Functional morphology of the mouthparts and proventriculus of the rock crab Nectocarcinus tuberculosus (Decapoda: Portunidae)

Abstract

Mouthpart and proventriculus structure of the brachyuran crab Nectocarcinus tuberculosus was described by scanning electron microscopy and histology and characteristics indicative of dietary preferences were identified. A qualitative stomach content analysis was conducted to verify structural interpretations with respect to diet. The mouthparts and proventriculus of N. tuberculosus have features that are typical of macrophagous crabs and are consistent with a carnivorous diet of hard items. However, the mandibular arrangement indicates that N. tuberculosus is also adapted to ingest soft plant material and fleshy items, revealing that this crab is omnivorous which is consistent with its habitat of rocky shores and sea grass beds. Carnivorous features of the feeding apparatus include large crista dentata on the third maxillipeds that grip food items during ingestion and cuspidate setae on the second maxillipeds and first maxillae. Nectocarcinus tuberculosus has a complex arrangement of proventricular ossicles, 5-denticulated accessory teeth, and a spiny cardio-pyloric valve with rough, calcified protuberances. It has a robust, strongly calcified gastric mill with a prominent anterior cusp and 11 roughly surfaced vertical ridges on each lateral tooth specialized for crushing and grinding. In contrast to true carnivorous crabs, the mandibles in this species are symmetrically arranged, with two sharp cutting edges and no teeth, an arrangement adapted to cut softer plant-like materials and flesh. Structural interpretations with respect to diet were confirmed by stomach content analysis where items identified included gastropods, bivalves and the sea grass Posidonia australis. Functionally, the filtering system in the proventriculus of N. tuberculosus is complex. The cardiac stomach has a well developed ventral filtration system or ‘cardiac filter’ which comprises coarse and fine filters. Different modifications of setation between the distal and proximal regions of the inner valve of the pyloric filter are unique to this crab species, with an intersetule width 600 nm in the distal region and 80–100 nm in the proximal region. This suggests that some differentiation in filtration occurs between regions within the pyloric filter.