External morphology patterns of marine planktonic copepods > Introduction
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External morphology patterns of marine planktonic copepods

Introduction | Morphologie générale | Morphologie des diverses parties du corps et des appendices | Dimensions | Formes | Principales modifications structurelles | Les antennules | Les antennes | Le cadre buccal : Labrum et labium | Les Mandibules | Les Maxillules ou premières maxilles | Les Maxilles (Maxilla) ou secondes maxilles | Les Maxillipèdes | Les pattes natatoires | Variation du nombre des segments corporels | Nomenclature des parties du corps et Métamérie | Structures et organes tégumentaires | Organes visuels | Variations de la morphologie | Sex-ratio | Gynandromorphisme et intersexualité | Accouplement | Parthénogénèse | Caractéristique de la ponte | Les formes larvaires | La durée du développement | La croissance | Régénération | Anomalies | Classification | Références | Photothèque


Introduction :

H. Milne Edwards created the class of Copepoda in 1830 (Order in Annales de Sciences Naturelles, 20, and in Histoire naturelle des Crustacés, 1840). For the authors who consider the Maxillipoda a valid super taxon (Bowman and Abele, 1982 and most recent authors in spite of the remarks of Hessler, 1982) the subclass Copepoda comprises the highest number of species in the phylum (or superclass) Crustacea, about 11500 according to Humes (1994). This number is most probably very underestimated.
With the Nematoda, copepods are the most abundant multicellular organisms on earth. Therefore, they are also referred to as the "insects of the sea", however, their structural organisational plan is poorly diversified.
Essentially marine, benthic and pelagic, copepods have also conquered continental terrestrial and subterranean freshwater systems. Their high adaptive capacity has allowed them to develop associations with other living beings, as exo- and endoparasitic forms, colonising invertebrates and vertebrates and even macrophytes. They can also be carriers of unicellular and multicellular parasites and thus constitute pathogenic agents for other organisms in the natural environment as well as in aquaculture and so indirectly influence human health.
In spite of their modest dimensions, from 20-30 microns to less than one centimeter for the greater majority (with exception of up to 25 cm! for a parasitic form in whales), copepods play an essential role in the pelagic food web, being an equivalent of terrestrial herbivores, and a source of food themselves. Copepod studies are essential for the understanding and modelling of the carbon cycle and material fluxes in seas and oceans as well as in continental waters. Pelagic forms can be indicators of water masses, marine currents and climate modifications.
The poor fossil record and the supposed post-Precambrian origin of the group (Sharov, 1966; Boxshall, 1983) make the phylogenetic understanding very difficult. The most ancient known fossil copepod is a fish parasite, dating from the Lower Cretaceous, about 110 - 120 MY BP (Huys and Boxshall, 1991).
The classification of copepods at all taxonomic levels has been more hypothetic and pragmatic than natural and subject to modifications.
As the free forms present variable structures, but relatively comparable, for the parasitic forms this is not the same. These forms show an extraordinary differentiation and reduction of morphologic features (Yamaguti, 1963; Kabata, 1979; Raibaut, 1996) so that it is extremely difficult to distinguish joint characters for this group.
The order of Copepoda appears for the first time in l'Histoire naturelle des Crustacés by Henri Milne Edwards in 1834. He considers here only the free forms, belonging to the sub-class Maxillidea, at the same level as the Ostracoda, regrouped in the Entomostracea, while the parasitic forms, sucking or swimming parasites, were placed in two orders: Siphonostoma and Lernéens.
The classifications of Zenker (1854), Thorell (1859), Claus (1863) approach the free forms and the parasites. Two sub-orders render account: the Eucopepoda (or Copepods) and the Parasita (or Siphonostoma).
The large divisions, founded by Thorell, and afterwards Giesbrecht (1892), are synthesised partially by G.O. Sars (1901-1918) and utilised until 1979 when Kabata (in Parasitic Copepoda of British Fishes) reconnects the parasitic forms to the free forms, and so breaks an artificial dichotomy. According to this author, the considerable systematic cuts and the systems, proposed initially by Thorell and Giesbrecht, are not to be abandoned. So, the two branches Gymnoplea and Podoplea correspond to a major event in the course of the evolution of copepods (in relation to the initial habitat of the group and the way of locomotion).
According to these authors, the chosen taxonomic level is variable as one considers the Copepods as a class or sub-class. Equally the number of orders (or sub-orders) varies according to the authors, especially concerning the parasitic and semi-parasitic forms.

Un consensus sur le schéma évolutif est désormais établi, bien synthétisé in Copepod Evolution par Huys et Boxshall (1991, chapter 4, p.371-416), et qui coïncide avec le schéma phylogénétique donné par Ho (1990).
Pour Huys et Boxshall, la sous-classe Copepoda comprend deux infra-classes: Progymnoplea (avec l'ordre des Platycopioida) et Neocopepoda comportant les deux super-ordres: Gymnoplea (avec l'ordre des Calanoida) et Podoplea (avec les ordres: Misophrioida, Gelyelloida, Harpacticoida, Mormonilloida, Siphonostomatoida, Monstrilloida, Cyclopoida, Poecilostomatoida, [ Thaumatopsylloida = Cyclopoida ].

Parmi ces dix ordres, cinq présentent une grande importance numérique. Certains ne renferment que des formes libres: Calanoida, Harpacticoida (avec rare cas de commensalisme), d'autres sont dans leur immense majorité commensaux et parasites, mais montrent quelques formes libres: Siphonostomatoida, Cyclopoida et Poecilostomatoida, un est totalement parasite interne à l'état juvénile et libre uniquement à l'état adulte ou/et juvéniles: Monstrilloida. Les quatre ordre restant: Platycopioida, Misophrioida, Mormonilloida et Gellyelloida, sont de moindre importance à l'exception de leur phylogénie, présents dans le milieu marin, sauf le dernier dans les eaux souterraines.

Ce sont dans les formes libres marines que s'expriment le plus nettement les caractères fondamentaux des Copépodes. Le genre marin Calanus (Calanoida) avec l'espèce Calanus finmarchicus (Gunnerus,1770) est pris généralement comme modèle, bien qu'il ne représente pas la forme la plus primitive. Il convient de noter que les terminologies usitées ont varié selon les auteurs, et par conséquent dans les faunes (Cf. Kabata, 1979, p.15; in Monoculus, 1982 (5):15-20; 1983 (6): 15; 1984 (8): 4-9; 1985 (10): 15-23; Huys et Boxshall, 1991: 19-20, 455-459).


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Razouls C., de Bovée F., Kouwenberg J. et Desreumaux N., 2005-2017. - Diversity and Geographic Distribution of Marine Planktonic Copepods. Available at http://copepodes.obs-banyuls.fr/en 
[Accessed September 26, 2017]

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