Polychaeta
Introduction

As some other marine phyla that comprise mostly benthic forms, the Annelida contain a small minority of species that have evolved to spend all their life in the pelagic realm. These holoplanktonic Polychaeta have developed a number of special adaptations to live in the planktonic environment including small size, long setae, enormous and complex eyes, flattened or gelatinous bodies, a high degree of transparency, sperm storage in females, etc. Pelagic polychaetes are found mainly in the open sea, but they also occur in neritic regions. They inhabit the entire water column from surface layers to abyssal depths. Most research has been undertaken on their distribution and taxonomic relationships, and very little is known about other aspects of their biology and ecology.

body plan pelagic polychaete
anterior region ventral view
type of parapodial gland
type of parapodial gland 2
type of parapodial gland 3
types of setae
type of setae 2

The holoplanktonic species are mainly active predators which attack their prey with the rapidly everted proboscis. However, filter-feeders or phytophagous forms are also known, for example some species of the family Iospilidae (Day, 1967). Judging from their behavior and their strong proboscides, the Alciopidae are active predators, but there are no direct observations of alciopid feeding. They are assisted in their search for prey by telescopic eyes, which enable them to orient themselves in different directions, as well as to recognize the size and form of the prey. The tomopterid polychaetes have very short pharynges, and can either consume their prey whole or suck out the internal body fluids. The long cirriform appendages of the second segment in the tomopterid polychaetes may play a role in prey (or predator) detection. Remains of herring fry, fragments of siphonophores, chaetognaths and appendicularians have been found in their digestive tracts (Rakusa-Suszczewski, 1968). Many other species have no obvious prey-catching organs, and possibly they feed on microscopic prey or eggs in the plankton (Day, 1967). The aberrant polychaete Poeobius meseres captures falling detrital matter using a mucous web (Uttal-Cooke, 1992). Members of the family Typhloscolecidae may be neotenic forms in connection with their transition to ectoparasitism, and their pharynx is modified into a characteristic suctorial organ.

A few studies have been undertaken on the physiology and biochemistry of pelagic polychaetes (Ikeda, 1974; Thuesen and Childress, 1993; Childress and Thuesen, 1993). The active forms of pelagic polychaetes have very high metabolic rates, leading Thuesen and Childress (1993) to suggest that they may have the highest rates of all deep-sea pelagic animals. High levels of activity by alciopid and tomopterid polychaetes are supported by in situ observations (Video Archive Library, Monterey Bay Aquarium Research Institute, Monterey, California).

As in almost every other pelagic marine phylum of animals, bioluminescence has been observed in pelagic species of the Annelida. In the Alciopidae, Alciopina and Krohnia are bioluminescent, but this ability is probably present in other genera as well. Clark (1970) describes the histology of the mucus glands of Rhynchonerella angelini, and he mentions the possible relationship of these organs with the bioluminescence in this species. In the Tomopteridae, there are photogenic organs located on the parapodia which give off an unusual yellow light. This phenomenon is present in Tomopteris nisseni. One of the biological functions of bioluminescence is likely the attraction between males and females, which would be of great importance for pelagic forms since they usually do not form dense populations. Dales (1971) has reviewed this vital function in pelagic worms.

One of the most unique aspects of pelagic polychaetes is the evolution of eyes in the family Alciopidae. These complex eyes are very large and telescopic, and each eye has a lens that is controlled by muscles which can change the direction of its axis. The eyes can apparently determine not only the intensity and direction of light, but also the outline and size of objects. This must be of great advantage to the species in this pelagic family since it enables them to survey their surroundings in different directions. The first studies of these complex structures were undertaken by Greeff (1876) and Demoll (1909), and their function has been described by Hermans and Eakin (1974) and Wald and Rayport (1977). The eyes of these annelids are remarkably similar to those of cephalopods and vertebrates, and because of their different developmental origin, they have been cited as an extreme example of convergent evolution (Salvini-Plawen and Mayr, 1977). Wald and Rayport (1977) have emphasized the extraordinary evolutionary convergence represented by the possession of accessory retinas in alciopid polychaetes, cephalopods and deep-sea fishes. Recent findings of a small multi-gene family of packed box-containing genes (Pax genes) in different metazoan phyla, which play an important role in embryonic development in the nervous system, has revealed that their expression is structurally similar in different visual systems, yet ontogenically distinct (Zuker, 1994).

Uschakov (1972) pointed out, correctly, that in the general economy of the sea, the role of the 8000+ species of Polychaeta is tremendous because they represent a very important link in all processes of the production of living matter, and in particular, they are the basic food, rich in calories, of numerous fishes. Although Uschakov was referring to benthic or meroplanktonic species, it is clear that holoplanktonic polychaetes play an important role in the pelagic ecosystem.