Morphological character changes through decapodid-stage larva and juveniles in the ghost shrimp Nihonotrypaea harmandi from western Kyushu, Japan: Clues for inferring pre- and post-settlement states and processes

Abstract

Some decapod crustaceans release larvae from estuarine or coastal shores to coastal oceans. Decapodids (last-stage larvae) return home, settle, and metamorphose into juveniles I, which go through further instars. Several morphological characters degenerate or develop in response to lifestyle transitions. Using a burrow-dwelling callianassid shrimp, Nihonotrypaea harmandi, inhabiting an intertidal sandflat in a coastal boundary layer adjacent to coastal ocean, this study described morphological changes and drew inferences about states and processes in its early life history. Decapodid and juveniles were differentiated by linea thalassinica on carapace. In the laboratory, the decapodid stage lasted 3–6d. Given choice between sandy sediments from coastal ocean bed and adult habitat, decapodids exhibited no preference, suggesting broad receptiveness. The shortest durations of the juvenile-I, -II, and -III were estimated at 6d each. Starved decapodids normally metamorphosed into juveniles I, showing secondary lecithotrophy. The non-feeding state could be extended through the juvenile I (and possibly juvenile II), where the mean total lengths of shrimps reared in groups with food (diatoms) remained about 4.6mm. Post-settlement decapodids reared individually resumed feeding. The rostrum lengths in decapodids were ≥0.36mm on the initial two dates (Days 0 and 1), after which the mean values rapidly reduced to 0.30–0.36mm on Day 2 and below 0.30mm thereafter. The pereiopod exopods disappeared through the decapodid stage from four pairs to almost zero by Day 4. In the coastal ocean, no juveniles occurred, and almost all decapodids had rostrum lengths ≥0.30mm and 3 or 4 pereiopod exopods, suggesting their pre-settlement state. Of the smallest shrimps collected on the sandflat for their subsequent rearing there, 74% were estimated to be Day-0 or -1 decapodids based on their rostrum lengths. This and the laboratory experimental results suggest (1) the acquisition of competence for settlement by newly-metamorphosed decapodids while in the coastal ocean, which was nevertheless realized there and (2) their rapid transport by flood tidal currents from coastal ocean to sandflat. The newly-settled decapodids grew steadily at 0.2mmd−1 in total length. The reared juveniles reached the smallest adult size in 80d. The uropod expod changed from elliptical to sub-circular in shape markedly around the termination of the juvenile II or III, suggesting the acquisition of ventilating function for benthic life. Overall, the post-settlement shrimps can be staged by total-length ranges as 4–5.5mm for decapodids, 5.5–10mm for juveniles, and 10–20mm for sub-adults.