الفهرس 
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Abstract
The present study aimed to investigate some ecological and biological aspects of the edible cockle Cerastoderma glaucum (family: Cardiidae) in two areas; Lake Qarun and Lake Timsah. Cockle samples were collected monthly from Lake Qarun during February 2008- May 2009 and collected seasonally from Lake Timsah [Spring, Summer, Autumn 2008 and Winter 2009]. The following aspects were mainly employed:-
(1) Population structure and growth.
(2) Length-weight relationships and changes in body weights.
(3) Reproduction.
The results are summerized as follow:
(1) Length frequency distributions for Lake Qarun populations indicated
that newly settled juveniles (≤ 8 mm shell length) were recorded in
February and May 2008, March and May 2009. For these months, the
percentages of these juveniles were 16.7, 11.5, 3.5 and 8.9% of the
total population, respectively. Length frequency histograms were
polymodal in Lake Timsah population indicating a continuous
admission of new cohorts.
(2) Largest cockles ever found were 27.9 and 32 mm shell length for
Lake Qarun and Lake Timsah, respectively.
(3) Monthly growth rate of the different cohorts of C. glaucum at Lake
Qarun ranged from 1.3 to 2.5 mm / month (X̅ = 1.6 mm/ month).
Very little or no growth was observed during summer.
(4) Linear regressions fitted between shell length and other shell
dimensions (shell height and shell breadth) were highly correlated
(p˂ 0.0001). For both Lakes, the growth rate in height and breadth is
higher than that of length.
(5) Total we weight, digestive gland weight and gonad weight increased
isometrically with shell length throughout the study period.
Spawning may explain negative allometric pattern for wet flesh
weight–shell length relationships during February–March 2008,
December 2008 and April–May 2009 for Lake Qarun population.
(6) C. glaucum is dioecious. It is possible to sex individuals from the
colour of their gonads that is orange in males and light brown in
females.
(7) Reproduction occurred throughout the year in a poorly defined pattern
with some annual variation in the timing and intensity of spawning
(i.e. the lack of reproductive periodicity). The individuals of C.
glaucum spawned in four episodes (multiple spawnings) through the
year. Mass spawning of lake Qarun population occurred during April–
May, July, November 2008 and February–March 2009. spawning
activities were not closely synchronized and some individuals may be
in spawning condition at nearly all times of the year.
(8) The end of the spawning periods was usually marked by gamete
artesia (degeneration and phogocytosis). The state of sexual
quiescence (inaction stage) was not recorded in Lake Qarun
population.
(9) The depletion of ripe ova in female gonads during spring and winter
was rapidly compensated by the onset of oogenesis. This indicates
that spawning was followed by rapid development of the gonad.
(10) Sex ratio of Lake Qarun population differed significantly from the
expected 1:1 ratio indicating that males outnumbered females. For
Lake Timsah population, the departure from equal proportions of
females and males was not significant.
(11) The length at the onset of sexual maturity (SM50) varied seasonally.
The values ranged from 6 mm (in winter) to 10.1 mm ( in summer)
shell length in Lake Qarun population. Lower temperature in winter
is important for an early recruitment into the breeding population.
SM50 was relatively larger in Lake Timsah population. The lowest
value (9 mm shell length) was recorded in summer 2008 and the
highest value (11.4 mm shell length) was recorded in winter 2009.
Thus the higher temperature in summer in important for an early
recruitment into the breeding population.