Salpida
Introduction

Salpida is the most highly diversified order of the class Thaliacea, exhibiting the greatest variation in morphology and distribution. The first description of a salp dates back to the 18th century (Browne, 1756), and there are at least 25 references prior to 1850 (cf. Braconnot, 1973).

Like all members of the class Thaliacea, salps are metagenetic animals (Brien, 1948; Godeaux, 1990). The solitary individual (oozooid) produces by budding a stolon, which strobilates into chains of aggregate individuals (blastozooids). These chains detach and become free-swimming, pseudo-colonial groups. Each blastozooid in the chain is a hermaphroditic protogynous individual. Only one or a few oocytes succeed at maturing in the ovary, where they are fertilized by the sperm released to the surrounding water by older blastozooids that bear mature testes. Fertilization occurs shortly after the detachment of the chain from the solitary individual.

The fertilized egg develops into an embryo attached to the atrial body wall of the parent zooid by a kind of placenta. Young solitary zooids detach from the placenta at birth and exit through the atrial opening of the parental individual.

Several features of the ontogenetic cycle enable these animals to attain rapid population increases in response to sudden and unpredictable increases in food supply (Silver, 1975, reviewed by Alldredge and Madin, 1982). Among these characteristics, the most remarkable ones are: 1) the asexual production of hundreds of descendants per individual (Heron, 1972a, b; Heron and Benham, 1984, 1985; Esnal et al., 1987; Madin and Purcell, 1992); 2) the extraordinarily high growth rates that, in Thalia democratica, are the highest recorded among metazoans, approaching those of protozoans (Heron, 1972a, b; Deibel, 1982a; Heron and Benham, 1984, 1985; Le Borgne and Moll, 1986); and 3) the short generation times that, also in Thalia democratica, vary from 50 hours to about 14 days (Heron, 1972a). Unfortunately, except for the above-mentioned species, more or less detailed information on these traits are available only for Salpa fusiformis (Braconnot et al., 1988), and Cyclosalpa bakeri (cf. Madin and Purcell, 1992).

Salps feed by filtering suspended particles from a stream of water through a continuously renewed mucous net secreted by the endostyle, which is then ingested together with the particles entrapped (Madin, 1974). Feeding proceeds during swimming, by pumping water into the oral siphon by muscular action, and exhaling it through the atrial siphon. Madin (1995) reviewed the sensory ecology of salps emphasizing that swimming is their fundamental and constant behavior, as well as the basis for all the energetic and distributional aspects of their ecology.

Salps normally swim and feed continuously, but feeding may cease under special conditions. Vertically migrating Cyclosalpa bakeri stop feeding at the surface at night, this behavior probably being with reproduction (Purcell and Madin, 1991). Other non-migratory species cease swimming, and therefore feeding, at low temperatures (Harbison and Campenot, 1979). In general, salps stop feeding in response to adverse mechanical or chemical stimuli (Fedele, 1933), but there is no clear evidence yet that feeding rates may be modulated at short time scales by the type or concentration of food (Madin, 1995). Harbison et al. (1986) reported that Pegea confoederata do not slow or stop feeding, even under particle concentrations that may clog the filter and the esophagus. They attribute the generalized absence of salps in neritic environments (except for species of the genus Thalia) to this disadvantage. Madin (1995) suggested that Thalia species may perhaps avoid clogging by detecting particle concentrations by means of a special sensory structure, and by adjusting mucous production rates or water pumping accordingly.

Salps are food generalists, able to filter a wide particle size-range (from about 1 mm to less than 1 mm) with different degrees of retention efficiency (Madin, 1974; Harbison and Gilmer, 1976; Harbison and McAlister, 1979; Kremer and Madin, 1992); and at high rates (Madin, 1974; Harbison and Gilmer, 1976; Deibel, 1982a, 1985b; Mullin, 1983; Caron et al., 1989; Madin and Kremer, 1995). This capacity, in addition to the aforementioned population characteristics, results in the dominant role of these tunicates in cycling phytoplankton carbon (Andersen and Nival, 1988).

Salps sometimes represent a significant proportion of the diet of mesopelagic fish, but probably much of their nutritional value resides in the phytoplankton and microzooplankton packed in their stomachs, thus by-passing a trophic level (Kashkina, 1986; Fortier et al., 1994). Other salp predators include medusae, siphonophores, heteropods, and amphipods (Hamner et al., 1975; Madin et al. 1996). Recently, Kawaguchi and Takahashi (1996), reported krill (Euphausia superba) predation on salps, which were preferred over other prey types.

To complete this brief review of the important role of salps in pelagic ecosystems, their contribution to the vertical flux of carbon must be considered. They produce large (1 to 10 mm long), fast-sinking fecal pellets which, given the non-selective nature of salp feeding, reflects faithfully the composition of the available food-particle assemblage (Iseki, 1981; Silver and Bruland, 1981; Madin, 1982; Matsueda et al., 1986; Andersen and Nival, 1988; Morris et al., 1988; Caron et al., 1989; Pfannkuche and Lochte, 1993; Fortier et al., 1994). Fernex et al. (1996) found that organic nitrogen concentrations and ammonification rates in bottom sediments of the northwestern Mediterranean were highest immediately after salp blooms. The ability of some species to perform extended vertical migrations also contributes to the direct transport of fecal material to considerable depths (Wiebe et al., 1979).