Superficial ecosystem similarities vs autecological stripping: the "twin species" Mesocyclops leuckarti (Claus) and Thermocyclops oithonoides (Sars) - seasonal habitat utilisation and life history traits

Mesocyclops leuckarti and Thermocyclops oithonoides, among the most common European species of cyclopoid copepods, immigrated to central Europe from eastern refuges after the last glaciation. M. leuckarti arrived prior to T. oithonoides. In a border region of T. oithonoides in southern Norway, the species was found exclusively below the highest postglacial marine limit, whereas it had spread to other neighbouring lakes above the former marine limit close to its more central region of distribution in eastern Norway. The habitat of M. leuckarti is characteristically both littoral/profundal and planktonic, whereas T. oithonoides is a true planktonic species. The egg sacs of the larger species M. leuckarti protrude from its genital segment, likely increasing water friction. M. leuckarti has probably developed strategies to reduce predation on eggbearing females, such as staying in littoral, profundal or oxygen boundary regions where fish are either absent or experience hunting difficulties. We hypothesise that the numerical suppression of M. leuckarti, its sex ratio, the habitat distribution of adult females, and its life cycles in many eutrophic lakes, is strongly affected by fish predation. M. leuckarti is considerably larger than T. oithonoides; total body length: 1.0-1.3 mm vs 0.7-1.0, respectively. The negligibly coloured and smaller adult T. oithonoides may be outside the prey range for many fish species. In the lowland region, both species completed several numbers of reproductive cycles annually. There were various patterns of diannual and triannual life cycles. Some populations exhibited a conspicuously delayed revival from sediment diapause, others in eutrophic lakes developed slowly during the summer (probably due to naupliar competition from cladocerans), or stayed in the plankton during prolonged periods during autumn. At higher altitudes and in large cold lakes, one generation a year was recorded. In its northern range, M. leuckarti showed sediment diapause in all types of localities, even the deepest lakes, usually in the upper littoral region. In more shallow lakes, deeper diapause sites were observed. T. oithonoides diapaused in either the lower littoral, or the profundal regions. M. leuckarti showed different life cycles in localities within the same geographical region, especially in its southern range. In the shallow part of Bodensee in Germany it entered sediment diapause, whereas in the much deeper main basin it showed plankton diapause (also called "active diapause"). The period of diapause for M. leuckarti (especially in the sediment) decreased from north to south. At about 45º N, sediment and plankton diapause were non-existent, and the species exhibited continuous development, even with relatively low winter temperatures (in Lago Maggiore). T. oithonoides, whose southern distribution in western Europe extends to about 50º N, showed winter sediment diapause throughout its distribution, but frequently with a fraction of the local population in plankton diapause. The combined effects of these different abiotic and biotic parameters help explain the variations of life histories observed in the field.