Abstract

In Finland, signal crayfish live at more northerly latitudes than in their native distributional area in North America. The amount of daylight in Finland is longer from the spring to the autumn equinox, but the growing season is shorter due to the cooler climate. Since daylight and water temperature regulate the moulting and reproduction of crayfish, these environmental factors may affect the distribution of signal crayfish in many ways in Finland. Low water temperatures can delay hatching, and the juveniles may leave the female very late in the summer. In this case, the growing season may be too short for the juveniles to survive the following winter, and for the females, too short to reproduce again. However, a long daylight period does not prevent the females from moulting, even in late summer, but late moulting may prevent reproductive activity in that year. Decreasing water temperatures also induce the mating and spawning of signal crayfish and yearly variations in temperature may affect the success of these events. When the reproductive cycle is interrupted or unsuccessful over several successive years due to environmental reasons, self-sustaining signal crayfish populations cannot develop. A more detailed understanding of the temperature requirements for ovary and sperm developmental processes, mating and fertilization processes, egg development, and the optimal growth and living conditions for both juvenile and adult signal crayfish is needed. An evaluation of the resulting areas of suitable habitat, and the probable distribution limits of this introduced species, is necessary in order to avoid any non-viable stocking attempts and for the protection of the native noble crayfish.

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