Abstract
Paphies ventricosa is a large surf clam endemic to New Zealand with a patchy distribution and whose populations have substantially declined during the past century owing to overfishing and habitat degradation. Poor recruitment is now evident, and therefore, understanding the larval recruitment of P. ventricosa is key to developing and implementing conservation strategies for the species. In order to identify factors driving larval recruitment in toheroa, Paphies ventricosa, from Oreti Beach, Southland, New Zealand, the southernmost known extent of the species, various studies were carried out from 2011 to 2014 in the field and the laboratory
In 2011, the reproductive cycle of P. ventricosa was examined over one year in a population at Oreti Beach. In 2012, the spatial variation in reproduction among four sites along Oreti Beach, including the site from 2011, was quantified from body indices and the histological examination of gonads. Based on changes in oocyte size, gametogenic stage and condition index, we observed a species with a primary spawning in spring and a second spawning event in late summer/autumn, with no resting phase but minimal reproductive activity over winter. Seasonal reproduction corresponded with warmer sea surface temperature and a peak in chlorophyll-a concentrations in the region. Small-scale (< 15 km) variation in the timing of spawning was also evident along Oreti Beach, with a degree of asynchrony that could affect fertilisation success in the population. These patterns may be an important consideration when identifying areas that may be considered for conserving source populations.
Using scanning electron microscopy and light microscopy, fertilisation, and embryonic and larval development were observed at three culturing temperatures (12, 16 and 20 ºC). The progress of development follows that previously described for the family Mesodesmatidae, with P. ventricosa having a small egg (63–70 mm), with an 83–102 mm trochophore stage observed at 15 h, and a 100 mm D-veliger larva observed at 22 h at 12 and 16 ºC, and 37 h at 20 ºC. At 20 ºC, 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 ºC 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 ºC and Q10 = 2.33 for the range 16–20 ºC 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.
Temperature is one of the most important environmental factors controlling development in marine invertebrates, and thus likely plays a critical in recruitment dynamics. The temperature thermal tolerance of fertilisation and early larval development in Paphies ventricosa was examined to understand the role of temperature in early larval recruitment success. Fertilisation was examined across a thermal gradient of 10.5 to 30 ºC in an aluminium heat block. Fertilisation was considered successful by microscopic observance of the breakdown of the germinal vesicle, and the appearance of the fertilisation envelope and polar body. The thermal tolerance of development was examined across a thermal gradient of 8.0 to 25.5 ºC in an aluminium heat block at 2, 15, 22, and 37 h post-fertilisation. Fixed samples were examined using light microscopy and classified into the developmental stages of unfertilised, fertilised, embryonic (2-64 cell embryos), blastula, trochophore, veliger, and abnormal. There was a significant effect of temperature on the fertilisation success which ranged from 4.6% to 46.7%. Fertilisation was > 30% successful between 16.0 and 21.0 ºC, and was successful beyond the natural temperature range of the species. P. ventricosa larva were tolerant to temperatures beyond the naturally occurring temperatures during spawning/development periods, but were most successful around 15 ºC.
While temperature is important in the recruitment of marine invertebrate larvae, feed availability is also crucial, and often thought to be more important in overall larval development. The combined effects of temperature (12, 16, and 20 ºC) and feed concentration (1:1 mixed algal diet of Tetraselmis chui and Isochrysis galbana; 1,000, 10,000, and 20,000 cells ml-1) were examined in P. ventricosa larvae over 17 days. There was found to be significant combine effect of sampling day, temperature, and feed concentration on larval shell length. By 17-d post-fertilisation, the combined effect of feed concentration on larvae in each temperature became more apparent. Unlike the results of Chapter 3 when larvae were fed a single species (T. chui) diet at 10,000 cells ml-1, larvae reared at the colder temperatures had the largest shell lengths by 17-d post-fertilisation. At 17-d fertilisation, larvae at 12 ºC grew best when fed 20,000 cells ml-1, and 16 and 20 ºC grew best at 10,000 and 1,000 cells ml-1, respectively.
Overall, the results of this research fill in many gaps in our knowledge about the life history of Paphies ventricosa. In particular, the reproduction of the local Oreti Beach population and the early larval ecology, with implications for both the northern and southern populations, have been explored in depth. This is the first study of its kind for many of its components, including the detailed microscopic (both scanning electron and light microscopy) examination and description of the early larval stages of toheroa, and the identification of the fertilisation and thermal development windows in the species. In addition, it is the first study in toheroa to examine the combined effects of temperature and feed concentration. The present study has greater implications in regards to other bivalves of similar distributions and habitats, and provides insight into the conservation and management of the species.