General aspects of external morphology of marine planktonic Copepods

Nomenclature of body parts and Metamerism

Subsequently, the terms cephalosome (cephalon + maxilliped somite), metasome and urosome were introduced, the last two representing a subdivision of the primitive abdominal tagma.

Taking into account the position of the main body articulation (in Traité de Zoologie, tome VII, fasc. 1, p.24), Gooding (1957) defined anteriorly the prosome (cephalosome + metasome) and posteriorly the urosome. Most authors accept that, from a functional standpoint, the head comprises the cephalon plus the first thoracic somite, and that the seventh thoracic somite together with the first abdominal belongs to the urosome, the intermediate region comprising the remaining thoracic somites.

The existence of 16 or 17 segments is generally accepted (fig. G13). The general plan comprises, in addition to the acron, 5 cephalic segments (or 6 if the labrum is considered a transformed appendage), 7 thoracic and 4 abdominal, which, together with the structure of the mandibles, maxillules and maxillae, places Copepods within a higher-order taxon. Dahl (1956, 1963) defines it under the term Maxillopoda, considered as a class of the Diantennata or Crustacea (in Traité de Zoologie, tome VII, fasc.1, p.4, 5, 8, 19, 293).

The presence of a seventh pair of appendages (P6 in current numbering) on the trunk of various Harpacticoids, as well as other characters analysed by Hessler (1982) and Boxshall (1983), scarcely allow the maintenance of this taxon.

Under these conditions, the origin of Copepods (then considered as a class) is monophyletic. From an ancestral type, various more or less extensive adaptations have induced modifications of the initial ground plan that are at the origin of the various groups or orders.

For Marcotte (1983) the plan is of the type: Acron + 4 segments for the head, 7 thoracic, 4 abdominal + telson (with dorsal anus) + caudal rami.

It is important to emphasise the distinction between the apparent segmentation of the adult body, whose nomenclature differs from that used for the somites derived during embryonic development. Terminologies often vary according to authors, but tend at present to become unified.

Remarks:

1. It is accepted that the original cephalon of Arthropods comprises six primary somites bearing in succession: stalked eyes (not developed in Copepods), antennules (A1), antennae (A2), mandibles (Md), anterior maxillae (Mx1), posterior maxillae (Mx2).

   The question of whether the pre-somitic acron does or does not include homologous structures, and whether or not it includes the antennular somite, is not yet resolved. Similarly, it is still unknown whether the labrum corresponds to a sternal structure or to a pair of fused appendages and forms part of the acron or of an intercalary somite.

   Vaupel-Klein (1982) suggests that the pair of frontal tubercles bearing the rostral threads (derived from the underlying frontal organs) could indicate the existence of a first pre-antennular embryonic somite, owing to their extreme anterior position and their innervation by a primary branch of the optic nerve.

   The case of the antennule, uniramous, and its homology with the biramous appendages of the following somites is still under discussion.

   The often striking resemblance between the last cephalic appendages (Mx2) and the maxillipeds (not joined to each other by an intercoxal plate, except in particular cases, unlike the swimming legs) is interpreted as a functional convergence.

2. The thoracic nature of the first somite of the urosome is inferred by reference to thoracic segments 6 and 8 which bear the gonopores in the female and male of Malacostraca, as well as by the presence in certain podoplean Copepods of a sixth pair of vestigial legs.

   It is generally accepted that the seventh thoracic somite (commonly noted Th6 or Abd1 in the literature) is a male genital somite in Calanoids, which have one more segment in the urosome than the female. In the female, this segment is composite, also including the first original abdominal somite, the whole constituting what should properly be called the genital complex or double genital somite (and not, as is often found in the literature, the genital segment or the first abdominal segment). Gurney (1931) considers, however, that the genital complex is formed by two thoracic somites (Th7+Th8).

3. The complex formed by the anal somite, the anal operculum and the furca is variously interpreted.

   Bowman (1971) considers the anal metamere as the true terminal somite bearing a telson, reduced to the form of the anal operculum, and the furcal rami which are regarded as homologous to the pair of uropods of Malacostraca, followed by Kabata (1979) and Vaupel Klein (1982). Huys and Boxshall (1991, p.315) contest this interpretation; for them, ontogenetic development shows that the anal "somite" is a telson (not a true somite), and one notes moreover the absence of an anal operculum in the primitive group Platycopioida as in the most primitive family of Pseudocyclopidae (Calanoida).

   The current discussions on the probably polyphyletic origin of the various crustacean groups (Lang, 1948; Dahl, 1963; Manton, 1977; Yager, 1981) and Vaupel-Klein (1982) stress that it is appropriate to question the fundamental difference between thoracic and abdominal somites, as well as the secondary nature of tagmosis in Crustacea, including Copepods.

   The correspondence of segments and tagmosis among the various authors, as well as the homologies with Malacostraca and during development between Gymnopleans and Podopleans, have been summarised by Dudley (1986).

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