Feeding and burrowing in a North Island New Zealand population of the estuarine mud crab, Helice crassa

Abstract

Benthic organisms interact with and modify the sediment in which they live by their feeding and burrowing activities. These two activities result in bioturbation, a major ecological process in soft sediment habitats. Bioturbators themselves can be affected if they live in polluted estuaries, as their feeding and burrowing may result in accumulation of contaminants in their tissues, which might ultimately affect their behaviour or physiology. This thesis documents aspects of the feeding and burrowing of the mud crab Helice crassa, a common inhabitant of intertidal sediments throughout New Zealand, that relate to these two processes: bioturbation of the sediment and effects of any contaminants in that sediment. The field component of study was carried out in the mud flats of Whangateau Harbour, and represents the first comprehensive account of the behaviour of a North Island population of this crab. The temporal activity pattern of H.crassa was investigated in both field and laboratory situations. H.crassa exhibited temporal organisation for different activities according to different tides and times of the day. It is postulated that H.crassa has tidal and circadian endogenous clocks that may enable it to anticipate and prepare the sense organs for the changes that occur on its home beach. However, much of a crab's activities appear to be a response to immediate and exogenous stimuli. Of particular relevance was the finding that they were most active during daytime low tides. Fine structures of mouthparts were analysed using scanning electron microscopy to determine whether H.crassa's distribution on the shore could be explained by its ability to feed on different substrata. With the exception of a lack of setae on the inner surface of the chelae, H.crassa is well adapted to feeding on a wide range of material. It possesses the full array of setae that were identified as necessary to feed on muddy and sandy substrata at both low and high water. Foraging pattern in terms of the size and shape of home ranges was quantified with respect to different densities of crabs and different sizes of individuals. Home range size increased with crab size. The radius of the foraging area decreased with an increase in crab density. The position and size of any neighbouring crabs influenced the directions of feeding excursions; the majority of feeding excursions took place away from neighbouring crabs. The findings indicated that the shape and size of individual feeding areas of H.crassa results from intra-specific interactions and that the population has a 'despotic' distribution (i.e.they are not “free”, to move). This aspect was further tested by examining the spatial dispersion of burrows. Organic and water content of the surficial sediment samples were also analysed to determine the relationship with crab density. Dispersion patterns are also likely to be density dependent; at higher crab densities, dispersion was uniform but this changed to random at lower densities. There was no consistent relationship found either between crab density and organic content of surface sediments or crab density and water content of the sediments. The factors which determine the burrowing activity, were examined as the depth of burrows may influence the rate of bioturbation by burrowing. While the depth of burrows increased with the size of the crabs, not all burrows reached down to the water table. In addition, H.crassa possesses well-developed setae to suck up interstitial water and crabs are likely to depend on this interstitial water particularly in sandy substrata. In order to quantify the rate of bioturbation, burrow characteristics such as burrow: crab ratio, residence time in burrows, longevity of burrows and burrow architecture were investigated for all four seasons, and sediment turnover rates were then estimated. The bioturbational activity by burrowing is likely to be greatest in summer-spring (a sediment turnover rate of 126-l35%) than in autumn-winter (a sediment turnover rate 48-91 %). The seasonal values were then converted to annual estimates on both a "per crab" basis and a "per unit area" basis. These estimates of sediment turnover rate, and consequently the total amount of new surface area exposed per unit time were greater compared to published studies that have estimated the bioturbational activity of other crab species. The responses of H.crassa in terms of accumulation in the tissues and changes to burrowing activity as a result of exposure to different levels of copper, zinc and lead contaminated sediments were examined. The results indicated that H.crassa is a robust species that can regulate to a wide range of ambient zinc and copper bioavailabilities at an approximately constant level. Therefore, H.crassa is not likely to be an effective bioindicator for the generally low levels of zinc (< 300 µg Zn g-1) and copper (< 500 µg Cu g-1) that are found currently in New Zealand, estuarine sediments. However, H.crassa could be used as an indicator for higher levels of lead pollution in estuarine sediment, because they accumulated lead in proportion to the concentration of lead in the sediment. The resilient behaviour that H.crassa showed for the metal contaminated sediments suggests that H.crassa would turbate the contaminated sediments and may increase the bioavailability of the contaminants in polluted estuaries. Overall, H.crassa is an efficient bioturbator and can be considered as a pivotal species in the natural functioning of estuaries in New Zealand.