Digital Keys to the Calanoid Copepods

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Key to the genera of the family Megacalanidae

Author: Janet Bradford-Grieve

The Megacalanidae are large (9-17 mm long), red pigmented, bathy-, abysso- and even hadopelagic copepods with a wet weight up to 0.09 g (Thuesen et al. 1998; Bradford-Grieve et al. 201x). Bradford-Grieve et al. (201x) have revised this family. Megacalanids are widespread and sparsely distributed in the deep sea and some species have been taken only rarely (Bradford-Grieve et al. 201x).

Their vertical distribution may vary with developmental stage (Sewell, 1947; Gueredrat & Friess 1971) but few studies have recorded the developmental stage and precise depth of capture. Little is known about reproduction in the Megacalanidae. Megacalanids seem to lay large yolky eggs freely into the water (Bradford-Grieve pers. obs.) and the egg hatches into a large nauplius larva: in Megacalanus the nauplius stage VI is xx mm long (Bradford-Grieve et al. 201x). The Megacalanidae are generally thought to be carnivorous (Arashkevich, 1969) and have some of the modifications to mouthparts necessary to make this mode of feeding successful. The mandible has a strongly sclerotized gnathobase with the ventral tooth stronger and set apart from the other teeth. Maxillae and the maxillipeds are modified in some genera. The long, curled terminal setae of the maxillae in Bathycalanus suggest they function as tangling devices. This observation led Bradford-Grieve et al. (201x) to deduce that detrital feeding on marine snow may be the feeding niche of Bathycalanus. The terminal setae of the maxillipeds of Elenacalanus are spine-like and reduced in number suggesting a more raptorial feeding mode. The megacalanid copepod body is divided into two major parts: the prosome and the narrower urosome with the major articulation occurring between the fifth pedigerous and the genital somite (gymnoplean position) (Huys and Boxshall, 1991). The prosome encompasses the cephalosome and 5 pedigerous somites. The urosome (Ur) is formed from 4 free somites in female and 5 free somites in male and is terminated by a pair of caudal rami. The female genital double-somite is composed of the fused genital somite and the first abdominal somite. The rostrum (R) is composed fused to the cephalosome and may be bifurcate or have a pair of rostral filaments attached. The paired limbs of the cephalosome are, from anterior to posterior, antenna 1 (antennule), antenna 2 (antenna), mandible (Mn), maxilla 1 (maxillule – Mx1), maxilla 2 (maxilla – Mx2), and maxillipeds (Mxp). When detail of the morphology of the limbs is referred to in the key, the part concerned is indicated by a red box. Swimming legs 1-5 (P1-5) are biramous with a basic 3-segmented plan for each ramus and branches of each pair of legs are joined by an intercoxal sclerite. The fifth legs are sexually dimorphic. Further information may be obtained from publications such as Bradford-Grieve et al. 201X), Mauchline (1998) and Huys and Boxshall (1991) and the references therein.