Abstract
This study set out to determine the effectiveness of tetracycline tag-recapture methods for measuring growth in sea stars, and to specifically quantify the growth in situ of the eleven-armed sea star Coscinasterias muricata. C. muricata is a large, common sea star found in shallow subtidal environments throughout coastal New Zealand. In New Zealand’s southwestern fiords, it reaches a large body size and exercises an important influence on community composition. Despite this ecological importance, many aspects of its biology remain little-known, including its post-larval growth and population biology.
Studying growth in sea stars is problematic, owing to their lack of increment bands or lines on their skeletal elements. Historically, field studies of sea star growth have focused on the analysis of size-frequency distributions, a method which, while useful, is inherently inexact, given that it requires validation of the age of cohorts, and also works entirely with population averages rather than individual growth rates. The tag-recapture method, where study animals are injected with a chemical compound which binds to their calcifying skeletal elements, thus marking their size at the time of tagging, has been successfully applied in the past to many marine invertebrates, including sea urchins.
In December 2022, a sample of 261 C. muricata was tagged in Doubtful Sound with the fluorescent dye tetracycline, and a survey of the size-frequency distribution of the population was conducted. Return trips were made to the site in May 2023 and December 2023, to record subsequent instalments in the size-frequency distributions, and to collect a final sample of animals for tetracycline tag analysis. In a simultaneous laboratory experiment, around 100 C. muricata were tagged with tetracycline and raised under two feeding (weekly and fortnightly) and three water temperature regimes (ambient, +1.5°C, and +3°C above ambient). The aim of the laboratory study was to measure growth in captivity to provide a baseline to compare with the field results, and to investigate the relative importance of different environmental drivers of growth. Samples of pyloric caeca and gonads were taken from both the field and laboratory animals, to investigate differences in pyloric and gonadal indices between these populations.
At the end of the study period, after one year, arms were removed from the study animals, processed by bleaching, and tetracycline tags on the ambulacral ossicles interpreted. In the field, 20% of the animals checked in May had visible tags, while in December (one year after tagging) 8% of recovered animals had usable tags. Recovery rates in the lab were higher, where 62% of the processed animals had usable tags. The size of the ossicles at time of tagging and at time of reading was measured and converted to total body diameter by a previously calculated allometric relationship between the two variables. Body size values at the beginning and end of the study were analysed using a Walford plot, which allowed the parameters of the Brody-Bertalanffy growth equation to be calculated. While several other growth functions were tried (e.g., the Richards), the Brody-Bertalanffy was the only one which could be reliably fitted to the data.
The Brody-Bertalanffy growth model showed that C. muricata in the fiords reaches an asymptotic size of 419 mm diameter and has a growth constant K of 0.37. It grows rapidly, reaching just under one-third of its asymptotic size in its first year (130 mm diameter), beginning to reproduce in its second year at a diameter of around 200 mm, and reaching adult size around four or five years of age (>250 mm diameter). The size-frequency data was analysed to identify year classes (modes), providing an independent measure of growth in the population. Evaluation of the model and comparison with the size-frequency data suggests that the model may slightly overestimate growth in smaller individuals, although the growth values determined from the size-frequency data are still within the 95% confidence limits of the model.
The growth model is primarily limited by its small dataset, owing to a low recovery rate, especially for smaller individuals. This lack of growth information for smaller animals means that a potential lag phase in C. muricata’s growth pattern, which would account for the model’s overestimation, could not be quantified. Such a lag phase is documented for several other echinoderm species, including sea stars, where it is often associated with ontogenetic diet changes. The laboratory growth experiment found that nutrition had a large impact on growth, while that of water temperature was insignificant. These results are consistent with previous research establishing that food intake is the primary determinant of growth in sea stars, and that temperature (within the species’ range of tolerance) has little effect on growth. The organ index analysis found patterns consistent with previous research, with high pyloric and low gonadal index values in winter, and a reversed pattern in summer. This pattern is due to C. muricata’s seasonal reproductive cycle. The index analysis also found that although pyloric and gonadal weight are positively correlated with diameter, there was no association between diameter and index value. This lack of correlation suggests that all the animals in each population were similarly well fed, and that larger individuals are not proportionally more fecund than smaller ones.
This study is the first reported to apply the chemical tag-recapture method to a sea star. It has shown that this method is a viable approach for studying sea star growth in the field. The study reveals that C. muricata is a fast-growing animal, with growth patterns similar to that of the crown-of-thorns sea star, Acanthaster cf. solaris. This similarity has implications for C. muricata’s ecology and life history, which should be investigated by future research. The chemical tag-recapture method provides a tool for future research to compare growth between different populations of C. muricata, as well as between C. muricata and other sea star species.