Reproduction and larval development of the New Zealand scallop, Pecten novaezelandiae
The New Zealand scallop Pecten novaezelandiae is an important fishery. Wild scallop spat are used extensively to enhance and maintain scallop beds. The current yields of scallops have decreased by over a third in the last decade. A potential solution to increasing the scallop yield and conserving stocks are hatchery grown spat. Most studies related to P. novaezelandiae have concentrated on their ecology and the process and effects of scallop enhancement. This study aimed to fill two gaps in our knowledge of P. novaezelandiae biology by exploring the effects of diet ration on the animal’s gonad maturation and to characterise the larval development from zygote to pedivileger.
Wild scallops were collected and maintained in the laboratory for seven days and fed ad libitum. An initial sample of scallops was taken and reproductive condition was compared to that of scallops fed one of three algal diets for thirty days: The diet rations were 0.5g (low), 0.92g (medium) and 1.9g (high) of microalgae per day.
The results of the study showed there was a significant increase in the gonadalsomatic index (ANOVA; F3=49.3; p<0.05; n=91), wet gonad weight (Steel, p<0.05), egg size (ANOVA, F3=2.8; p<0.05; n=3773) and colour between the male and female portions of the gonad (p<0.05; n=117). However, colour, wet gonad weight and acini area were not significantly different after 30 days of conditioning.
Three lots of scallops were spawned to characterise the key phases of P. novaezelandiae from zygote to pedivileger. Two spawnings characterised P. novaezelandiae from gamete to D-larvae. The embryos were cultured in 900ml of fresh sea water, filtered to one micron for 3 days and sampled randomly. There was 100% mortality by day four. The third spawning saw fertilised eggs placed in a 170L conical tank for three days. The D-larvae were then transferred to six Cawthron Ultra Dense Larval rearing systems (CUDLs), from which larvae were selected every two days for imaging with Light Microscopy and Scanning Electron Microscopy.
A strong linear growth for P. novaezelandiae larvae diameter corresponded to 6.5895 × age in days + 74.337. A mean growth of 5.92 µm a day was experienced over 30 days for P. novaezelandiae larvae. A mean (±SE) clearance 65.4±2.59% of the Chlorophyll A was cleared per day over 30 days of feeding. P. novaezelandiae sperm morphology, 2.14 ±0.05 length and 61 ±0.07µm breadth, is similar to the Pecten maximus. Unknown micropores are visible on the nucleus of the sperm. P. novaezelandiae eggs are spawned with irregular shapes and experience meiosis in the first hour. A polar body emerges 15min after fertilisation. The embryogenesis of P. novaezelandiae is similar to other scallop bivalves comprising of a polar lobed, cleavage, cell division, blastula, gastrula and trochophore phases. D-larvae emerge after three days. However, an umbo veliger phase is not obvious and only emerges before around when an eyespot is visible around 23 days post fertilisation [PF]. Approximately 30 days PF a pedivileger emerges.
This is the first study in broodstock P. novaezelandiae, and which characterises key growth phases. It is also the first time semi quantitative colour measures have been conducted on the diet concentration of the gonad. Spawning was found to be a barrier to successfully spawning large quantities of scallops.