The bioaccumulation of metals and the induction of moulting in the Blue Swimmer Crab, Portunus pelagicus (Linnaeus 1766)

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This thesis presents work outlining the development of the Blue Swimmer Crab (Portunus pelagicus) as a biomonitor of available cadmium and also presents a proposed method for the production of “soft-shelled crabs” utilising P. pelagicus. The first experiments established that the Blue Swimmer Crab accumulates only cadmium in the hepatopancreas after being presented elevated levels of the metals Cd, Cu, Zn, As, Fe and Al via a food source, the mussel Trychomya hirsuta. Over eight weeks, crabs were fed a controlled diet to determine the accumulation of metals. Significant (p = 0.05) Cd accumulation was detected after four weeks of feeding. Food mussels were sourced from either “contaminated” or “uncontaminated” sites. Mussels from uncontaminated sites had an average Cd level of 0.07 µg/g wet weight and contaminated mussels had an average of 5.2 µg/g wet weight. Having established that the crab accumulated cadmium it was then necessary to determine the animal’s residency in a given area for it to reflect the available cadmium associated with a particular spatial scale. To do this it was necessary to test and develop a suitable tag for use in population studies. When applied as tested the standard “T” bar anchor tag (Hallprint Australia TBA-1, TBA-2), similar to the Floy anchor tag, was not successful for tagging P. pelagicus. A modified “T” Bar anchor tag, with the labeled barrel extending almost the complete length of the tag, was developed and data presented which show this tag to be superior for use with P. pelagicus. The development of this tag then allowed the conduct of a series of population studies to assess the residence of P. pelagicus in estuarine areas. The distribution of P. pelagicus was found to equate with depth. This distribution results in a series of geographically distinct populations in the small bays along the Cowan Creek estuary during summer. Mark-recapture assessments in one of these bays, using the Weighted Mean Method over four consecutive capture release days, on two assessment periods, gave population estimates of 26.0 (±14.9) and 15.2 (±7.6). On two other sampling occasions during winter no crabs were captured suggesting the animals either move to deeper waters during colder months or lower their metabolism and remain in the sediment for these periods. During the population assessment work late stage berried females in the study area raised the question of female crab movements relating to the release of egg masses. An initial survey of the plankton for P. pelagicus larvae suggested that the life cycle could be similar to other swimming crabs such as the Blue Crab, Callinectes sapidus, which moves to the mouths of estuaries and releases larvae on the out going tide. This information casts some doubt on the residency of berried females and their use as a biomonitor of available cadmium. Previous work had established that this animal could accumulate cadmium and was at least resident on a scale of estuaries during summer months; the organism was then employed as a biomonitor of environmental cadmium levels. By examining the level of cadmium accumulation in the hepatopancreas of P. pelagicus, a number of New South Wales estuaries were assessed for potential contamination by this metal. Crabs assessed from Lake Macquarie, on the New South Wales central coast, were found to be significantly (p<0.05) higher in cadmium in the hepatopancreas than levels found in crabs in other estuaries. This result is consistent with other studies which reflect metal contamination of the Lake by a number of industrial and domestic sources. Thus proving the utility of the Blue Swimmer Crab as a biomonitor of available cadmium in estuaries. Potential methods for the production of soft-shelled crabs were trialled using P. pelagicus. It was found that crabs held physically separated in a single aquaria, with both eyestalks ablated, resulted in the highest percentage of crabs moulting per unit time, (mean of 48% as opposed to 20% for controls). Crabs held as individuals, and completely isolated from others, again with both eyestalks ablated, resulted in the fastest mean days to moult when compared to crabs held in groups with double eyestalk ablation (19.0 days +/- 2.4). The injection of moulting hormone, 300C temperatures, single eyestalk ablation and male only treatments were found to have little effect on the initiation of moulting in this species. A method for the treatment and holding of P. pelagicus for the production of softshelled crabs is proposed.
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