Euphausiacea
Morphology of larval development

The larval period of the euphausiid life cycle is divided into three phases which are distinguishable in terms of their characteristic gross morphology (Gopalakrishnan, 1973). The phases are, in order of development, metanauplius/nauplius, calyptopis and furcilia. Each phase is made of up a series of stages, 3 in the metanauplius/nauplius, 3 in the calyptopis and between 3 and 8 in the furcilia (the number varying according to species and to author).

In the nauplius stage, the larva is essentially just the head of the definitive animal, consisting of 3 pairs of cephalic appendages, antennae 1 and 2 and the mandibles, which are natatory in function. The posterior part of the body is undifferentiated and contains only the rudiments of future major body parts (tagmata):

Euphausia crystallorophias 2
Euphausia frigida 2
Euphausia superba 1
Thysanoessa macrura 2

There are two distinguishable nauplius stages: nauplius I (with one pair of seta on the posterior edge of the body) and nauplius II (with more than one pair of seta). At the metanauplius stage, the carapace and the precursors of the thorax and abdomen become developed and the buds of maxillae 1 and 2 and thoracopod 1 begin to form. Meanwhile, the mandibular rami become reduced and the natatory capabilities of the mandibles become diminished (see illustrations above).

On becoming calyptopes, the larvae acquire compound eyes which remain under the carapace during the entire phase. The development of other parts of the body is rather gradual (anamorphic). The antennae continue to have a natatory function and the tagmata (i.e. the head, thorax, and abdomen) are generally differentiated:

Euphausia crystallorophias 3
Euphausia frigida 3
Euphausia longirostris 1
Euphausia superba 2
Euphausia spinifera
Euphausia triacantha
Euphausia vallentini 2
Thysanoessa macrura 3

In calyptopis I, the abdomen is still unsegmented whilst in calyptopis II, there are 5 segments with the distal segment containing the precursors of the telson and uropods. In calyptopis III, this distal segment separates to form a 6th abdominal segment. At this stage, some species with accelerated development such as Euphausia superba, may have several pairs of thoracopods but not pleopods (Euphausia superba 2).

At the furcilia phase, the compound eyes are no longer under the carapace, the antennae are losing their natatory function and undergoing a significant change in shape; the thoracopods, pleopods, and abdominal photophores gradually develop and the terminal spines of the telson reduce in number.

Euphausia crystallorophias 4
Euphausia frigida 4
Euphausia longirostris 2
Euphausia superba 3
Euphausia spinifera 1
Euphausia triacantha 1
Euphausia vallentini 3
Thysanoessa gregaria 3
Thysanoessa macrura 4

There is both inter- and intraspecific variation in the sequence of these processes (Fraser, 1936; Einarsson, 1945; Mauchline and Fisher, 1969, Mauchline, 1980; Makarov et al., 1986; Menshenina, 1990) but, in most cases, dominant pathways of development may be traced (Makarov, 1974; Mauchline, 1980; Knight, 1984) allowing the array of different furcilia forms to be grouped into stages according to the most frequently occuring characters. Using this approach, we propose two developmental schemes for furcilia. The first scheme is based on Fraser (1936) and covers the developmental sequences of the Euphausia and Thysanoessa species that occur in the temperate South Atlantic and the Antarctic:

Scheme I
Furcilia I. Developing pleopods without swimming setae, their number varying from 0 to 5. Telson with 7 (or 8 in Euphausia triacantha) terminal spines and 3 subterminal spines of similar width.
Furcilia II. At least one pair of pleopods setose. Telson with 7 terminal spines.
Furcilia III. Most or all pleopods setose; telson with 7 (sometimes 6) terminal spines; terminal part of telson bearing setae projecting posteriorly; distalmost pair of subterminal spines becomes distinctly wider than first 2 pairs.
Furcilia IV. Telson with 5 (sometimes 4) terminal spines.
Furcilia V. Telson with 3 (sometimes 2) terminal spines.
Furcilia VI. Telson with 1 terminal spine. Three pairs of subterminal spines are present, the most distal of which is very wide whilst the proximal pair is reduced.

The Nematoscelis and Stylocheiron species exhibit a more abrupt developmental pathway in which the sequence of appendage and telson development differs considerably from that of Euphausia and Thysanoessa (Mauchline and Fisher, 1969; Gopalakrishnan, 1973). A second scheme for Nematoscelis is therefore given based on Gopalakrishnan (1973):

Scheme 2
Furcilia I. As Scheme 1.
Furcilia II. One or two pairs of pleopods setose, number of terminal spines varying between 5 and 7.
Furcilia III. All or most of pleopods setose, the number of terminal spines varying between 3 and 5.

At the postlarval phase, euphausiids usually acquire a definitive scaphocerite and a fully segmented antennal endopod; sometimes development in the thoracopods and photophores continues; the number of subterminal telsonic spines falls to two. The loss of the proximal subterminal spines conventionally marks the onset of the juvenile phase of development.