Introduction

Phylum Annelida
Class Polychaeta ('Poly' = many; 'chaeta' = hairs)

        Polychaetes (bristleworms) are marine annelids with parapodia bearing numerous setae in distinct fascicles.   They are dioecious and multi-segmented.    They have simple exit ducts from the gonads.   Most of them live in soft or rocky environment on sea floor from intertidal to depth.   They may be, but rarely, found in freshwater and terrestrial (humid), or parasitic.

        The sex of most polychaetes are separate.   The gametes produced in a few specialized segments are shed into the coelom and leave the body through the nephridia.   After the external fertilization, a ciliated free-swimming trochophore larval is developed.

        Other common features of polychaetes include: bisymmetry, metamerism, segmentation, schizocel, closed vascular system, chain nervous system, spiral and determinant cleavage.

        Traditionally, polychaetes are separated into two large groups: Errant Polychaetes and Sedentary Polychaetes.

Errant Polychaete

Sedentary Polychaete

The Differences between Errant Polychaetes and Sedentary Polychaetes

Hong Kong Polychaetes

        The polychaete fauna of Hong Kong belongs to the tropical Indo-Malayan subregion of the Indo-west-Pacific province (Shin 1982).   Since the 1970's, about 180 species have been reported.   They belong to 146 genera and 47 families.   Of these, 70 genera were not determined to species level due to the poor understanding of this fauna locally.

        Polychaetes are abundant in mid and outer Tai Tam Bay and Hoi Ha Wan (Shin 1998).    They can also be found in Tai Long Wan, Mirs Bay, Victoria Habour, Tolo Harbour, Tolo Channel, Cape d'Aguilar Marine Reserve, western waters and the vicinity of Waglan Island.   The polychaete diversity in Victoria Habour and Tolo Harbour is low because of the pollution from sewage and development activities and the dominance of a few environmentally tolerant species such as Minuspio cirrifera (Spionidae) and Capitella capitata (Capitellidae).   The diversity is also low in the western waters due to freshwater discharge from the Pearl River.

General Morphology

        The body of polychaetes can be divided into three parts: Head, Trunk, and Tail.

Diagram showing the major morphology features of a generalized polychaete (adapted from Fauchald 1977). Top: Entire worm.  Bottom: Cross section of setiger.

1.  Head
a.  Prostomium
     It is a pre-segmental part of the body anterior to the mouth that includes antennae and palps.   Antennae are sensory.    Palps may be sensory or used as feeding appendages.   Some species have one or two pairs of eyes on prostomium.

b.  Peristomium
     It is the first distinct post-prostomal region around the mouth that includes tentacular cirrus and proboscis.

c.  Pharynx
     It is the anterior part of digestive tract for feeding and sometimes for burrowing.   Most polychaetes have eversible pharynx.

2.  Trunk
     Each segment generally has its own local nerve center called ganglion and a pair of nephridia for excretion.

a.  Parapodia
     The parapodia are the flatlike projections on both sides of each segment for locomotion and gas exchange.   It can be biramus with both notopodia (upper division)and neuropodia (lower division) or uniramus with only neuropodia.

b.  Setae
     It is the chitinous bristle bearing on the parapodia and used for locomotion, feeding and building tubes.   There are many kinds of setae: simple, compound, capillary, limbate, bifurcate, trifurcate, pinnate, harpoon, pectinate and spatulate, etc.

3.  Tail
     The posterior section of the body is the truncated or tapered pygidium which contains a dorsal or terminal anus.    Cirri may be present.

The Importance of Polychaetes

1.   The dominant benthic fauna in marine environment
          Polychaetes are the most abundant and diverse group in all marine sediments from intertidal to deep-sea.   Around 10 000 species have been described worldwide (Yang and Sun 1986).   They show great variations in morphology, feeding and reproductive modes that make them adapt to different marine condition especially in the sand and mud.    They are vital to the structure, production, dynamics, and health of the marine benthos and environment.   They aid the deposition, breakdown, incorporation and turnover of the organic matters in the seabed that help to recycle nutrients to the overlying water column.

2.   As food resource
        Most polychaetes are small and short-lived with a high secondary production.   They are an important link in marine food webs.   Due to the high calorific value and rich protein content, both the adult and larvae of polychaetes are the main food supply of many economically important fishes (Yang and Sun 1986).   They can be harvested artificially or directly.   In China, a large amount of Nereidae are exported to Japan as bait for recreational fishing.    They are also a delicacy in South China and Southeast Asia.

3.   As an indicator of toxic materials and pollution
        Polychaetes are very useful organisms that can be used as indicator for monitoring the marine environment.    They are sensitive to changes in environmental conditions.   For example, the dominance of tolerant species, such as Capitella capitata (Capitellidae) in Victoria Habour and Minuspio cirrifera (Spionidae) in Tolo Habour, Tolo Channel and Mirs Bay, are indicators of organic pollution (Shin 1990).    Other species such as Diopatra chiliensis (Onuphidae), Marphysa sanguinea (Eunicidae) and Perinereis aibuhitensis (Nereididae) can be used as indicators of seawater temperature (Yang and Sun 1986).

4.   Production of pesticide
        Nereistoxin extracted from Lumbrineris heteropodais (Lumbrineridae) is used to produce a pecticide (Yang and Sun 1986).   It is useful to kill pests but safe to humans and domestic animals because it can be decomposed and excreted.

5.   As harmful organisms       
        Some polychaetes are fouling organisms that damage to ships' hulls, piers, water pipes and any artificial surfaces submerged in seawater, causing increased maintenance costs and the risk of mechanical failure.   These polychaetes include Serpulidae, Syllidae, Spirorbidae, Tellbellidae, Sabellidae and Nereididae.   In China, the yield of economically important seashells is reduced by Serpulidae which usually attaches to rocks, coral, ships' hull, piers and seashells.
        The Nereididae, Tylorrhychus heterochaetus and Perinereis nuntia, are pests that damage crops (Yang and Sun 1986).   They can live in low salinity and freshwater.   They feed on crops in the fields near the coast of South China.