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Originally classified within the Phylum Annelida, together with the earth-worms (Class Oligochaeta) and the leeches (Class Hirudinea), recent studies have failed to show that the annelids are a monophyletic group. This means that currently, the term ‘polychaetes’ should be used, rather than ‘Annelida’. Polychaetes now include all the traditional polychaete families as well as the leeches, earthworms, and the siboglinids, sometimes known as ‘beard worms’.
What is a polychaete worm?
Polychaete worms are segmented invertebrates that have bodies consisting of:
- two presegmental regions: the prostomium and peristomium (the 'head' region)
- a segmented trunk (the 'body')
- a postsegmental pygidium (the 'tail' region)
The body wall consists of circular and longitudinal muscle layers enclosing a body coelom (fluid-filled space). Usually, poychaetes have a well defined head with sensory and/or feeding appendages, followed by numerous body segments that may be divided into thoracic and abdominal regions.
Typically, each segment has a pair of parapodia - 'feet' - with chaetae (bristles).
Polychaete body shapes are as diverse as their lifestyles, and are adapted to the habitats that they live in. For example, species that burrow through sediments (like the capitellids) tend to have few, if any, sensory or feeding appendages and reduced parapodia; in contrast, nereidids that crawl actively over the substrate (bottom) have well developed sensory appendages and parapodia.
Traditionally, polychaetes consist of about 72 families and these are grouped into a series of clades. Clade is the preferred term 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 lack of a good fossil record for polychaetes, because they are soft-bodied worms that don't often leave useful fossils, means that evolutionary relationships can be hard to determine. However it is clear that they are an old group, with some fossil polychaetes known from the Ediacaran period (around 580 – 545 million years ago). Since that time, many polychaetes would have evolved and become extinct, making today’s polychaetes the descendants of a very ancient lineage.
Many morphological (external) characters, while closely resembling each other, may not necessarily indicate that species are closely related (i.e. are homologous structures - look the same, but arise from different origins). Instead, these features 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. This makes the coding of characters for phylogenetic (relationships) analysis difficult. In addition, many species have lost certain characters, which also makes coding difficult.
Few polychaetes have common names, so it is more common, and more useful, to use scientific names when discussing families, genera and species.
Closer to arthropods or molluscs?
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, polychaetes were thought to be most closely related to the arthropods (insects, spiders etc), based upon the presence of segmentation in both groups. Recently, however, it has been shown that they may be closer to molluscs (snails, slugs, squid et.c.). Polychaetes remain one of the last large groups of invertebrates for which a widely accepted classification is still not available.
Where do polychaetes live?
Polychaetes occur throughout the world in all habitats from the supralittoral (above the shore line) 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 polychaete species have been described worldwide and many more remain to be described. They range from species a few millimetres or less in length with few segments to those many centimetres in length and hundreds of segments.Many benthic (seafloor) marine communities are dominated by polychaetes, not only in the overall number of species but also by the sheer number of individuals present. In estuarine environments with fluctuating salinity levels, the species diversity is lower, but individual worms may be abundant.
On coral reefs such as the Great Barrier Reef, polychaetes are diverse and abundant in inter-reef sediments and also as borers or nestlers in dead coral substrate. Some polychaete 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, molluscs or crab carapaces. The plankton around the reef contains pelagic (free-living) adults, 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.
Reef polychaetes are important at all levels of the ecosystem. Many of those living in the substratum are important because they play a major role in 'bioturbation', which is the breaking down of organic sedimentary matter as it passes through their bodies; others also shred plant material, making it more available to other living things. 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.
Polychaetes are also abundant in seagrass beds and mangrove areas, where large concentrations of organic matter accumulate from shed leaves. On intertidal reef flats, these soft-bodied worms are an important food source for wading birds at low tide, and for fish and crustaceans at high tide.
The distribution of polychaetes is largely dependent on the type of substrate present. For example:
- the size and type of sediment for burrowers
- the presence of suitable reef substrate for the borers and nestlers
- hard substrates for the encrusting species to settle on
- suitable algal substrate for species that live in seaweed
Additional 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 beach environments are typically low in the number of species and individuals.
Protection and predation
As all polychaetes are soft-bodied, they need protection from predators. This is achieved either by secreting a tube into which they can rapidly retract or by living in habitats where they can burrow and avoid predators.
A few species have developed additional anti-predator strategies. For example, the numerous spiny chaetae (bristles) of amphinomids may make them unpalatable to fish and other predators.
However, fish and bird gut contents reveal that polychaetes are an important prey item for many species and, even when buried in sediments, polychaetes can be preyed upon, providing an easily digestible and abundant food source.
How do polychaetes reproduce?
Polychaetes exhibit an amazing diversity of reproductive strategies, including both sexual and asexual reproduction. While most polychaetes are dioecious (males and females separate), some species are hermaphrodites (possess both male and female organs), and others may be males at particular times of their life and then become females later.
Polychaetes may live for a few weeks or months to many years. Some breed continuously over several months, while others are restricted to spawning on a single day.
Some species mate actively, with the male fertilising the eggs as they are laid and, in some cases, the fertilised eggs are then placed under the scales. In some tube-dwellers, the eggs are brooded within their tubes, or placed in the chamber below the operculum, while 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. The fertilised eggs develop into free swimming larvae, or trochophores, that spend from a few hours to many weeks in the plankton before settling and metamorphosing into adult worms. Some larvae feed while in the plankton while 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. Instead, crawling bottom-dwelling 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 (hormone-producing) systems to co-ordinate this.
How are polychaetes found and identified?
It is not easy to see many polychaetes while diving, but sometimes the tentacles of some species can be seen spread over the substrate, or the expanded branchial crowns of sabellids and serpulids. At night more tentacles can be seen spreading out over the substrate or outstretched, fishing, in the water column, but all can rapidly retract back into their burrows when disturbed. However, polychaete diversity is revealed when sediment samples are collected and sorted under the microscope, or when pieces of dead coral substrate are split open to reveal borers and nestlers. However, a permit is required to collect such samples.
Polychaetes as marine pest
Marine species are being transported accidently around the world via ballast water and as hull fouling organisms on ships. While some of the species introduced never become established in their new environment, some species prosper and become pests. A range of animals have become marine pests including several species of polychaetes and it is very difficult to eradicate such species once they have become established. Just think of how much time and money has been spent trying to eradicate rabbits or cane toads on the land, imagine how much more difficult it is to eliminate marine species.
Pat Hutchings has been involved in various port surveys which have been conducted around Australia to identify introduced species. Correct identification of the fauna is critical and in many areas the native fauna is poorly documented. It is not always easy to distinguish between introduced species and undescribed native species, and some of the work which Pat and Maria Capa are undertaking is trying to resolve some of these questions and often requires using both morphological and molecular characters.