Population Biology of Mussels (Aulacomya Maoriana, Mytilus Galloprovincialis and Perna Canaliculus) From Rocky Intertidal Shores in Wellington Harbour, New Zealand

Abstract

This study examines the population ecology and dynamics of three co-existing mussel species (Aulacomya maoriana, Mytilus galloprovincialis and Perna canaliculus) in Wellington Harbour, New Zealand. The present study investigates the role of multiple environmental factors and their multiple effects on the intertidal mussel population. Wellington Harbour is a complex system, supporting speciose intertidal invertebrate communities. CTD data loggers recording seawater temperature, turbidity, chlorophyll a concentration and salinity at Evans Bay, Seatoun, Matiu-Somes Island and Petone provided the environmental data. The data suggest the existence of distinct zones within Wellington Harbour, with different hydrological regimes present at each zone. Consistently high salinity (35.08 + 2.9 PSU) and chlorophyll a concentration (9.42 + 4.33 μg l-1) were found at Evans Bay, while these parameters displayed a degree of temporal variation and were significantly lower at Seatoun (31.5 + 4.17 PSU and 2.15 + 2.1 μg l-1) and Matiu-Somes Island (33.26 + 0.99 PSU and 1.23 + 1.79 μg l-1). At Petone, a site located near the Hutt river mouth, salinities were reduced (31.59 + 3.21 PSU) while chlorophyll a levels were similar to those at Matiu-Somes Island (1.64 + 1.08 μg l-1). Mean turbidity values were similar at Seatoun and Evans Bay (11.51 + 18.53 FTU and 11.89 + 5.52 FTU, respectively), with mean turbidity slightly reduced at Petone (8.20 + 11.16 FTU) and elevated at Matiu-Somes Island (15.35 + 11.12 FTU). Further, CTD data revealed similar seawater temperature at all sites, with mean values oscillating around 13 - 15ºC. The ecology of larval stages was expressed in this study by quantifying the rates at which mussel larvae settled on the experimental substrate. A year-round spawning, as well as temporal and spatial variability in mussel recruitment at four experimental sites was revealed. Evans Bay was the site with consistently higher recruitment rates but not the mean recruit numbers (721 + 879 larvae), while the highest number of recruits (9851 larvae) was recorded at Petone (1041 + 2112 larvae). Recruitment rates were lower at Seatoun (729 + 536 larvae) and Matiu-Somes Island (410 + 636 larvae). However, only at Seatoun was this variability clearly linked to the environmental conditions of water turbidity, chlorophyll a concentration, and salinity. The post-larval ecology part of this study concentrates on the condition index and gonad mass, and the degree of infestation with a parasitic pea crab Pinnotheres novaezelandiae studied at four sites. Spatial and temporal variation in condition index and gonad mass was revealed in all three species investigated, with both condition index and gonad mass of adult mussels being highest at Matiu-Somes Island (14.59 + 4.41 and 0.21 + 0.16 g), followed by Kau Point (13.47 + 6.99 and 0.17 + 0.10 g), Seatoun (13.32 + 7.79 and 0.11 + 0.10 g) and Evans Bay (11.99 + 2.78 and 0.14 + 0.14 g). Condition index was significantly correlated with gonad mass, and was highest in Aulacomya maoriana (15.85 + 9.38), followed by Perna canaliculus (12.52 + 4.39) and Mytilus galloprovincialis (11.66 + 5.91). The condition was generally reduced in mussels infested with the pea crab Pinnotheres novaezelandiae, although the overall infestation rate was low (3.28%). In order to describe the pattern of mussel community development, patches of bare rock were experimentally created in the mid-intertidal zone. Subsequently, predatorexclusion cages were set up in those areas and monitored regularly. The abundance of main groups of intertidal taxa settling on the cleared substrate was expressed in terms of percent cover, and was highest at Evans Bay (59.57 + 80.27%), lowest at Kau Point (13.96 + 26.18%) and intermediate at Seatoun (22.56 + 41.64%). However, the bottomup factors were visibly linked to the community development at Seatoun. The full cage experimental treatment provided the maximum protection from predation and desiccation, therefore the community recovery was most pronounced under this treatment. Mytilus galloprovincialis was revealed as the most competitive mussel species, in some cases able to colonise the entire available substrate and exclude other two mussel species. Further, seasonality of mussel response to wave action and desiccation was investigated. Mussel species-specific strength of attachment to the rocky substrate was expressed in kg (effectively the force) required for the mussel to be removed from the rocky substrate at shores facing south and north in Wellington Harbour. The strength of attachment was highest in Perna canaliculus (5.81 kg + 2.27), followed by Aulacomya maoriana (3.63 kg + 1.63) and Mytilus galloprovincialis (3.44 kg + 1.70). Mussel strength of attachment was generally higher at south-facing sites, due to stronger waves generated by southerly winds. In a separate experiment, in which mussels were exposed to air at six different shore levels within the intertidal zone, desiccation tolerance was highest in Mytilus galloprovincialis on the south-facing sites (LD50=0.62m and 0.87 for north- and south-facing sites, respectively), followed by Aulacomya maoriana (LD50=0.65 and 0.75m for north- and south-facing sites, respectively) and Perna canaliculus (LD50=0.20 and 0.35m for north- and south-facing sites, respectively). LD50 desiccation exposure values were lower in all three species found on the north-facing shores, indicating that mussels on those shores are less tolerant to desiccation-induced stress and therefore died more rapidly.