Introduction

Introduction

Polychaetes (Gr. polys, many, + chaite, long hair) are annelids. They are the largest class of annelids. There are about 10,000 described species, most of them being marine. Most polychaetes are about 5-10 cm long, However, there are many that are smaller than a millimetre, and that some can be as long as 3 metres. Some of them have very bright colour, such as green or red. Others remain mostly dull in colour.

Habitats of Polychaetes

Polychaetes are mostly found under rocks, in coral crevices or in abandoned shells. They can also burrow into mud or sand. Some of them also build their own tubes in bottom material. Polychaetes are abundant in some areas. For example, there can be thousands of polychaetes in a square metre of mud flat.

The Structure of Polychaetes

Polychaetes have well-differentiated head and specialized sensory organs and appendages on most segments called parapodia. There is no clitellum present in polychaetes. Also, they have many forms of setae which are arranged in bundles on the parapodia.

Fig 1. The general morphology of polychaetes

(Adapted from Fauchald 1977)

Prostomium

The prostomium is usually distinct and may have appendages. In some families, the prostomium may fuse with the peristomium and the first segment. However, it is difficult to determine the degree of fusion, so that the taxonomic value of this character is low.

Prostomial appendages include antennae and palps. Antennae are innervated through single roots directly from the brain. Palps always have double roots either from the brain or from the circumesophageal ring (Akesson 1963; Orrhage space 1966). Antennae are sensory organ. Palps may be sensory in function or act as appendages which are used for feeding. The position of palps also varies. They can be ventral to dorsal position or from frontal to occipital. The position or function of the palps is an important taxonomic character.

Fig 2. The general morphology of prostomium

Amphinomidae

Goniadidae

Lumbrineridae

Peristomium

The structure of peristomium consists of larvally derived structure and one or more true segments. The peristomium may carry a single pair of dorsal cirri called peristomial cirri. The fused segment may carry parapodial remnants called tentacular cirri. In Hesionidae, the number of tentacular cirri varies from one to four pairs. Other families mostly have a constant number of tentacular cirri.

Eversible pharynx

Eversible pharynx is part of the anterior digestive tract which everses from polychaetes. The morphological features of the pharynx vary in different families. There are jaws, teeth or other chitinized structures associated with the anterior end of the pharynx. These features are useful characters in classification of polychaetes.

In errantia families, the eversible buccal cavity forms a proboscis which may be covered with papillae or made with hard chitinous elements such as paragnaths and jaws. The proboscis is used for feeding and burrowing.

In filter-feeding tubicolous polychaetes the buccal cavity is not eversible and there is no proboscis.

Parapodia

The parapodia of polychaetes can be biramous or uniramous. Biramous parapodia refer to the division of the foot into notopodia and neuropodia. In uniramous parapodia, polychaetes would only have neuropodia developed. The notopodia are considered secondarily reduced (Fauchald 1974a). This is the main morphological character to classify the families of polychaetes. The detailed features of parapodia such as parapodial lobes or cirri are key characters to classify polychaetes at the generic and species level. In addition, the presence or absence of branchiae associated with the parapodia is another feature on the parapodia.

Fig 3. The general morphology of parapodium

Biramous parapodium

( Nephtyidae)

Uniramous parapodium

( Lumbrineridae)

Setae

There are numerous kinds of setae that can be found in polychaetes, such as uncini, limbate setae and acicular hooks. This is usually a family character for taxonomic identification. For example, all members of Phyllodocidae have composite setae.

Acicular compound setae Compound setae

Capillary setae Bipennate neurosetae

Denticulate setae Forked setae

Acicular setae Hook setae

Polychaetes are divided into two orders, ERRANTIA and SEDENTARIA. This classification is based on the morphological features of the anterior end of polychaetes and the living habit of the species.

ERRANTIA polychaetes have:

	Large number of body-segments.
	Fewer number of anterior appendages which are differentiated into palps, antennae or tentacular cirri, etc.
	Mostly free-living with rapacious habit.
	All polychaetes with jaws are considered in this order.

Errant polychaete

SEDENTARIA polychaetes have :

	Limited number of body-segments.
	Body may be separated into different regions.
	Anterior appendages may be absent or similar appendages are present in a few to many in numbers.
	Short parapodia associated with tubicolous or burrowing habit.
	Usually deposit or filter feeders.

Sedentary polychaete

Developed by: P. L. Chan Last modified: 4 April,00