is now regarded as belonging to the family Serpulidae. These small worms also have an operculum. In some species a brood chamber develops on the undersurface where the fertilised eggs develop, hatching as miniature adults. They may breed continuously over several months. Serpulids are commonly transported across oceans by drift algae and as fouling organisms on the hulls of ships (Fig. 22.3B, C). Some species are gregarious and larvae are attracted to settle close to adults of the same species.

Another group of filter-feeding polychaetes are the sabellids, or fan worms, which also have a branchial crown, but are always found in muddy/sandy tubes and lack an operculum (Fig. 22.2G, H). Again, a tremendous diversity is found in the sabellids, which range from fab-riciniids a few millimetres in length living among algal turfs to large conspicuous fan worms with branchial crowns 2-3 cm in diameter. All these fan worms can retract rapidly back into their tubes in response to a passing shadow or vibration, but sometimes they are not quick enough and are eaten by predatory fish. Providing the worm can rapidly contract the body wall to prevent loss of coelomic body fluids, they can regenerate the crown. The larger sabellids, belonging to the sabellines, are found in sediments with the tubes anchored onto a small piece of hard substrate such as shell fragment; others actively bore into coral substrate and line their burrows with fine chitinous tubes.

Species of eunicids may be found underneath large boulders. They may reach several centimetres in length, have numerous segments, often an iridescent epithelium and, along the body on restricted segments, are bright red tufted branchiae (Fig. 22.2I). One species of eunciid, Eunice aphroditois, is found in the low intertidal zone deeply embedded in the reef crest, and at early morning low tides extends out of the reef crest and grazes on algae (Fig. 22.2J). All eunicids have a well developed set of jaws and are regarded as scavengers, feeding on both animal and plant matter. Some species bore into dead coral substrate using both mechanical and acid secretion to excavate their burrows deep within the coral, maintaining an opening to the exterior through which they obtain their food and oxygen from the water column, as well as discharging their gametes.

Another family, the nereidids or ragworms, are common on the reef, occurring in many habitats: in the

Figure 22.3 Polychaetes: A, Serpulidae, Eulaeospira sp., firmly attached to a blade of seagrass. These small animals filter feed and they are able to seal off their tubes as one of the branchial filaments is modified to form a plug or an operculum. In some species a brood chamber is developed underneath the operculum. (Photo: T. Macdonald.) B, Serpulidae, Hydroides sanctaecrucis. Close-up of calcareous tubes on a fouled yacht. (Photo: J. Lewis.) C, Family Serpulidae, Hydroides sanc-taecrucis. This is an introduced species that has colonised a yacht in a marina in Cairns. (Photo: J. Lewis.) D, Family Terebellidae, Eupolymnia koorangia. This is a surface deposit feeder living under rocks or at the base of bommies. Size: 30-50 mm in length, excluding tentacles. (Photo: R. Steene.) E, Family Polynoidae. A scale worm, probably a species of Iphione, crawling over coarse sediment. Scale worms are active carnivores, catching prey using their eversible pharynx. Encrusting barnacles are present on some elytra in this specimen. (Photo: R. Steene.) F, Hesionidae, Hesione sp. or Leo-crates sp., crawling over the substrate. Hesionids are active carnivores that feed by everting the pharynx and sucking up their prey. (Photo: R. Steene.) (Continued)

Figure 22.3 Polychaetes: A, Serpulidae, Eulaeospira sp., firmly attached to a blade of seagrass. These small animals filter feed and they are able to seal off their tubes as one of the branchial filaments is modified to form a plug or an operculum. In some species a brood chamber is developed underneath the operculum. (Photo: T. Macdonald.) B, Serpulidae, Hydroides sanctaecrucis. Close-up of calcareous tubes on a fouled yacht. (Photo: J. Lewis.) C, Family Serpulidae, Hydroides sanc-taecrucis. This is an introduced species that has colonised a yacht in a marina in Cairns. (Photo: J. Lewis.) D, Family Terebellidae, Eupolymnia koorangia. This is a surface deposit feeder living under rocks or at the base of bommies. Size: 30-50 mm in length, excluding tentacles. (Photo: R. Steene.) E, Family Polynoidae. A scale worm, probably a species of Iphione, crawling over coarse sediment. Scale worms are active carnivores, catching prey using their eversible pharynx. Encrusting barnacles are present on some elytra in this specimen. (Photo: R. Steene.) F, Hesionidae, Hesione sp. or Leo-crates sp., crawling over the substrate. Hesionids are active carnivores that feed by everting the pharynx and sucking up their prey. (Photo: R. Steene.) (Continued)

sediments, associated with filamentous and encrusting algae, as nestlers in dead coral substrate, and in the infralittoral zone underneath decaying vegetation. Members of this family have an eversible pharynx with well developed jaws that they use to collect prey and scavenge opportunistically (Fig. 22.2A). A closely related family, the Phyllodocidae, is also common and they can be abundant on reef flats at particular times of the year when the mature worms congregate on the surface to spawn, producing egg masses that are attached to pieces of algae or substrate (Fig. 22.2K). Other phyllodocids spawn in the water column (Fig. 22.2L).

Scaleworms, belonging to the polynoids, are common underneath coral substrate and are characterised by a series of overlapping scales (or elytra) that cover the dorsum and which may be brightly coloured, with distinct pigment patterns, and/or heavily ornamented

(Fig. 22.3E). This group is highly speciose on the reef and one group lives commensally on the underside of some species of holothurians.

The hesionids (Hesionidae), like the scaleworms, are another group of active carnivores (Fig. 22.3F) found on the reef.

Common in the sediments both at the base of the reef and in lagoonal sediments are capitellids, which superficially resemble earthworms with poorly developed parapodia and a blunt head with no obvious sensory structures. They have an eversible pharynx that is used to swallow the sediment, obtaining nutrients from the algae and bacteria on the surface of the particles. These thread-like worms are typically bright red in colour due to the haemoglobin that is dissolved in their blood that allows them to absorb oxygen from the pore water in between the sediment particles.

Figure 22.3 (Continued) G, Questidae, Questa ersei. Line drawing of anterior end of the animal, showing its simple body lacking parapodia, paired bundles of chaetae on all segments and the head reduced to a simple palpode with an eversible pharynx. Found living within sediment, it can reach lengths of up to 10 mm. (Figure reproduced with permission from Polychaetes and Allies (2000): A. Murray.) H, Family Chaetopteridae, Mesochaetopterus sp. One is removed from its tube and the other is still in its flimsy tube made of sand grains. These worms are common in reefal sediments. The anterior palps are used for feeding. The enlarged parapodial lobes in the mid body create water currents that pass through the tube and particles of food are collected. Size: up to 10-20 mm in length. (Photo: K. Atkinson.) I, Phoronid. These animals are always associated with burrowing sea anemones. (Photo: K. Attwood.) J, Line drawings of syllids undergoing asexual reproduction. (Source: after M. Durchon 1959). K, Line drawings of a marine oligochaete. (Artwork: K. Attwood.) L, Line drawing of digenetic trematode. (Artwork: K. Attwood.)

Figure 22.3 (Continued) G, Questidae, Questa ersei. Line drawing of anterior end of the animal, showing its simple body lacking parapodia, paired bundles of chaetae on all segments and the head reduced to a simple palpode with an eversible pharynx. Found living within sediment, it can reach lengths of up to 10 mm. (Figure reproduced with permission from Polychaetes and Allies (2000): A. Murray.) H, Family Chaetopteridae, Mesochaetopterus sp. One is removed from its tube and the other is still in its flimsy tube made of sand grains. These worms are common in reefal sediments. The anterior palps are used for feeding. The enlarged parapodial lobes in the mid body create water currents that pass through the tube and particles of food are collected. Size: up to 10-20 mm in length. (Photo: K. Atkinson.) I, Phoronid. These animals are always associated with burrowing sea anemones. (Photo: K. Attwood.) J, Line drawings of syllids undergoing asexual reproduction. (Source: after M. Durchon 1959). K, Line drawings of a marine oligochaete. (Artwork: K. Attwood.) L, Line drawing of digenetic trematode. (Artwork: K. Attwood.)

Sometimes occurring in the sediment, conical tubes made of cemented sand grains resembling 'ice cream cones' are found, inside which live pectinariids. Their golden spines, which are used to dig the sediment head down to collect food particles, cover the opening of the tube.

Representatives of one family, the Questidae, which are common in reefal sediments, have often been referred to as 'oligochaetoid polychaetes'. While they show some resemblance to marine oligochaetes, especially the Tubificidae, they share more characters with the polychaetes (Fig. 22.3G). One species, Questa ersei, has been recorded from Wistari Reef at Heron Island and also from the Houtman Abrohlos in Western Australia. These elongate and slender worms up to about 10 mm in length with up to 60 chaetigers are usually reddish-orange in colour. They have a blunt head without appendages and feed by swallowing sand grains using their ventral eversible pharynx and digesting the bacterial/diatom film on the surface of sand grains. This species is probably common throughout reefal sediments. Another family common in reefal sediments are members of the family Chaetopteridae, belonging to the genus Mesochaetopterus (Fig. 22.3H). These animals build sandy tubes that protrude from the surface of the sediment and water is pumped through these tubes, trapping particles that are used by the animal for either tube construction or are eaten.

The distribution of polychaetes is largely dependent on the type of substratum present; for example, the size and type of sediment, presence of suitable reefal substrate for the borers and nestlers, hard substrates for the encrusting species to settle on, and suitable algal substrates. Factors such as exposure and water currents are important for filter-feeding organisms. Species living in sediments need to have stable sediments so high energy beaches are typically low in the number of species and individuals. As all polychaetes are soft-bodied they need protection from predatory organisms, either by secreting a tube into which they can rapidly retract or habitats in which they can burrow and avoid predation. A few species have developed anti-predator strategies, for example, the numerous spiny chaetae of amphinomids (Fig. 22.2B-D) may make them unpalatable to fish and other predators. However, examination of fish and bird gut contents reveals that polychaetes are an important prey item for many species and obviously even when buried in sediments they can be preyed upon. Lacking an external skeleton or shell they provide an easy to digest source of food.

Polychaetes exhibit an amazing diversity of reproductive strategies, including both sexual and asexual reproduction. While most polychaetes are dioecious, that is, with separate males and females, some species are hermaphrodites, and others may be males at particular times of their life and then become females subsequently. Species may live for a few weeks or months to many years, some breed continuously over several months, whereas others are restricted to spawning on a single day. Some species actively mate, with the male fertilising the eggs as they are laid. In some the fertilised eggs are then placed under the scales. The eggs of some other species are brooded within their tubes, or placed in the chamber below the operculum, others lay their fertilised eggs in brood capsules that are then attached to the substrate where development occurs. In some cases, the developing embryos are cannibals, eating some of the other embryos, but in other cases the large yolky eggs have sufficient nutrients for them all to hatch as miniature adults. Other species are broadcast spawners with gametes being released into the water column where fertilisation occurs. In these cases, free swimming larvae, or trochophores, are produced that spend anything from a few hours to many weeks in the plankton before settling and metamorphosing into adult worms. Some larvae feed while in the plankton whereas other species, which produce large yolky eggs, do not. Most of the terebellids, for example, produce large yolky eggs and the pelagic larval stage is almost non-existent. Crawling benthonic larvae are produced that settle near where they hatch. In all cases it is important that males and females are ripe at the same time, and that spawning is synchronised—polychaetes have well developed endocrine systems to co-ordinate this. (Box 22.1 and Box 22.2.)

In summary, while not too many polychaetes will be observed when diving on the reef, on closer inspection tentacles can be seen spread over the substrate, as well as the expanded branchial crowns of sabellids and ser-pulids. At night more tentacles can be seen spreading out over the substrate or being held up fishing in the water column, but all can rapidly retract back into their burrows. Only when sediment samples are collected and sorted under the microscope, or if pieces of dead coral substrate are split open to reveal borers and nestlers, will the diversity of polychaetes be revealed. However, you must have a permit to collect such samples (see Chapter 12). Even though most of the diversity of this group is not readily visible, its members play an important part in the functioning of coral reef ecosystems, with roles including bioturbating sediments, breaking down organic matter, and settling and boring into coral substrate (see Chapter 8). They are also important prey items for a wide variety of organisms.

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