الفهرس 
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Abstract
The studies on the status of the sea urchin populations of the Red
Sea are scarce. This thesis aimed to provide an overview of some aspects of
sea urchins in the Northern Red Sea, Hurghada. Those marine world wide
invertebrates provide myriad of opportunities for investigation. Among the
Red Sea species, T. gratilla was selected to be the subject of the following
investigation. It was a target species due to its high economic value in
addition to the lack of studies on this species in Egypt. Sampling site was
selected in front of the branch of the National Institute of Oceanography
and Fisheries (NIOF). It is considered the most suitable sampling site for
studying the biology of T. gratilla according to many criteria. It contains a
reasonable abundance of the targeted species, suitable for snorkeling and
sampling, and close to the NIOF laboratories to prepare the samples
immediately after collection. Although, the collected sizes were limited to
obtain more accurate estimation on growth and mortality for each size
group, growth and mortality were calculated for the obtained size range.
The limited sample sizes were mainly due to, the scarcity of small
individuals, especially those less than 6cm total length and hiding behavior
of T. gratilla under stones and in the crevices, as well as reducing the
potential of collection to conserve the exited populations. This explains the
dominance of large species in the sampling and might explain also the
dominance of the female individuals (208 individuals, 61%) over males
(131 individuals, 39%).