Sex ratio in the pelagic copepods Temora stylifera Dana and Centropages typicus Krøyer

From the study of several plankton samples from the surface of southern Aegean Sea it is found that the sex ratio in Temora stylifera Dana and Centropages typicus Krøyer varies considerably (from 40 % to 132 % for Temora and from 24 % to 178 % for Centropages). The mean sex ratio shows a seasonal fluctuations. For these species the highest sex ratio is observed during the period of greatest abundance, that is when the environmental factors are optimal for the species, and the minimum sex ratio during the period of minimum abundance. During the latter period there are adverse environmental factors and a scarcity of food; a large number of females and hence high fecundity will favour the survival of the species. © 1972

Sex ratio in the pelagic copepods Temora stylifera Dana and Centropages typicus Krøyer

From the study of several plankton samples from the surface of southern Aegean Sea it is found that the sex ratio in Temora stylifera Dana and Centropages typicus Krøyer varies considerably (from 40 % to 132 % for Temora and from 24 % to 178 % for Centropages). The mean sex ratio shows a seasonal fluctuations. For these species the highest sex ratio is observed during the period of greatest abundance, that is when the environmental factors are optimal for the species, and the minimum sex ratio during the period of minimum abundance. During the latter period there are adverse environmental factors and a scarcity of food; a large number of females and hence high fecundity will favour the survival of the species. © 1972

The cladocerans of the Aegean Sea: Occurrence and seasonal variation

The little-known cladoceran population of the Aegean Sea was studied. One hundred and seventy-six planktonic samples were collected during 8 cruises in the Aegean Sea (4 in the northern part, 4 in the southern part). From these samples, the following 4 species of Cladocera were determined: Evadne spinifera Muller, Evadne tergestina Claus, Penilia avirostris Dana, Podon intermedius Lilljeborg. All species were found both in the north and in the south Aegean Sea, with the exception of P. avirostris, which was found only in the northern Aegean Sea. Its absence from the southern part is attributed to the higher salinities and the pelagic character of the area. With regard to abundance, E. spinifera is by far the commonest in this area; P. avirostris and E. tergestina could be characterised as frequent forms; P. intermedius is scarce. P. avirostris, E. spinifera and E. tergestina are thermophilic; their occurrence is more or less restricted to the warm season; P. intermedius is psychrophilic and appears only in winter. © 1973 Springer-Verlag

Effects of seven diets on the population dynamics of laboratory cultured Tisbe holothuriae Humes (Copepoda, Harpacticoida)

The harpacticoid copepod Tisbe holothuriae was collected from Saronicos Gulf (Greece) and reared under constant laboratory conditions. In order to study the effects of food on the population dynamics, seven diets were tested: the seaweed Ulva; five artificial compound feeds: the liquid Fryfood® (Waterlife), a powder of Mytilus, yeast, soya and Spirulina, respectively; and a mixed diet consisting of Ulva and Fryfood. The life cycle parameters (mortality, sex ratio, generation time, offspring production) were measured, and the demographic variables [mean generation time (T), net reproductive rate (Ro), and intrinsic rate of natural increase (rm)] were determined. As to their efficiency regarding population dynamics, the diets ranked as follows: (1)Ulva+Fryfood, (2), Ulva, (3) Fryfood, (4)Mytilus, (5) soya, (6) yeast, and (7)Spirulina. In this order they cause a progressive increase of both larval mortality and generation time, a progressive decrease of sex ratio, number of offspring per egg sac, number of egg sacs per female and, consequently, of Ro and rm. The observed differences between diets were most pronounced with respect to offspring production. Of the compound diets, those containing animal extracts were more efficient than those containing vegetable materials. Ulva plays an important role in the nutrition of T. holothuriae, favouring offspring production as well as larval survival, development and pigmentation. Ulva in combination with Fryfood led to a greater copepodid survival and offspring production. This mixed diet proved to be the most favourable for rearing the Greek population of t. holothuriae, resulting in an efficient intrinsic rate of natural increase (rm=0.304) of the population. © 1991 Biologische Anstalt Helgoland

Toxicity of chromium to the marine planktonic copepod Acartia clausi, Giesbrecht

The toxicity of chromium to the marine planktonic copepod Acartia clausi, Giesbrecht was studied. The LC50 48 h values (concentration of chromium lethal to 50% of the test animals) vary with the experimental temperature, the form of chromium compound tested and the annual generation to which the Acartia specimens belong. The elevation of temperature resulted in a considerable increase of Acartia's sensitivity to chromium. Cr6+ in the form of Na2CrO4 was more toxic to A. clausi than in the form of CrO3 · Cr3+ in the form of Cr(NO3)3 9 H2O precipitated to the bottom and was not toxic to Acartia. Acartia specimens belonging to the summer generation were more sensitive to chromium than those belonging to the winter or autumn generation. The exposure of Acartia to sublethal concentrations of chromium resulted in a reduction of its longevity proportional to the chromium concentrations used. Furthermore, when A. clausi was exposed to sublethal chromium concentrations it showed a decrease of feeding capacity and increase of respiratory rates, which became more pronounced with increasing chromium concentrations. © 1982 Dr W. Junk Publishers

Pollution effects on the phytoplankton-zooplankton relationships in an inshore environment

The quantitative and qualitative relationships between the phytoplankton and zooplankton populations were examined for one year in an inshore environment (Elefsis Bay) of the Aegean Sea. It was found that quantitative relationships among these populations were influenced by the trophic level of the environment and their stocks were significantly correlated only in non-polluted conditions. The quantitative relationship between the principal grazers in the zooplankton population (Acartia clausi, Oithona nana) and the phytoplankton dominant species Exuviaella baltica, upon which these copepods were believed to graze preferentially, was tested and the results were discussed. © 1980 Dr. W. Junk b.v. Publishers

Pollution effects on the phytoplankton-zooplankton relationships in an inshore environment

The quantitative and qualitative relationships between the phytoplankton and zooplankton populations were examined for one year in an inshore environment (Elefsis Bay) of the Aegean Sea. It was found that quantitative relationships among these populations were influenced by the trophic level of the environment and their stocks were significantly correlated only in non-polluted conditions. The quantitative relationship between the principal grazers in the zooplankton population (Acartia clausi, Oithona nana) and the phytoplankton dominant species Exuviaella baltica, upon which these copepods were believed to graze preferentially, was tested and the results were discussed. © 1980 Dr. W. Junk b.v. Publishers

Thermal tolerance of two populations of Acartia clausi (Copepoda) living at differently polluted areas

The temperature tolerance of three annual generations of the planktonic Copepod Acartia clausi was measured at the upper (30°C) and lower (0°C) temperature limits. The specimens of Acartia were collected at one heavily polluted and another relatively non polluted area. Acartia tolerates better the lower than the upper temperature limit. Differrences in the temperature tolerance have been observed between the three annual generations. At the upper temperature limit the Acartia of the cold period was less resistant than the Acartia of the warm and intermediate period, while no significant differences have been observed in the longevity of Acartia at 30°C between the warm and intermediate period. At the lower temperature limit the Acartia of the cold period proved the most and the Acartia of the warm the least resistant. The specimens of the polluted area proved more resistant both to the upper and lower temperature limits. © 1981 Dr. W. Junk b.v. Publishers

Differentiation of the sensitivity to copper and cadmium in different life stages of a copepod

Copper is more toxic to all life stages of the copepod Tisbe than cadmium. The most sensitive life stage of Tisbe to both copper and cadmium is the one-day-old nauplius. The resistance of larval stages of Tisbe increases with age (one-day-old nauplii 48h lc50=0.3142 mg Cu l-1. and 0.5384 mg Cd l-1, 0.3415 mg Cu l-1. and 0.645 mg Cd l-1. for five-days-old nauplii and 0.5289 mg Cu l-1. and 0.9061 mg Cd l-1. for ten-days-old nauplii. The two reproductive stages of Tisbe tested (females with ovigerous bands and females bearing the first ovigerous sac) demonstrated an increased sensitivity to metals and proved more sensitive than the ten-days-old copepodids (only females with ovigerous bands had a similar sensitivity to copper with the ten-days-old copepodids). © 1982

Thermal tolerance of two populations of Acartia clausi (Copepoda) living at differently polluted areas

The temperature tolerance of three annual generations of the planktonic Copepod Acartia clausi was measured at the upper (30°C) and lower (0°C) temperature limits. The specimens of Acartia were collected at one heavily polluted and another relatively non polluted area. Acartia tolerates better the lower than the upper temperature limit. Differrences in the temperature tolerance have been observed between the three annual generations. At the upper temperature limit the Acartia of the cold period was less resistant than the Acartia of the warm and intermediate period, while no significant differences have been observed in the longevity of Acartia at 30°C between the warm and intermediate period. At the lower temperature limit the Acartia of the cold period proved the most and the Acartia of the warm the least resistant. The specimens of the polluted area proved more resistant both to the upper and lower temperature limits. © 1981 Dr. W. Junk b.v. Publishers