The functional morphology of stomatopod Crustacea

Abstract

A comparative study has been made of the mouthparts, mandibular mechanism, feeding mechanism and proventricular functional morphology of stomatopods. The mandibles have well developed, cusped, molar processes that extend into the proventriculus. In A. laevis , the mandibles retain a near-vertical axis of swing and a promotor-remotor rolling action. The mandibular musculature is basically similar to those of Chirocepalus, Anaspide, Paranaspides and Hemimysis but is uniquely modified for mastication. Functionally important differences in the stomatopod mandibular arrangement include the replacement of the transverse mandibular tendon with an extensive endophragmal bridge and the enlargement of muscles 4, 5 b and 6 (Manton’s terminology). The persistence in stomatopods of well developed molar processes and a primitive musculature is related to the use of the molar processes as the major masticatory structures within the proventriculus. Prior to ingestion, food is held between the incisor processes of the mandibles and torn apart by the maxillipeds. These fragments are passed into the proventriculus by the rolling action of the mandibles in combination with anterior movements of the labrum. Mastication is achieved by the powerful promotor-remotor rolling actions of the mandibular molar processes operated by enlarged muscles 3, 5 b and 6. Muscles 4, 5 a and 5 c are also large and provide strong transverse adduction of the incisor processes in the absence of a wide mandibular gape. Transverse movements of the incisor processes grip the food; they do not masticate it. Comparisons of the diet and structure of the mouthparts of A. laevis with those of Anchisquilla fasciata, Oratosquilla nepa, Harpissquilla stephensuni, Odontodactylus cultrifer and Gonodactylus graphurus indicate differences in trophic specialization both within and between the families Squillidae and Gonodactylidae. Feeding in stomatopods is subdivided into three phases: prey-capture by the raptorial limbs; manipulation by the third, fourth and fifth maxillipeds; and ingestion. Morphological and functional differences are associated with the ‘spearing’ and ‘smashing’ mechanisms of prey-capture in squillids and gonodactylids respectively. Similarly, the degree of food manipulation and associated movements of the maxillipeds differ between the two families. The palaeontological, ontogenetic and morphological evidence concerning the structure and function of feeding limbs in extinct and extant hoplocaridans is assessed. There is no evidence for a filter-feeding ancestor of the Hoplocarida. The lack of specialized endites or exopods in extinct and extant forms, the simple setal structure and arrangement in larval and adult stomatopods and a consideration of functionally possible intermediate forms indicate that stomatopods probably arose from simple raptatory ancestors. The anatomy and function of the proventriculus of A. laevis is described in detail. The cardiac stomach lacks masticatory ossicles. Mastication is achieved by the actions of the molar processes of the mandibles together with contractions of the gastric mill. The breakdown of food is aided by digestive juices pumped into the cardiac stomach from the digestive gland. The posterior cardiac plate is a complex filtratory structure through which all macerated food passes before entering the pyloric stomach. The dorsal pyloric stomach is vestigial and does not provide direct communication with the midgut. Finely suspended material from the cardiac stomach flows through the ampullae directly into the digestive gland. This posterior flow is a result of contractions of gastric muscles investing the wall of the cardiac stomach. Although partial filtration of material occurs during posterior flow, the material in both the upper and lower ampullary chambers is admixed in the postampullary chamber before passage into the digestive gland. The ampullae act primarily as a mechanism to filter digestive fluids flowing forwards from the digestive gland and to pump these into the cardiac stomach. Partially digested food particles are thus prevented from passing anteriorly into the cardiac stomach. The digestive cycle from ingestion to defaecation is phasic, characterized by discrete sequences of ampullary forward pumping, posterior flow from the cardiac stomach to the digestive glands and transfer of unassimilated particles into the midgut. The entire process occupies between 24 and 48 h in experimental animals. Indigestible fragments stored in the folds of the cardiac stomach are regurgitated when all digestible material has been pumped into the digestive gland. The structure and function of the proventriculus in the Hoplocarida is uniquely different from those of other Malacostraca. A.fasciata, 0 . nepa, H. stephnoi, 0 . cultrifer and G. graphurus are generally similar in pro ventricular structure to A. laevis. Minor differences in the structure of the ossicles of the cardiac stomach, cuticular processes and relative proportions of the cardiac and pyloric regions are related to trophic specializations and the size of the animal. Histological, histochemical and transmission electron microscope investigations of the digestive gland and midgut of A. laevis indicate that the digestive gland is the sole source of digestive enzymes and the major site of absorption and storage of the products of digestion. E-, R- and B-cells are present in the epithelial lining of the digestive gland. No F-cells were found. Secretion is holocrine, correlating the intermittent feeding and digestive cycle in stomatopods. The diagnostic characters of the Eumalacostraca and the available information on hoplocaridan evolutionary relationships are reviewed in the light of the information obtained in the present study. The removal of the Hoplocarida from the Eumalacostraca is supported and a polyphyletic origin of the Phyllocarida, Hoplocarida and Eumalacostraca is proposed.