Ctenophora
Introduction

Ctenophores are a small, well defined phylum of biradially symmetrical animals with about 150 species distributed world wide. They are exclusively marine, planktonic or benthic (one order), solitary organisms with gelatinous soft bodies. The general body plan is closely akin to, but more advanced than, that of Cnidaria (Harbison and Madin, 1982; Harbison, 1996).

The locomotory apparatus is composed of eight symmetrical tracks (subtentacular and substomodeal comb rows), distinguished by fused ciliary plates (ctenes) on the body surface (Hernández-Nicaise and Franc, 1993) (Ct. 1A , Ct. 1D). Metachronal beating of the ctenes produces locomotion in all but creeping forms. Ctenophores are the largest animals using cilia for locomotion. Swimming is coordinated by the apical sense organ or statocyst (Ct. 1A). Tentacles, when present, usually bear adhesive structures called colloblasts (Ct. 1C); colloblasts are designed to capture prey, and are another unique feature of the phylum.

The gastrovascular system (Ct. 1E) can be divided into two portions: axial and peripheral. The axial portion consists of a large stomodeum (pharynx), the infundibulum (gut), infundibular canal, and the anal canals (excretory) and pores. The peripheral portion consists of perradial, interradial, adradial and meridional canals, the tentacular canal and the paragastric canals. Except for the adradial and meridional canals, any of them may be missing (Harbison, 1985).

Ctenophores comprise two classes: Nuda (without tentacles during their entire life), with the single order Beroidea (Ct. 1F); and Tentaculata, with five orders. The most primitive Tentaculata, the Cydippida, keep their tentacles during their entire life (Ct. 1A). The order Platyctenida is, with few exceptions, a benthic group, having lost the ctenae as an adaptation to bottom condition. The order Thalassocalycida is monogeneric; it was proposed for forms that did not fit the definitions of the other orders (Harbison, 1985). The order Lobata is easily recognizable by the presence of large oral lobes and auricles (Ct. 1B), and by the absence of tentacular sheaths. Members of the order Cestida have a ribbon-like body and modified tentacles (Ct. 1G).

Most of the ctenophores are simultaneous hermaphrodites, capable of self fertilization. Only the genus Ocyropsis has been reported as dioecious (Harbison and Miller, 1986). The Platyctenida, as an exception to the rule, are protandrous, and asexual reproduction has also been recorded (Harbison, 1985).

Bioluminescence is a common feature in most of the species, occurring generally along the comb rows. In some cases, clouds of bioluminescent substance are generated, probably as a means to avoid predation (Harbison, 1996).

All ctenophores are carnivorous, feeding on zooplankton of various sizes, including copepods, euphausiids, fish eggs and larvae. The Beroida also feed on gelatinous forms such as other ctenophores, medusae and salps (Fraser, 1962; Swanberg, 1974; Kremer, 1979; Alvariño, 1985; Purcell, 1985; Mianzan and Sabatini, 1985; Monteleone and Duguay, 1988; Hoffmeyer, 1990). Their role in marine trophic webs is more complex than previously thought. Long considered a carbon dead-end in marine food webs, recent studies indicate that the group can be consumed by various fish species (Oviatt and Kremer, 1977; Arai, 1988; Ates, 1988). Recent information from the Argentine continental shelf (35-47°S) stresses the seasonal trophic significance of ctenophores for 69 fish species between the coast and 200 m depth (Mianzan et al., 1996). More complex trophic relationships also imply a more active role in the transmission of parasites; the first record of a metacercarian encysted in ctenophores was recently published (Martorelli, 1996).

Ctenophores are among the most difficult marine animals to study (Harbison, 1986), and represent the greatest challenge to conventional oceanographic sampling (Harbison et al., 1978). Standard "quantitative" samplings give the impression that ctenophores represent an insignificant fraction of the open ocean fauna (Harbison, 1986). Of the few specimens that can be collected with nets, only a small fraction can be preserved in recognizable fashion (Harbison, 1986), mainly Cydippida and Beroida. In situ techniques like diving or submarines, on the other hand, have proven to be very effective for collecting these delicate animals, resulting in the discovery of a number of new species.

It is well known that ctenophores occur from time to time in vast numbers in coastal and estuarine regions of all oceans (Fraser, 1962; Harbison et al., 1978); and a number of authors have concluded that neritic species are important predators that can regulate the abundance of certain herbivores (Harbison et al., 1978; Deason, 1982; Feigenbaum and Kelly, 1984; Mianzan and Sabatini, 1985). Recently, new immigrant species, sometimes referred to as biological pollution, have invaded environments like the Black Sea that are not prepared for such organisms, leading to the collapse of local pelagic fisheries (Shushkina and Musayeva, 1990; Vinogradov et al., 1989; Zaika and Sergeyeva, 1990; see GESAMP, 1997, for review).

Coastal species are sturdier than oceanic ones, easily collected by plankton nets, so they are well known in several regions of the world. In South Atlantic waters, however, coastal faunas are still poorly known. Only a few papers on ctenophores from the South Atlantic have appeared (Moser, 1910; O'Sullivan, 1986; Hernández-Nicaise and Franc, 1993). Recent local information shows that coastal forms are an important macroplanktonic group in the Southwestern Atlantic. Seasonally, they can dominate other planktonic predators both in abundance and in biomass (Mianzan et al., 1996). A few species, including Mnemiopsis leidyi, M. maccradyi, Pleurobachia pileus and Beroe ovata, have been mentioned in local literature (Balech and Motti, 1941; Ramírez, 1973b; Hoffmeyer, 1983; Mianzan, 1986a, b; Mianzan and Sabatini, 1985; Ramírez et al., 1990).

Three species of ctenophores are known from the Southeastern Atlantic and the Benguela Current system (O'Sullivan, 1986), but only Pleurobrachia pileus and Beroe cucumis are common to both (Gibbons et al., 1992).

This chapter is aimed mainly at students and planktologists working with net samples. The information is based on local and historical papers, and on personal collections. A diagnosis and schematic figures of the external appearance (based on photographs, living specimens or published information) is presented for each taxon considered herein.