Local Adaptation of the Blue Mussel (Mytilus galloprovincialis) in Southern New Zealand

Abstract

Mussels are key organisms in many shallow coastal ecosystems, acting as benthic-pelagic couplers, providing habitat for other organisms, while their biogeochemical functions of improving water clarity and bio- deposition help maintain a healthy ecosystem. Despite their abundance and ecological importance, few studies have investigated the blue mussel, Mytilus galloprovincialis, in New Zealand. This study aims to fill a knowledge gap in this species of crucial information such as investigating the reproductive cycle and spawning periods of this species, and determining changes in biochemical composition throughout the reproductive cycle. This study compares M. galloprovincialis populations from two locations, the entrance of Otago Harbour (Aramoana) and Timaru Harbour, enabling spatial variation between M. galloprovincialis populations in southern New Zealand to be made.

The reproductive cycle of M. galloprovincialis was examined over a twelve-month period between July 2017 and June 2018 at both Aramoana and Timaru. The reproductive cycle was determined by quantifying tissue indices and histological examination of the gonad. The reproductive cycles of M. galloprovincialis from Aramoana and Timaru were found to be similar, in that spawning was observed in summer. However, a winter spawning event was also observed at Aramoana which was not observed at Timaru. This observed difference in the reproductive cycle between the two locations is most likely due to differences in environmental conditions. Water temperatures and day length were very similar at both sites, suggesting that food availability may be playing a significant role in the observed differences between the populations.

Whilst examining the reproductive cycle, biochemical composition of the gonad and viscera (all other soft tissues homogenised) was investigated over the same twelve month period between July 2017 and June 2018 at both Aramoana and Timaru. Protein, carbohydrate, and lipid content were determined for all mussels sampled during the twelve month period, enabling spatial and temporal differences to be identified between Aramoana and Timaru. Variation in the timing of accumulation and depletion of biochemical reserves was observed between the two sites. Carbohydrate content was found to be closely linked with observed changes in chlorophyll-a concentration, while variation in protein and lipid content was found to be more dependent on the reproductive cycle. The variation in periods of accumulation and depletion of biochemical components observed between the two sites, is most likely due to the observed variation in chlorophyll-a content between the two locations.

This study is the first to investigate the reproductive cycle of M. galloprovincialis in southern New Zealand, with key differences in the reproductive ecology between M. galloprovincialis from Aramoana and Timaru being identified. The observed spatial variation in this study highlights the need for more M. galloprovincialis populations to be investigated in detail across small spatial scales, enabling effective management decisions to be made for this species.