Studies of the age, growth and shell increment patterns in the New Zealand cockle (Austrovenus stutchburyi)

Abstract

Austrovenus stutchburyi were collected (n=1100) from Papanui Inlet on the Otago Peninsula and half were marked using the fluorescent dye, calcein, and half were marked by exposing them to an abrupt decrease in temperature which thermally stressed the animals. A proportion of the cockles (n=660) were returned to cages in Papa.nui Inlet while the remainder (n=440) were maintained in tanks at the Portobello Marine Laboratory. Once a month, ten marked cockles from each treatment from each treatment from both Papanui Inlet and the Laboratory were collected and sacrificed by immersion in hot fresh water. The thermal stress method failed to leave a discernible mark in the shell and, therefore, these cockles were used for allometric measurement only. The cockles that had been treated with calcein were used for the growth analysis. These shells were cleaned, measured and internal increments examined using the thin section method. Under an ultraviolet light source, calcein was found to be incorporated as a green line in the shell. Measurements from the calcein line to the shell edge allowed a growth rate to be calculated. Thin sectioning of the shells revealed that there are two types of growth increment: larger macro-increments and smaller micro-increments. The calcein mark allowed the periodicity of these to be examined. The results indicate that the macro-increments found in the shell are annual in nature. Analysis of the periodicity of the micro-increments revealed that during the warm months of summer the periodicity of these increments is tidal. However, this periodicity breaks down over winter. In the summer months there was a relationship between the width of micro-increments and the spring/neap tidal cycle, with wider, more complex, micro-increments being laid down during spring tides and narrow, simple micro-increments being laid down during neap tides. Seasonality of growth was determined by examining the marginal increment on the shells and by comparing the actual growth of samples collected in summer and winter. The results support the hypothesis that Austrovenus siuichburui grows slowly in winter and faster in summer. There is also a slowing of allometric growth in summer that may be related to the physiological stress of gametogenesis and spawning. Cockles maintained in the laboratory showed little or no growth for the duration of this study. This may have been due to inadequate or inappropriate food.