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This study examined aspects of the biology and distribution of the marine invasive bivalve Theora lubrica in the Lyttelton Basin from two distinct sampling areas. Samples were collected monthly, over a 13 month period (January 2004 and January 2005), with particular emphasis on seasonal sedimentary characteristics and contamination. The sample sites consisted of five Port sites and five Harbour sites. They were sampled using a 20 litre anchor dredge. In winter and summer, core samples from each of the sample sites were collected, and the top 1.5cm of sediment was used to obtain sediment characteristics. Sediment characteristics indicated a depositional setting, with an average grain size (diameter) of 8-10 μm, which did not vary seasonally. Analysis indicated that the sediments naturally flocculated, to the 100μm size range and were platykurtic. Organic content (OC) in the sediment in Lyttelton Port showed significant seasonal variation. OC was highest during the winter period corresponding to an increase of T. lubrica abundance. There were noticeable seasonal changes in the abundance of T. lubrica, with high winter and low summer population densities. The period for recruitment coincided with high T. lubrica densities, and the greater frequency of juveniles with shell length below 4.33 mm. Histological sections showed that spawning occurred during the summer, confirmed by the presence of high percentages of ovigerous females and mature males. Community analyses distinguished four distinct seasonal and spatial benthic populations. The populations exhibited greater total species richness during the summer (compared to the winter). T. lubrica, maintained high abundance during periods of low total species richness. Furthermore, there was evidence of a predator prey relationship between the crab Macrophthalmus hirtipes and T. lubrica. Trace metals and hydrocarbons showed seasonal and spatial variation between the Port and Harbour sediments. Only the arsenic, nickel and copper in the Port location were at concentrations likely to be toxic to the biota. No trace metals exceeded threshold limits in the Harbour. Levels of arsenic and nickel were highest in the summer and copper was at its highest concentration in the winter. However, there was little evidence to suggest that the sediment contamination affects T. lubrica or the rest of the benthic community directly. Thus, the significantly lower species richness at the Port location may be due to other influences (i.e. sediment disturbance from shipping). In conclusion T lubrica is regarded as an opportunistic marine invasive species which can inhabit areas not occupied by other marine macro-biota. T lubrica did not appear to negatively impact the other benthic species at the Lyttelton Port or Harbour sites. It provided a plentiful food source and increased species diversity in sediments that are frequently disturbed (by shipping and wave action). Furthermore, T lubrica may help reestablish native species to the Port area by filtering and processing contaminated sediments (bioturbation), potentially pioneering the way for greater species diversity in Lyttelton Port and Harbour. |
