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Population biology of the sand dollar, Peronella lesueuri, in Cockburn Sound, southwest Australia

Yeo, Sharon (2013) Population biology of the sand dollar, Peronella lesueuri, in Cockburn Sound, southwest Australia. PhD thesis, Murdoch University.

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Peronella lesueuri, commonly known as the pink sand dollar, is an irregular echinoid that can be found in the sandy sediment substrates of Cockburn Sound, southwest Australia. The numbers in which they are found indicate that they are likely to be an ecologically significant species. This is the first study of the biology and ecology of this species.

A survey of current spatial distribution and abundances of Peronella lesueuri populations at a range of sites within Cockburn Sound shows the presence of the sand dollars on the shallower, sandy sills that surround the Sound. P. lesueuri was notably absent from the deeper central basin of Cockburn Sound, where silt is a major component of the sediment. The population densities of P. lesueuri ranged from 0.3 to 2.5 individuals m-2 at sites no deeper than ten metres. Mean test lengths were larger at sites less than five metres in depth. Sediment grain size preference and food availability were likely factors that determined the spatial distribution of the sand dollars in Cockburn Sound.

Detailed monthly sampling for population demographics was conducted at Jervoise Bay over a 23-month period in which significant temporal variations in the densities of the P. lesueuri were recorded. Two spikes in population densities in December 2010 and January 2011 were indicative of clear annual recruitment events. The spike in densities caused by recruitment, however, were quick to return to what appeared to be the stable population density of 0.13 – 0.3 individuals m-2.

This study showed that the P. lesueuri reproductive cycle is annual, with peak spawning in summer. Gametogenesis was recorded in spring, spawning in summer, spent gonads in autumn and gonads recovered during winter. The gametogenic cycle of P. lesueuri underwent five distinct morphological stages; Recovery, Growth, Mature, Partially Spawned and Spent, similar to those described in other echinoderm gametogenic cycles.

In the recovery stage, nutritive phagocytes filled the lumen of both the testes and ovaries. Primary gametocytes (Stage 1 gametes) were present large numbers. Growing gonads contained increasing numbers of secondary gametocytes. Mature gametes are present in some gonads while nutritive phagocytes decreased in volume. In fully mature male gonads, lumens were densely packed with spermatozoa. Two variants of the mature stage were observed in the ovaries; one was packed full of ova with few oocytes, the other contained oocytes in all stages of development. Some female sand dollars produced only one cohort of ova which are spawned out once all the ova are released, while others can produce several cohorts of ova throughout the spawning period. Partially spawned gonads contained large numbers of mature gametes although spaces left by spawned gametes were observed. Lumens of spent gonads were mostly empty, although unspawned gametes in various stages of degeneration were occasionally present. Unspawned gametes eventually broke down and were incorporated into the nutritive phagocytes. The gametogenic stages were highly synchronous between sexes. Juveniles attained sexual maturity at between one and a half and two years of age.

Size frequency distributions as well as density variation over a 23-month period indicated that Peronella lesueuri recruits annually. Juveniles appeared in the adult population around May/June, five to six months after the start of the spawning period. Annual recruitment strength was variable and juvenile mortality was high. Up to 88% juvenile mortality was measured in the first year alone.

Pooled length-at-age data derived from the 2009 and 2010 cohorts of recruits was combined with length-at-age data for large “adult” sand dollars obtained from growth zone counts to construct a scatter plot to fit a growth curve. Growth in P. lesueuri is sigmoidal and is best described by the Richards’ Growth Curve. Juvenile P. lesueuri grow quickly, with a maximum growth rate of 60 mm yr-1 at two years of age. Intermediate-sized sand dollars were scarcely encountered. The rapid growth rate of sand dollars of intermediate size predicted by the Richards’ growth curve coupled with high juvenile mortality provides an explanation for the scarcity of sand dollars between 50 and 100 mm. The growth rate slowed to close to zero at approximately two and a half years of age, and this may be associated with the attainment of sexual maturity. Maximum size in the population studied was 182 mm. The maximum life expectancy is approximately five years.

This study indicated that P. lesueuri has diurnal and seasonal patterns of activity throughout the year, with greater movement rates in summer (mean of 5.3 cm hr-1, day; 3.9 cm hr-1, night) than in the winter (mean of 2.7 cm hr-1, day; 2.0 cm hr-1, night). Seasonal changes in temperature and physiological requirements by the sand dollar are the most likely reason for the seasonal differences; however reasons for diurnal movement variation were unclear. Direction of movement was found to be random at both times of the year. Based on the movement rates, and at a density estimate of 0.5 sand dollars per m-2, it is estimated that the sand dollars can rework the entire area of the sediments in the habitats they occupy in approximately 20 days.

This study provided the first histological description of the gametogenic cycle in Peronella lesueuri and established that the reproductive cycle is annual. The growth rate and life expectancy of P. lesueuri was also determined. The study of diurnal and seasonal patterns of activity in P. lesueuri indicated that it was ecologically significant as a sediment bioturbator. While further research is required to fully assess the biological and ecological significance of P. lesueuri in Cockburn Sound, the densities in which P. lesueuri is found on the shallow sediment bottoms imply that they have a potentially important role as an agent in mediating biogeochemical processes in the surface sediments.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Veterinary and Life Sciences
Supervisor(s): van Keulen, Mike and Keesing, John
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