Euphausiacea
Introduction

Euphausiids are holoplanktonic crustaceans which were grouped together with mysids in the 19th Century as members of the Schizopoda (divided feet, a reference to the biramous pleopods shared by both taxa). Although differences to the contrary were recognised by Boas in 1883, this alliance of the two groups persisted in the literature until the end of the 1920s. Today, euphausiids are considered as members of the crustacean superorder Eucarida and there is thought to be a close affinity with the ancestral decapods by way of the penaeid family Sergestiidae (Mauchline and Fisher, 1969). Similarities between these deep-sea prawns and the euphausiids are seen in the larval stages with both exhibiting a free nauplius and an abrupt metamorphosis at the end of each developmental stage. As adults, both groups possess a petasma, spermatophore and thelycum (see definitions below), although these structures themselves are probably not homologous. Both sergestiids and euphausiids also possess photophores. The order Euphausiacaea is significantly less diverse than the Decapoda, however, and contains some 86 species distributed in two families and amongst 11 genera (Baker et al., 1990).

There is little agreement between the few phylogenies which have been drawn up for euphausiids, although discrepencies may be explained by the different information used in their construction (Casanova, 1984; van der Spoel et al., 1990). Having said that, however, all agree that the monospecific family Bentheuphausiidae is the more primitive of the two families. Bentheuphausia ambylops is a deep-water species of cosmopolitan distribution which lacks photophores, has a simple biramous and relatively undifferentiated petasma, and a full compliment of 8 pairs of thoracic legs. It is thought to have originated in the North Atlantic during the early Cretaceous and dispersed elsewhere at a later date (van der Spoel et al., 1990). All the other genera possess elaborate petasmae, photophores and a reduced number of thoracic legs. While still exclusively marine and distributed throughout the coastal seas and oceans of the world, euphausiids now occupy the epi- and mesopelagic realms, as well as the bathypelagic (where they may be benthopelagic).

Although few euphausiids have very restricted patterns of distribution they are frequently associated with particular water masses or environments (Dadon and Boltovskoy, 1982; Gibbons et al., 1995; Tarling et al., 1995), and have been used as tracers of water movement. Given this, one would anticipate a genetic homogenity among populations of the same species in the same water mass. Such has been demonstrated for species such as Euphausia superba, where there is extensive mixing between populations in the Indian and Atlantic sectors of the southern Ocean, probably owing to the unique nature of the water flow around Antarctica (e.g., Kuhl and Schneppenheim, 1986). However, in other species significant genetic differences can exist even between populations as close to each other as 400 km in a relatively dynamic system, such as the California Current (Bucklin, 1986). The exact reasons for the restricted gene flow between populations are unknown, but they must include some sort of physical or hydrographic barrier to dispersal.

By plankton standards, euphausiids are relatively large and they frequently dominate zooplankton communities, especially over the continental shelf and in regions of high environmental productivity. Euphausiid assemblages in neritic areas are generally of low diversity (e.g., Barange et al., 1992; Gibbons et al., 1995), whilst those of frontal areas and oceanic waters may be species-rich (e.g., Barange, 1990). There is evidence to suggest that the high diversity observed in assemblages at fronts or in oceanic waters may be maintained by biological interactions between species (Barange et al., 1991).

Many species of euphausiids display pronounced diel vertical migration (DVM) and frequently traverse distances in excess of 200 m at night (see Table IV in Mauchline, 1980). Although this behaviour is thought to be cued by light (Forward, 1988), it is strongly affected by a number of intrinsic and extrinsic factors. Most species display ontogenetic vertical migration, whereby developmental stages of increasing age can move increasingly greater vertical distances (e.g., Pillar and Stuart, 1988; Pillar et al., 1989). The depth of nocturnal occupation appears to be influenced by both the thermal structure of the water column as well as the stratification of the food environment (see references in Andersen and Nival, 1991). The quantitative food environment also seems to influence the amount of time spent by euphauiids in the upper water layers, which can result in asynchronous patterns of individual movement (Gibbons, 1993). While predation is currently thought to be the driving force behind the evolution of DVM, it enables populations to take advantage of differential horizontal water flow in the vertical dimension and so maintain themselves in productive and shelf areas (Pillar et al., 1989; Barange and Pillar, 1992). Euphausiids in multi-species assemblages (at fronts and in the open ocean) appear to partition the water column vertically, possibly as a way of reducing interspecific interactions (Barange, 1990).

Euphausiids are truly omnivorous, although different genera tend towards either herbivory or carnivory (Roger, 1973). Diets are broad and range from dinoflagellates, diatoms and tintinnids to fish eggs and larvae, as well as detritus (see e.g. Table VIII in Mauchline and Fisher, 1969). Feeding behaviour and rate processes are stongly influenced by the quantitative and qualitative nature of the food environment (Stuart and Pillar, 1990), and there is a firm correlation with DVM which is often reflected by feeding rhythms (Ponomareva, 1971; Roger, 1975; Gibbons et al., 1991a; Gibbons, 1993). Selective feeding has been demostrated for some euphausiid species (Stuart, 1989; Gibbons et al., 1991b) and the switch between a herbivorous and carnivorous diet appears to be dictated by the food environment (Stuart and Huggett, 1992).

The sexes in euphausiids are separate, and mating is thought to involve the transfer of a spermatophore by the male to the female using modified pleopods. Fertilisation is internal and eggs are either released directly into the water column or they are retained by the female in egg sac/s carried between the thoracic legs. In most oceanic species, once the eggs have been spawned they sink to a considerable depth during which time embryonic development takes place. When the larvae eventually hatch they migrate back to the upper layers (Einarsson, 1945; Marr, 1962; Makarov, 1982). In neritic species (including Euphausia crystallorophias which lives in the relatively deep waters of the submerged Antarctic shelf), the eggs remain suspended in the water column (Zelikman, 1961; Makarov and Maslennikov, 1981; Makarov, 1982). Eggs typically hatch into a first nauplius which then develops into a second nauplius and then a metanauplius. In the case of those species which retain the eggs, the naupliar stages are by-passed and it is the pseudometanauplius that emerges from the egg (Zimmer and Gruner, 1956; Casanova-Soulier, 1968; Mauchline and Fisher, 1969; Gopalakrishnan, 1973). These first developmental stages have non-functional mouthparts and are thought to be nourished by yolk reserves. A series of three calyptopis stages follow, with the first calyptopis stage being equipped with functional mouthparts and the further two stages exhibiting greater eye development and increasing segmentation in the abdomen.The third calyptopis larva moults into the first of a number of furcilia stages, the exact number of which varies greatly with both the species and the environment (see e.g. Pillar, 1984a, 1985). Generally though, as the furciliae moult and increase in size so they become more complex: pleopods develop and become setose, photophores are produced and there is a reduction in the number of telson spines. There is no clear transformation from furcilia to adolescent, although mouthparts which may be incompletely formed in furciliae develop fully during adolescence. Adulthood is attained following the development of reproductive organs and secondary sexual characteristics.

The age of sexual maturity appears to be temperature dependent, and is generally earlier for species and individuals at temperate or tropical latitudes than it is for those nearer the poles (Mauchline and Fisher, 1969). Breeding is tightly linked to the environment: species at high and mid latitudes tend to be seasonal whereas those in equatorial or upwelling areas may reproduce throughout the year (Mauchline and Fisher, 1969; Pillar and Stuart, 1988; Barange and Stuart, 1991). The number of successive broods produced by an individual per "season" is largely unknown, although it may be high for species in upwelling areas (Ross et al., 1982; Stuart, 1992). Good positive relationships between brood size and body mass have been established for many euphausiid species (Mauchline and Fisher, 1969). Deep-living species tend to produce fewer but larger eggs than those of more superficial water layers and species at higher latitudes are likely to produce larger broods than species from lower latitudes (Mauchline and Fisher, 1969).

Owing to their large size, often high abundance and omnivorous diet, euphausiids can play a vital role in the functioning of some pelagic ecosystems. Whilst they are generally considered to have a negligible impact on phytoplankton production and standing crop (e.g. Stuart and Pillar, 1990), euphausiids are capable of transferring surface production to the sediments directly (via DVM) and may play an important role in carbon flux (Noji, 1991). Euphausiids are thought to exert considerable predatory impact on mesozooplankton populations (Stuart and Pillar, 1990), and might compete directly with fish larvae for food under certain circumstances. They can form a central component of the diet of many nekton species (see reviews of Mauchline and Fisher, 1969; Mauchline, 1980) and so act as important conduits through which primary production is channeled to fish. Euphausiids are not only prey for fish but they can also be significant predators on fish larvae, and may influence the magnitude of pelagic fish recruitment (Theilacker et al., 1993).

Despite the fact that euphausiids can occur in vast luminescent swarms that must have been obvious to even the first seafarers, it was not until the reports of the early whalers who observed small "insects" or "fish" in the stomachs of their kills that records of these creatures first became documented. Euphausiids were only collected and studied for the first time on the Great European expeditions that variously criss-crossed the globe. The exploratory nature of these cruises was such that the work arising therefrom was mostly of a descriptive and taxonomic nature although later reports dealt with larval development and general biology. A number of these expeditions included the South Atlantic in their itinerary, even though their efforts were, for the most part, concentrated in and around the whaling grounds of the Southern Ocean. Reference to these expeditions has been made elsewhere.

More recently there has been a move away from strictly taxonomic and biological studies of euphausiids to work which is more ecological and process orientated. Once again though, research effort has not been spread evenly across the region, and the Southern Ocean has received the lions share of interest. Studies on the dominant euphausiid species occurring around the continental margins of South America, and especially southern Africa, have been ongoing though, and useful information in subject areas like production, life-history strategies, feeding, migration and population maintenance, as well as biogeography, has been forthcoming (see reviews of Pillar et al., 1992; Tarling, 1995). Nevertheless, this knowledge base falls behind that which has originated from latitudes further south. Whilst the focus on Antarctica partly reflects a management need for information on the exploitable Euphausia superba (Miller and Hampton, 1989), it is mostly driven by a political willpower to be seen to be involved in Antarctic research. This effort has unfortunately resulted in an ignorance concerning euphausiids and euphausiid ecology elsewhere in the region, especially around the equator and in the central water masses, and this imbalance needs to be redressed if we are to get comparable information therefrom.