Abstract
Paphies ventricosa is a large (up to 150 mm shell length) surf clam endemic to New Zealand, with a geographically patchy distribution. Using scanning electron microscopy and light microscopy, its fertilization, embryonic and larval development were observed at three culturing temperatures (12, 16 and 20 °C). The progress of development follows that previously described for the family Mesodesmatidae, with P. ventricosa having a small egg (63–70 µm), with a 83–102 µm trochophore stage observed at 15 h, and a 100 µm D-veliger larva observed at 22 h at 12 and 16°C, and 37 h at 20 °C. At 20 °C, the pediveliger larval stage was reached by 31 d. While the morphology of the embryonic and larval stages of P. ventricosa is typical for bivalves, we show that in this species the shell field invagination occurs in the gastrula stage and that the expansion of the dorsal shell field occurs during gastrulation, with the early trochophore having a well-developed shell field that has a clearly defined axial line between the two shell lobes. The growth of P. ventricosa larvae cultured at 12, 16 or 20 °C over 39, 33 and 31 d respectively, was faster at warmer temperatures. Using the temperature quotient Q10 at day 27 to quantify the response to temperature, values of Q10 = 1.82 for the range 12–16 °C and Q10 = 2.33 for the range 16–20 °C were calculated. Larval shape was not temperature dependent, suggesting that the smaller larvae found at colder temperatures reflect a slowing of larval development, rather than physiological damage by temperature resulting in abnormal larval development.