Abstract |
This dissertation is a contribution to the study of the reproductive biology of two species: the
lessepsian migrant Lagocephalus sceleratus (Gmelin, 1789) and the mesopelagic fish Argyropelecus
hemigymnus (Coco, 1829). The first is a coastal, benthopelagic species originating from the Red Sea
and the study of its reproduction aims at better understanding its invasion success. The second is a
small-bodied, pelagic fish with poorly studied reproductive biology, like most mesopelagic fishes.
The comparison of the number and the development of oocytes from the anterior, middle and
posterior part of the ovary showed that, despite their large size, the gonads of L. sceleratus are
homogenous. The histological analysis showed that the examined fish, which were sampled during
the reproductive period, had either recently spawned (presence of post ovulatory follicles), with signs
of atresia in the remaining vitellogenic oocytes, or they had not yet matured, being in the secondary
growth (vitellogenesis) stage. Only one individual examined was at final maturation (germinal vesicle
migration). The oocyte size frequency distributions (OSFDs) revealed that, in fish at advanced
vitellogenesis or final maturation, the most developed oocytes (advanced batch) were fully separated
in size from the remainder, less developed oocytes. These results suggest that L. sceleratusis a species
with determinate fecundity, and, possibly, it spawns only once during the reproductive period.
In the case of A. hemigymnus, the study focused on the comparison of somatic and reproductive
parameters of the species between the Corinthian gulf and the Cretan Sea. The histological analysis
showed that the species was reproductively active in both regions, even though the sampling took
place in April in the Corinthian gulf and December in the Cretan Sea. Furthermore, the analysis of
OSFDs from both regions showed that the pattern of development and recruitment of oocytes was
the same: The recruitment of a new batch starts during the final maturation of the advanced batch
and ends during spawning. The condition factor, the gonadosomatic index, the oocyte sizes, the
relative number of oocytes >200 μm and the relative batch fecundity (eggs g-1) had significant higher
values in Corinthian gulf compared to the Cretan Sea. Even though these differences could be
attributed to various causes, they are more likely resulting from the higher temperature of
indermediate (mesopelagic) waters in the Cretan Sea (15 οC) compared to the Corinthian gulf (14 οC).
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