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Seasonal changes in the reproductive cycle and condition index of the surf clam Mactra stultorum (Mollusca: Bivalvia) in the Gulf of Gabes, Tunisia

Derbali Abdelkarim

Oceanological and Hydrobiological Studies

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2023

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Vol. 52, No. 3

264--272

EN

The reproductive biology of Mactra stultorum fromthe Gulf of Gabes was investigated monthly during a one-year period (2017). This study is the first study examiningits reproductive biology during one-year in the south of Tunisia. The overall samples presented a balanced sex ratio, with males dominating among smaller individuals (< 22mm) and females predominating in larger size classes (> 34 mm). The obtained total sex ratio (F:M) was equal to 1.03:1.The size at first maturity (SL 50%) was 20.43 and 22.10 mm for males and females, respectively. The seasonal variations assessed through macro- and microscopic characteristics in the gonads of both sexes indicated a clearly defined annual reproductive cycle with a principal spawning peak during summer and a resting period during December and January. The reproduction of M. stultorum was strongly influenced by fluctuations in the seawater temperature, as reflected through the temporal variation in the gonadal cycle, condition index (CI) and tissue weight rate (TiWR). If managed properly, M. stultorum is expected to meet thedomestic market demand through artisanal fishery. The adoption and implementation of rules, such as limiting the size of clams, is required to protect this new exploitable fishery resource.

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Feeding activity and diet composition of round goby (Neogobius melanostomus, Pallas 1814) in the coastal waters of SE Baltic Sea

Skabeikis A., Lesutienė J.

Oceanological and Hydrobiological Studies

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2015

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Vol. 44, No. 4

508--519

EN

Feeding activity and diet composition of round goby were investigated in the south-eastern Baltic Sea, the Lithuanian coastal waters during May-October 2012 in order to determine main feeding objects and seasonal periods when native fauna could be most affected by predation of this highly invasive species. In total, prey represented by 18 taxa was found in the gut contents of dissected fish. Feeding activity of round goby varied depending on the body size, sex and stage of the reproduction period. The gut contents of <50 mm specimens were dominated by zooplanktonic and meiobenthic organisms, whereas larger individuals (50- 99 mm) shifted to amphipods and mollusks. Individuals of the intermediate 100-200 mm length had a variable diet, changing depending on the season; in spring they mostly preyed on Macoma balthica, in summer − on polychaetes, while in autumn the contribution of Mytilus trossulus and fish considerably increased in their diet. Diet composition of individuals ≥200 mm was relatively constant in the course of the study with substantial preference to M. balthica. These findings imply that benthic fauna, particularly a newly settled generation of epibenthic mollusks in autumn is under strong predatory pressure of the round goby.

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Egg number-female body weight relationship in freshwater snail (Viviparus viviparus L.) population in a reservoir

Jakubik B.

Polish Journal of Ecology

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2007

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Vol. 55, nr 2

325-336

EN

Studies on the fecundity of V. viviparus were carried out in the years 1990-1994 in selected littoral sites of the Zegrzyński Reservoir (Central Poland) (60 km long, mean depth ca. 3.5 m, maximum depth at the dam up to 9 m, mean retention time from 1 to 15 days) and in outflow stretches of its tributaries. All sites were dominated by fertile females (i.e. having embryos) that lived in aggregations. Mean numbers of females ranged between 30 and 250 ind. m[^-2]. Females of III (shells 12.1 - 20.0 mm wide and 12.1 - 25.0 m high) and IV (20.1 - 25.0 mm width and 25.1 - 35.0 mm height) size class dominated. The embryos were noticed in females of II (8.1 - 12.0 mm width and height of the shell), III and IV size class and mean number of embryos per female was from 0.9 to 6.7, from 1.1 to 9.6 and from 2.0 to 9.1 for II, III and IV class respectively. Analysis of embryonic development revealed the presence of three growth stages in oviducts of females: the oval, transparent egg capsules called the youngest embryos), egg capsules with visible contour of a shell (called medium embryos) and snails with a shell (the oldest embryos). Embryos in a form of oval, transparent egg capsules and snails with developed shell dominated in particular size classes in analysed sites. Analysis of the correlation coefficient showed significant positive relationship between the mean sum of embryos and shell height and width, and dry weight of female's body and shell weight. Most significant correlation was found between dry body weight and mean number of embryos. The number of embryos in particular growth stages was also related to biometric indices as shown by the analysis of correlation coefficients. The number of oldest embryos (fully developed snails with a shell) increased with the increase of shell height and width. Female.s dry body weight, on the other hand, was best correlated with the youngest growth stage of embryos (oval, transparent egg capsules). Described relationships indicate the importance of the size of V. viviparus females for reproductive success and for survival and "condition" of young snails.