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Some evolutionary and ecological implications of colour variation in the sea urchin Heliocidaris erythrogramma

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Growns, J (1989) Some evolutionary and ecological implications of colour variation in the sea urchin Heliocidaris erythrogramma. PhD thesis, University of Tasmania.

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Abstract

An investigation into the evolutionary and ecological implications of variation
in the external colouration of the sea urchin Hefiocidaris erythrogramma was
made. Two different pigment systems create a complex polymorphism; red
granules of echlnochrome A in the dermis occur in varying densities, and purple
and green naphthoquinone pigments are found in the calcareous test and spines.
Many morphs may occur within one population, but the proportions of morphs vary
markedly between sites.
Evidence from the observed variability and chemistry of the pigments strongly
indicates that the variation has a genetic basis. Breeding studies which would
have resolved this question were unsuccessful, but did show that all crosses
between morphs developed and metamorph osed successfully.
Repeated sampling of 15 sites showed that morph proportions were stable at
most sites ove r the 35 months of the study. Geographic variationin the
proportions of morphs was determined from samples from 49 sites. Environmental
variables were recorded and the exposure of each site to wave action was
estimated using algal communities to develop an Algal Exposure Index (A.E.I.).
Stepwise linear regressi on analysis indicated that the A.E.I. and amount of algal
cover were the only environmental factors noted that were useful predictors
dermis colour proportions.
Five hypotheses were developed (two selective and three stochastic) of
processes which might be affecting morph proportions in the study area; these
were tested using Mantel's non-para metric test. The results suggest that fou r
geograph ical regions each ·have different patterns of morph distribution which are
controlled by unique combinations of selection (related to exposure) and gene
flow. These results are generally supported by what is known of water currents in
each region, as most gene flow in H. erythrogramma will occur due t o movement
of pelagic larvae.
Morphological data showed slight differences between urchins of different
dermis colour at one site, but no differences between urchins with different
coloured spines. Th ere were significant differences between urchins at different
sites. Surveys of urchin microha bitats indicated that (I) urchins of the same
derm is colour tend to occur next to each other, (2) white dermis urchins tend to
occur under rocks more often than red dermis urchins, and (3) urch ins which are
hidden under rocks tend to 'cover' wit h pieces of shell, algae or pebbles to a
lesser extent than urchins which occur on the upper surfaces of rocks. A
laboratory experi ment indicated that, although the podia (tu be feet) of red and
white dermis urchins were initially of compara ble strength, red dermis urchins
tended to tire more quickly. No diff erences between morphs were fo und in the
time of matu ration of gonads or the size of gonads re lative to body weight.

Item Type: Thesis (PhD)
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Date Deposited: 12 Dec 2012 01:18
Last Modified: 15 Sep 2017 01:06
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