Polychaetes

Polychaetes are multi-segmented worms. They have parapodia (small leg-like structures) for crawling, swimming and burrowing. Each segment has a pair of parapodia with chaetae (bristles).

Their bodies consist of circular layer of muscle around a coelom (fluid-filled space). Many have tentacles for feeding or feather-like branchial radioles for feeding and breathing.

Polychaete have diverse body shapes and lifestyles, and are adapted to the habitats that they live in. Some burrow into sediment (like the capitellids) and have reduced sensory or feeding appendages and feet. Those that crawl over the substrate (bottom) need well developed sensory appendages and feet.

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.

Some are free moving - active crawlers or swimmers are often predatory with well-developed eyes and jaws. Others are sessile worms that live in tubes of mucus, sediment, or calcium carbonate. They filter food using tentacles or gills.

It is not easy to see polychaetes. Divers may see the expanded feather-like branchial crowns of sabellids and serpulids. At night tentacles spread out over the seabed or extend out into the water to fish. They retract rapidly back into their burrows if disturbed.

Borers and nestlers are seen in sediment samples viewed under the microscope, or when pieces of dead coral substrate are split open. A permit is required to collect such samples.

Where do polychaetes live? We find them throughout the world and in all habitats - from above the shore line to the deepest parts of the ocean. They are mostly marine or estuarine but some species occur in moist terrestrial environments (on land). Most are free living although some need a host animal (commensal or parasitic).

Polychaetes dominate many benthic (seafloor) marine communities, with many diverse species and a high number of animals. In estuarine environments, where salinity (salt) levels fluctuate, there are fewer species but still abundant numbers.

Polychaete worms are common and varied on coral reefs such as the Great Barrier Reef. They live in the sand and mud between reefs, and many burrow into dead coral.

Some groups, such as serpulid worms, live inside hard calcium tubes. These tubes attach firmly to rocks, coral, algae, seagrass, shells and even crab shells.

The water around reefs also contains free-swimming adult worms and the young larvae of many species. Some live on floating debris, while others live with sea cucumbers or attach to buoys and ship hulls.

The distribution of polychaetes is largely dependent on the type of substrate present:

• burrowers need sediment of a certain size and type
• borers and nestlers - suitable reef substrate
• encrusting species - hard substrates to settle on
• species that live in seaweed - suitable algal substrate.

Exposure and water currents are important for filter-feeding organisms. Species living in sediments need to have stable sediments, so few are found in high energy beach environments.

Polychaetes is one of the last large groups of invertebrates without a widely accepted classification. Few polychaetes have common names, so scientific names are used to talk about families, genera and species.

Traditionally, polychaetes consist of about 72 families, which are grouped into 'Clades'. Clades are taxonomic ranks that are still being debated.

Some species have developed (external) characteristics over long periods of time that are similar to characteristics of other species found in comparable environments. These similarities do not mean the species are closely related. Others have lost certain characteristics. All of this makes the coding of characters for phylogenetic (relationships) analysis difficult.

Related to arthropods or molluscs?

Polychaetes were thought to be related to the arthropods (insects, spiders etc), as both groups have segmented bodies. More recently it has been shown that they may be closer to molluscs (snails, slugs, squid etc).

Polychaetes reproduce in many ways. Some reproduce sexually, others asexually. Some species are hermaphrodites, which means they have both male and female reproductive organs. Other species start as males and later change to become females.

Some polychaetes live for only a few weeks, others for many years. Some species breed over several months, but others spawn only once on a single day.

Species lay and care for their eggs in various ways. Some mate directly, with the male fertilising the eggs as the female lays them. Some tube-dwelling species keep and protect their eggs inside their tubes. Others place fertilised eggs into capsules and attach them to the sea floor to develop.

In some species, developing embryos eat other embryos inside the egg capsule. In other species, the eggs contain enough nutrients for all embryos to survive and hatch as tiny adult worms.

Some polychaetes release eggs and sperm into the water, where fertilisation happens outside the body. The fertilised eggs develop into free-swimming larvae. These larvae may stay in the plankton for a few hours or many weeks before settling onto the sea floor and changing into adult worms.

Reef polychaetes are important at all levels of the ecosystem. Burrowers aerate sediment and turnover organic matter. Many break down organic sedimentary matter as it passes through their bodies. Others also shred plant material, making it more available to other living things.

Fish, birds and crustaceans prey on polychaetes as an abundant food source.

Polychaetes are also abundant in seagrass beds and mangrove areas, where organic matter accumulates. 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.

As all polychaetes are soft-bodied, they need protection from predators. To avoid predators some burrow and others they secrete a tube into which they can retract rapidly.

A few species have developed anti-predator strategies. Amphinomids have spiny chaetae (bristles) that deter fish and other predators.

As they are soft-bodied, fossils of polychaetes are rare. This makes it hard to determine evolutionary relationships. Polychaetes are an old group, with some fossil polychaetes known from the Ediacaran period (around 580 to 545 million years ago). Since that time, many polychaetes have evolved and become extinct, making today’s polychaetes the descendants of a very ancient lineage.

Certain species of polychaetes thrive in disturbed, polluted environments. They are used to monitor organic pollution and environmental health.

Some species are transported around the world in ballast water and on the hulls of ships. Some of these introduced species prosper, become pests and can be difficult to eradicate.

Australian ports have been surveyed to identify introduced species. This can be difficult, particularly if native fauna is poorly documented or undescribed. AMRI scientists are trying to resolve some of these questions using both morphological and molecular characters.