The Comparative Anatomy of Two Copepods, a Predatory Calanoid and a Particle-Feeding Mormonilloid

G. A. Boxshall

Abstract

The skeletomusculature of a large predatory calanoid, Euaugaptilus placitus, is described and compared with that of a small, particle-feeding mormonilloid, Mormonilla phasma. The comparisons are extended to other copepod groups for which data are available, to identify any general patterns in copepod musculature. Anatomy has been related to presumed function wherever possible, and functional interpretations are offered of the feeding and swimming apparatus in both species. The trunk muscles of Euaugaptilus and Mormonilla conform to the typical copepod pattern of paired dorsal and ventral bundles but Euaugaptilus lacks the oblique muscles that originate on the postmaxillary apodemes in Mormonilla and other copepods. The prosome-urosome joint has a similar structure in both genera but the joints are not homologous because Euaugaptilus is a gymnoplean in which the prosome-urosome division occurs between thoracic somites 6 and 7 whereas in Mormonilla, a podoplean, it occurs between somites 5 and 6. Differentiation of the prosome-urosome joint during ontogeny and the development of trunk tagmosis are described. The difference between Podoplea and Gymnoplea is merely that these processes are completed by the second copepodid stage in the former, by the third in the latter. It is concluded that the primary factor affecting trunk tagmosis is the evolution of an efficient metasome, specialized for rapid swimming movements. Differences in metasome composition between gymnopleans and podopleans may be related to behavioural differences. The former jump by using their swimming legs primarily as an escape reaction whereas normal swimming in the latter involves repeated jumping movements of the swimming legs. The musculature of the cephalosomic limbs of Euaugaptilus is complex. Each limb, especially those involved in prey capture and manipulation, has the ability to perform several roles and it is this multiplicity of function that explains the relative complexity of the musculature. The ventral cephalic tendon system is correspondingly elaborate and additional apodemes, anterior to the mandibles and medial to the maxillae, are present in Euaugaptilus. Mormonilla has secondarily reduced musculature, especially in the cephalosome. It is a specialized particle-feeder but each limb performs relatively simple movements and has a restricted range of functions. The numbers of extrinsic limb muscles are reduced and the posterior ventral cephalic tendon is lost, resulting in a change in site of origin for some remaining muscles. Interpretations of the segmental composition of the mouthparts are summarized. Lack of evidence has made these highly subjective. A common pattern of homologies is established based on their musculature, which allows the coxa-basis joint to be identified as a reference point. It is concluded that the protopod of the postmandibular limbs (maxillules, maxillae and maxillipeds) primitively comprised three segments but that in most extant copepods the praecoxa and coxa are partly or completely fused, forming a syncoxa. There is no evidence of a praecoxa in the antennae and mandibles, which both have a protopod comprising coxa and basis only. The copepod antennule is primitively uniramous and multisegmented. Reports of a vestigial second ramus in some siphonostomatoids are examined and reinterpreted. Recent studies of planktonic feeding mechanisms and increased knowledge of the dominance of viscous forces in the environment experienced by copepods have led to the generation of a new model of copepod feeding. This is applied to the predatory behaviour of Euaugaptilus and to the feeding of Mormonilla on suspended particles. Fine scale adaptations of the feeding appendages, such as the button setae of Euaugaptilus and the interlocking setae of the filter basket walls of Mormonilla, are also described. The gross anatomy of the gut is described for both genera and compared with that of a range of other copepods. The skeletomusculature of the swimming legs is described. Both Euaugaptilus and Mormonilla display the characteristic copepod patterns of extrinsic and intrinsic muscles. Fine scale adaptations of the legs are also examined. Euaugaptilus feathers its oar-like rami so that they form a median longitudinal keel during the recovery stroke. Mormonilla closes up its rami and flexes its legs posteriorly during the recovery stroke, as does Euaugaptilus also, but is unable to feather its leg rami in the same way. Data on the ontogeny of the musculature during the nauplius stages are summarized for representatives of the Harpacticoida, Cyclopoida and Calanoida. There is no clear indication as to which of these taxa is the most primitive but the Calanoida appears to exhibit the most regular anamorphic developmental sequence.