On the anatomy of the misophrioid copepods, with special reference to Benthomisophria palliata Sars

Abstract

The misophrioids are a small but phylogenetically important order of copepods, comprising only three species, which exhibit a mosaic of characters drawn from both gymnoplean and podoplean lineages. The skeletomuscular system of Benthomisophria palliata is described in detail and comparative observations are made on B. cornuta and Misophria pallida . All were found to possess a carapace-like posterior extension of the cephalosome which completely encloses, both dorsally and laterally, the first pedigerous somite beneath. This structure is a posterior outgrowth of the maxilliped-bearing somite and is not derived from the tergite of the pedigerous somite that it encloses. It may represent a modification of the somitic hyaline frill common to many copepods. Its presence is apparently associated with the ability of Benthomisophria to gorge itself until virtually all the free space within the prosome is occupied by the gut. The gut contents of B. palliata reveal that it feeds on relatively large food particles. The probable feeding mechanism is inferred from the ranges of movement possible at each of the joints of the feeding appendages and from their musculature. The probable swimming mechanism is also inferred from similar data for the thoracic limbs. The skeletomusculature of other copepod groups is compared with that of Benthomisophria . The extrinsic muscles of the cephalic appendages and both the extrinsic and intrinsic muscles of the swimming legs were found to be relatively uniform throughout the Copepoda. The longitudinal trunk muscles and intrinsic muscles of the cephalic appendages were found to be more subject to modification during adaptive radiation in habit and feeding strategy respectively. Both species of Benthomisophria exhibit ornamentation of the integument. This is described and a strengthening role is attributed to the intricate system of lamellae and ridges present. Laterally on the cephalosome of Benthomisophria species are areas of funnel-shaped cone organs standing erect from the body surface and bearing a spherical globule of secretion distally. They are positioned so that the long setae of the reflexed antenna and mandibular palp sweep over them. It is concluded that the secretion is spread over the surface of the carapace-like structure by the setae. Its function may be protective. The gross anatomy of the other organ systems is described. The highly distensible gut has large lateral caecae which can expand to accommodate large amounts of food. The musculature of the hindgut is described, as is the postulated sequence of events during defaecation. Adult B. palliata possess paired antennary glands as the functional excretory organs. The maxillary gland is absent. In other copepod groups the maxillary gland is the functional excretory organ of the adult. The central nervous system is described. The complete absence of the nauplius eye throughout the life cycle of misophrioids is noted. The heart is absent from B. palliata . In Misophria pallida it is a barrel-shaped structure about 40 µm long, with weakly muscular walls. It has anterior and posterior ostia only and is suspended from the dorsal body wall by short muscle fibres. The reproductive system is described in both sexes. Male and female have paired gonads, ducts and genital openings. The oviducts open into cavities (genital antra) within the genital somite, which are closed off externally by the plate formed from the fused sixth legs. The male possesses relatively simple vasa deferentia which are poorly differentiated into functional zones. The phylogenetic position of the misophrioids is discussed. They exhibit an unusual combination of ancestral, unique and convergent characters which makes assessment of their affinities difficult. It is tentatively suggested that the Misophrioida, which merits ordinal rank, is more closely related to the Harpacticoida than to any other order.