General features of the external morphology of marine planktonic Copepods
Morphology of the various body parts and appendages
The cephalosome, in dorsal view (fig. G), presents anteriorly a free margin bearing two small "hairs": the rostral filaments (fr), very discrete, sensory filaments or sensilla. On the ventral surface, the front curves into a rostrum directed posteriorly, bifurcated and terminating in two points, the rostral filaments (FR).
On the ventral surface (fig. G2), the head bears symmetrically, in a pre-oral position, two multi-segmented appendages longer than the body: the antennules (A1) (or first antennae) (fig. G3 A). These consist of twenty-five more or less equal segments in the female (fig. G3, A, a), bearing simple or plumose setae and sensory organs (the aesthetes or aesthetascs). In the male, the two proximal segments are fused, slightly enlarged, and bear more developed sensory organs than in females. The antennules are oriented at 90° to the body axis.
Posteriorly, the antennae (A2) (fig. G3 B, a) show a biramous appendage structure with a sympodite or basipodite (B) composed of two successive segments (B1 or coxa, and B2 or basis). On the basis, two multi-segmented branches articulate: one internal, the endopodite (Enp) with two segments; the other external, the exopodite (Exp) formed of seven segments. All these segments bear setae.
The mouth opens posterior to the antennae, on the medio-ventral line. It is protected by an upper lip: the labrum (Ls), a muscular lobe bearing fine, short sensory setae. Its posterior margin is delimited by the labium (Li), consisting of a pair of lobes derived by fusion of the paragnaths.
The mandibles (Md) (fig. G3 C, a) are disposed on either side of the mouth. They display a fundamentally biramous structure, comprising a coxa bearing at its base a well-developed chitinous expansion whose distal extremity shows a molar process (pars molaris) that partially conceals the buccal opening. This masticatory border presents a row of silica-tipped teeth. The basis is reduced; its axis at 90° to the masticatory blade bears a multi-segmented exopodite of five segments and an endopodite of two segments, both bearing setae.
The maxillules (Mx1) (fig. G3 D, a) are highly modified appendages, foliaceous in form with very unequal outer and inner lobes, armed with robust and fine setae. The praecoxa bears an arthrite (inner lobe) equipped with 16 robust setae; a coxa with an endite (inner lobe) bearing 3 setae and an epipodite (outer lobe) bearing 9 setae; the basis bears 2 endites, the proximal (a well-individualised lobe) with 4 setae, the distal (incorporated into the segment) bearing 4 setae, and an outer lobe (exite) bearing 1 seta; the exopodite (in the form of a single-segment outer lobe) bears 11 setae; the endopodite shows three short segments each bearing 4, 4 and 7 setae respectively.
The maxillae (Mx2) (fig. G3 E, a) appear uniramous. The praecoxa possesses 2 endites bearing 6 and 3 setae; the coxa possesses 2 endites bearing 3 setae and 1 robust outer seta; the basipodite bears an endite with 3 setae. The endopodite has 3 segments (the first being fused with the basis = allobasis) bearing 2 or 3 setae. According to Lang (1947), the three distal segments are considered as the endopodite while the single outer seta would represent the vestigial exopodite. The number of setae on the various lobes is difficult to count.
The maxillipeds (Mxp) (fig. G3 F, a) are also elongated uniramous appendages. They belong to the first thoracic somite and are homologous to the swimming legs, but the right and left appendages are not joined at their base. They present a praecoxa; a coxa with 3 weakly marked endites bearing 3, 4, 4 setae; a basis bearing 3 setae (on the inner medial side) and the first segment of the endopodite, more or less incorporated and reduced to 2 setae, followed by five segments each bearing 4 setae. The maxillipeds are curved ventrally and anteriorly.
Five "rings" delimited by dorso-lateral sutures, and the presence of a pair of biramous legs, define the "thorax" or metasome. They are numbered from front to back (Th1 to Th5). The "last" thoracic segment (Th5) presents rounded postero-lateral angles.
Five pairs of swimming legs are well developed ventrally, each corresponding to a thoracic segment. They are designated from front to back (P1 to P5). They all display the typical biramous appendage structure with a basal part of three segments (the praecoxa being barely identifiable) and two rami of three segments each (fig. G4). A median chitinous plate (pm), fused to the praecoxa and the basal part of the coxa (with the exception of the Gelyelloida), connects the two appendages, ensuring synchrony of the beat.
Each appendage is composed of three parts (fig. G4) :
- A basipodite of two segments: coxa (B1) and basis (B2), the praecoxa being reduced to a sclerite. The coxa bears an inner plumose seta, except in P5 which shows a serrated inner margin. The basis bears an inner plumose seta only in the case of P1, and articulates with the two rami.
- The exopodite (Exp) or outer ramus, comprises three segments of unequal length bearing long inner setae (Si), spines or outer setae (Se) of variable robustness and length depending on the leg, and a terminal seta (St) of variable development that extends the distal segment.
- The endopodite (End) or inner ramus has three segments, shorter than the exopodite, bearing setae.
The differences between the five pairs of legs are minor, but each presents a specific number and arrangement of setae and spines that constitutes a rigorous nomenclature forming a kind of diagnostic fingerprint of the species and sex, used in identification keys.
Example for P4: Coxa (B1): 0-1 — Basis (B2): 0-0 — Endopodite (End): 0-1; 0-2; 2-2-3 — Exopodite (Exp): I-1; I-1; II-I-5
Setae (Arabic numerals) and spines (Roman numerals) are counted from outside to inside for each article, from proximal to distal (Sewell, 1949).
The first pair of legs (P1) is shorter than the second (P2), which is itself slightly shorter than the following pairs; maximum size is reached by the fourth (P4). The fifth pair (P5) is somewhat different in the two sexes and shows slight asymmetry in the male, where the exopodite of the right leg is slightly shorter.
The posterior body (or urosome), bears no appendages. It is cylindrical in shape and articulates with the fifth thoracic segment (Th5). It presents four segments in the female; the first (the most anterior) is swollen on the ventral surface, showing a thickening covering a median unpaired genital aperture. In the male it has five segments, with on the first, a small slit slightly to the left of the midline corresponding to the genital opening.
The last segment of the urosome, bearing the anus in a dorsal position covered by an operculum, is the anal somite, on which two independent rods articulate: the furca or caudal rami, extended distally by six setae of unequal length.
The description and nomenclature used above apply in broad outline to the great majority of free-living copepods, although numerous variations in body and appendage form and structure can be observed among them.
Despite its great diversity, the group conveys an impression of both structural unity and plasticity, which has allowed it to colonise all marine and freshwater aquatic environments, but also biological unity through their mode of reproduction (mating, and deposition of a spermatophore by the male onto the genital "segment" of the female) and development, which includes, in addition to the egg, three phases showing metamorphosis (naupliar, metanaupliar, copepodite), each of several stages (respectively and generally 3 or fewer, 3 or fewer, 6), the last copepodite stage being the adult that has reached its maximum size and no longer undergoes moults.
The naupliar form, ovoid or pyriform, is characteristic, generally presenting a single median nauplius eye and three pairs of appendages: A1 uniramous, A2 and Md biramous.
The main structural modifications occurred during the evolution of the principal taxa and within each of them under the influence of:
- habitats, which condition body form: reductions by fusion of segments of the anterior and posterior body, position of the articulation between these two parts, modification of certain appendages by adaptation.
- the mode of nutrition in relation to available trophic sources and more or less pronounced modifications of the mouthparts.
- the mode of reproduction imposed by sexual dimorphism and the necessity of mating.