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How To Describe A Nereid

Nereids are common predators in marine systems. They are numerous and difficult to identify. So care needs to be taken when describing the worm.

Starting with the anterior end of the worm, note the general shape of the prostomium, number and shape of the antennae found on the front edge, and number of tentacular cirri on each side of the prostomium (usually counted in pairs, Figure 1).

Fig 1

Next observe the general form of the body. The shape of parapodial structures and the types of setae (the setal signature) varies down the body and this must be noted. On the forms, we ask you to observe setae and parapodia from the anterior part, the mid section and the rear end of the worm.

A few genera of nereids have branchiae (gills), usually on the anterior setigers. These structures are obvious as bushy growths on the parapodia.

The parapodia comprise a series of lobes, termed ligules and cirri. The various combination of lobes and cirri, as well as their shape, are important characters both at generic and species level.

Fig 2

The first two setigers are uniramous, having only a neuropodia (Figure 2a). In some genera, all the parapodia are uniramous, while in the remainder they are biramous (Figure 2b).

Fig 3

To describe and draw the parapodia, it is necessary to examine it under a high powered microscope. Make sure when making the preparation that you mount the parapodia with the anterior side uppermost.

In a typical biramous parapodia, the main notopodial features are: the presence, shape and length of the dorsal cirrus (Figure 3-D.C>; the shape and relative position of the notopodial ligule - Figure 3-1a, for example, is it swollen or the same size as the inferior ligule? The shape and relative position of the inferior notopodial ligule (Figure 3-1b, found below the notopodial setae) should be noted.

In the neuropodia, note the size and shape of the setigerous ligule. This is a ligule which carries the neurosetae (Figure 3-2a). There are dorsal and ventral sections to this ligule separated by a stout acicula. Also, the end of the ligule may carry two lobes which lie one behind the other. These are the pre-setal (lying in front of the setae) and a post-setal lobe (lying behind the setae). They may be difficult to see. Ventral to the setigerous ligule is the inferior neuropodial ligule (Figure 3-2b) and ventral to this ligule is the ventral cirrus (Figure 3-V.C.).

Setal structures in nereids are important taxonomic characters and combinations of the different types and their occurrence in the different parts of the parapodia are useful generic characters. The actual shape of the setae is useful at the species level.

In addition to simple setae, there are four types of compound setae: homogomph falcigers and spinigers and heterogomph falcigers and spinigers. The terms homo- and heterogomph refer to the shape of the joint between the shaft and blade. A homogomph seta has a symmetrical even joint at the end of the shaft (Figure 4a), while on a heterogomph seta, one side is much longer (Figure 4b).

Fig 4

The terms spiniger and falciger refer to the blade of the setae; a falciger has a blunt or hooked tipped blade (Figure 5a), while on a spiniger the blade tapers to a point (Figure 5b).

Fig 5

The final series of characters relate to the eversible pharynx. In nereids, this is muscular and armed with two large jaws. The rest of the pharynx can carry small chitinous structures called paragnaths, soft papillae or may be plain. The form and pattern of the armament on the pharynx are important in the taxonomy of these worms.

The everted pharynx can be divided into an outer or Maxillary ring (Figure 6) and an inner or Oral ring (Figure 6) Within these rings are areas designated usually by a Roman numeral (Figure 6). On the dorsal side of the Maxillary ring there is a central area Group I, flanked by Group II, ventrally a central area Group III flanked by Group IV. On the dorsal side of the Oral ring the central Group V flanked by Group VI and on the ventral side a central Group VII flanked by Group VIII. The type and general pattern of paragnaths or papillae is important.

Fig 6

The main problem faced in describing nereids will be that the pharynx is not everted. To see the characters dissect out the pharynx. Make a slit along the anterior part of the ventral side (Figure 7), pull back the body wall to reveal the gut and, within the gut, the inverted pharynx. Make a cut down the pharynx and pull the sides apart to see the internal structure. You will be looking down on the dorsal surface and the oral ring will still be nearest the mouth. It will be difficult to see the arrangement of paragnaths clearly, so concentrate on the types of armament rather that their particular arrangement on each of the areas of the pharynx.

Fig 7

The above description and also the forms only deal with the Atokous (non–sexual) stage of adults. However, nereids undergo morphological changes on sexual maturity. Most noticeably, the body becomes shorter and the parapodia, particularly those of the posterior, become more conspicuous. The ligules of these parapodia become flap–like and accessory fleshy processes are developed. All the setae are replaced by paddle–like setae. Also, there are associated internal changes. This stage is called a Heteronereis or epitoke. These modifications enable the sexual stage to swim and seek a mate. Once mating has taken place the worm dies.

There are many species of nereid in SE Asian waters. The description of characters given here is based on:

Chambers and Garwood (1992) Polychaetes from Scottish waters. A guide to identification. Part 3 Nereidae. National Museums of Scotland. This reference has a good general description of the characters needed for identification.

Hylleberg et al. (1986) Polychaetes of Thailand. Nereidae (Part 1); Perinereis, Pseudonereis with notes on species of commercial value. Phuket Marine Biological Centre. Research Bulletin, 43: 1-22. This also gives a good account of the characters used, including an end on view of the arrangement of setae and ligules of parapodia.

Wu et al. (1986) The Nereidae (Polychaetous Annelida) of the Chinese coast. Chinese Ocean Press, Beijing & Springer-Zerlag, Berlin. 234pp. This is the English translation of the earlier Chinese publication and covers all the species found along the Chinese coastline.

Glasby, C. J. (1999) The Namanereidinae (Polychaeta: Nereididae). Pt.1 Taxonomy and phylogeny. Reords of the Australian Museum. Supplement 25: 1-129. This is an important revision of one of the sub-families of nereids, species of which are commonly encountered in freshwater or waters with low salinity.

An important paper on morphometric characters in nereids is Ben–Eliahu (1987) An approach to nereid morphometry. Bulletin of the Biological Society of Washington 7: 169-173.

Finally, Fauchald (1977) The polychaete worms. Natural History Museum of Los Angeles County, Science Series 28, 1–190. Gives keys to genera. He warns that many species cannot be separated using conventional characters.

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