Echinoids have an endoskeletal system which is ideal for studying calcified structures such as development of vertebrate skeletons. However, understanding echinoid skeletal (test) growth has proven challenging to analyse solely on the basis of any one approach or process. Therefore, theoretical models have been developed to understand growth and form of echinoid tests. Herein, Holotestoid, a computational model of echinoid test growth is described. The model incorporates mathematical principles (e.g., close-packing), physical principles (e.g., interface between coalescing bubbles) and biological processes (e.g., echinoid ontogenic processes). It is the first computational model that emulates all five ontogenic processes involved in test growth (plate growth, plate addition, plate interaction, plate gapping, and visceral growth) using a geometrical representation and three analogies (coalescing bubble, circle-packing, and catenary chains). The emulated processes are used to predict plate size, plate shape, and test shape. The results from the simulations of the growth zones show that the ambulacral column angle (e.g., for A. punctulata α_am= 22° and for S. franciscanus α_am = 32°) is a crucial parameter that distinguishes between species when varied. The. comparison of simulated data with those from real specimens yielded high accuracies, thereby validating the model. The combination of the simulated processes produced patterns mimicking real biological specimens. The model was further used to investigate the test morphological disparity observed among echinoids, specifically between. regular echinoid (sea urchin) tests and irregular echinoid (sand dollar) tests. Both exhibit morphological similarities as imagines, however, they develop different test morphologies as adults. Thus, Holotestoid was used to explore the influence of each parameter on test height-todiameter ratio (h:d). The results showed that both ambulacral column widening and increase in total plate number cause the test h:d to decrease thereby leading to test flattening. Whereas the absolute size of the apical system and peristome does not influence test h:d, however, their growth with respect to column length caused an increase in the test h:d. These results provide an explanation of how the different test shapes were obtained. / Thesis / Doctor of Philosophy (PhD)

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A plethora of naturally-produced steroid hormones, or artificial homologues of them, are being introduced into the aquatic and terrestrial environments each year. Two examples of these are the natural oestrogen 17-oestradiol (E2) and the oestrogen receptor antagonist, Bisphenol A (BPA), both of which target the ribonucleoprotein telomerase through upregulation of its telomerase reverse transcriptase component, TERT. The main objectives of this study were firstly to isolate and characterize the actual mRNA sequence for the telomerase catalytic subuninit, Tert, in rainbow trout (Oncorhynchus mykiss) (Walbaum, 1792) and European purple sea urchin (Paracentrotus lividus) (Lamarck, 1816), with the aim of developing qPCR assays for the amplification and quantification of Tert. Further objectives were to use these assays in controlled exposure studies to establish whether and to what extent the aforementioned chemicals regulate Tert transcription and by doing so further understand the mechanism of Telomerase gene expression and the extent to which environmental oestrogen can interfere. The initial step of sequence characterization and assay devlopment was successful in the case of rainbow trout where two possible splice variants of Tert mRNA are identified, omTertShort and omTertLong. Two qPCR assays were developed for the relative quantification of both of these splice variants in rainbow trout samples, the latter of these successfully amplifying its target in test samples. In order to demonstrate in vitro and in vivo modulation of telomerase activity and mRNA expression, early life-stages of rainbow trout and purple sea urchin, as well as rainbow trout hepatocytes, were exposed to a range of concentrations of E2 and BPA. Purple sea urchin embryos were exposed to 200, 20 and 2 ng E2/ml for 28 hours until they had reached the stage of pluteus larvaes. Rainbow trout embryos were exposed to 500, 20 and 0.1 ng E2/ml and 600 and 150 ng BPA/ml for 167 days from immediately after fertilization. Rainbow trout hepatocytes were exposed to 20 and 2 ng E2/ml for 48 hours. The results from this study show that telomerase activity as well as TERT mRNA expression can be significantly modulated by exposure to oestrogens and other oestrogenic chemicals. E2 concentrations as low as 20 ng/ml lead to an increase in telomerase activity early-life stages of purple sea urchin and upregulation in the transcription of Tert mRNA in unhatched rainbow trout embryos. BPA induced similar response (600 ng/ml) in hatched rainbow trout alevins larvae. Very high exposures to E2 (500 ng/ml) do however lead to downregulation of Tert mRNA in hatched alevins larvae. Differential regulatory response can be observed between different tissue types of 167 day old fry, with an upregulatory response observed at 0.1 ng E2/ml in liver and muscle tissues, but not in brain. Similarly, brain tissues were observed expressing significantly less mRNA than liver and muscle samples when exposed to BPA (150 ng/ml). It is evident that the previously observed link between environmental oestrogens and telomerase is also present in the two test species examined; purple sea urchin and rainbow trout.