General Features of the External Morphology of Marine Planktonic Copepods
Introduction
The class Copepoda, established by H. Milne Edwards in 1830 (as an Order, in the Annales des Sciences Naturelles, 1820, and in the Histoire naturelle des Crustacés, 1840), or as a subclass for authors who accept the validity of the supertaxon Maxillopoda (Bowman and Abele, 1982 and most current authors despite the remarks of Hessler, 1982), comprises the largest number of species within the phylum (or superclass) Crustacea, approximately 11 500 according to Humes (1994), but this is almost certainly a considerable underestimate.
Together with the Nematoda, copepods represent the most numerous multicellular organisms. Their sheer abundance in terms of individuals has led to them being called the “insects of the sea”, although their structural body plan is far less diversified.
Essentially marine, benthic and pelagic, they have colonised continental and subterranean freshwaters. Their remarkable adaptive capacity has led them to develop associations with other living organisms, giving rise to ectoparasitic and endoparasitic forms colonising both invertebrates and vertebrates, and even macrophytes. They can also act as vectors of unicellular or multicellular parasites, and may thereby function as pathogenic agents for other organisms in natural environments as well as in aquaculture, and thus indirectly affect human health.
Although modest in size, ranging from a few tens of micrometres to less than a centimetre in the vast majority of species (exceptionally up to 25 cm in a parasitic form of whales), copepods play an essential role in the pelagic food web, equivalent on land to herbivores, and themselves serving as a food source. Knowledge of copepods is indispensable for understanding and modelling the carbon cycle and matter fluxes in both marine and continental waters. Pelagic forms may serve as indicators of water masses, ocean currents and climatic changes.
The extreme rarity of fossil forms, the oldest known being a fish parasite dating from the Early Cretaceous (-110 to -120 million years ago) (Huys and Boxshall, 1991), together with the supposed post-Precambrian antiquity of the group (Sharov, 1966; Boxshall, 1983), makes the phylogenetic understanding of the group particularly difficult.
Classifications of Copepoda at all taxonomic levels have been more hypothetical and pragmatic than natural, and have been subject to repeated revisions.
Whereas free-living forms display varied but relatively comparable structures, the same cannot be said for parasitic forms, which exhibit an extraordinary degree of morphological differentiation and regression (Yamaguti, 1963; Kabata, 1979; Raibaut, 1996), hence the great difficulty in defining morphological characters common to the entire group.
The order Copepoda appears for the first time in the Histoire naturelle des Crustacés by Henri Milne Edwards in 1834. Under this term he included only the free-living forms belonging to the subclass Maxillides, alongside the Ostracoda, grouped within the Entomostraca, while parasitic forms were placed in the subclass Suceurs, legion of the swimming parasites, with two orders: Siphonostoma and Lernéens.
The classifications of Zenker (1854), Thorell (1859) and Claus (1863) brought free-living and parasitic forms closer together. Two suborders accounted for this: the Eucopepoda (or Copepoda proper) and the Parasita (or Siphonostoma).
The major divisions established by Thorell, then Giesbrecht (1892), were partly synthesised by G.O. Sars (1901-1918) and used until 1979, when Kabata (in Parasitic Copepoda of British Fishes) linked parasitic forms to free-living forms, thereby breaking an artificial dichotomy. According to this author, the major systematic divisions and systems initially proposed by Thorell and Giesbrecht should not be abandoned. Thus the two branches Gymnoplea and Podoplea correspond to a major event in the evolution of Copepoda (related to the ancestral habitat of the group and mode of locomotion).
Depending on the author, the taxonomic level chosen varies according to whether Copepoda are considered a class or subclass. Similarly, the number of orders (or suborders) varies between authors, notably regarding parasitic and semi-parasitic forms.
A consensus on the evolutionary scheme is now established, well synthesised in Copepod Evolution by Huys and Boxshall (1991, chapter 4, p.371-416), which coincides with the phylogenetic scheme given by Ho (1990). The classification is organised as follows:
• Progymnoplea — Platycopioida
• Neocopepoda
— Gymnoplea : Calanoida
— Podoplea : Misophrioida, Gelyelloida, Harpacticoida, Mormonilloida, Siphonostomatoida, Monstrilloida, Cyclopoida, Poecilostomatoida [Thaumatopsylloida = Cyclopoida]
Among these ten orders, five are of major numerical importance:
Calanoida, Harpacticoida (rare commensalism)
Siphonostomatoida, Cyclopoida, Poecilostomatoida
One order is entirely endoparasitic at the juvenile stage and free-living only as adults: Monstrilloida. The four remaining orders (Platycopioida, Misophrioida, Mormonilloida and Gelyelloida) are of lesser importance except in terms of their phylogeny; they occur in marine environments, except the last which inhabits subterranean waters.
It is in free-living marine forms that the fundamental characters of Copepoda are most clearly expressed. The marine genus Calanus (Calanoida) with the species Calanus finmarchicus (Gunnerus, 1770) is generally taken as a model, even though it does not represent the most primitive form. It should be noted that the terminologies in use have varied between authors, and consequently within the faunal works (cf. Kabata, 1979, p.15; in Monoculus, 1982 (5):15-20; 1983 (6): 15; 1984 (8): 4-9; 1985 (10): 15-23; Huys and Boxshall, 1991: 19-20, 455-459).