Heller Joseph
Department of Zoology, The Hebrew University of Jerusalem, Jerusalem, Israel.
Oecologia. 1979 Dec;44(1):98-104. doi: 10.1007/BF00346406.
The Israeli freshwater snail Theodoxus jordani exhibits high variability both in shell colour and shell pattern.In most localities in Israel, more than 80% of the snails are black regardless of the colour of their background. Within Lake Kinneret, however, there is a significant association between the relative frequency of dark patterned shells and the distribution of a black (basalt) background.Predators of jordani include the fishes Barbus longiceps, Blennius fluviatilis, and the crab Potamon potamon. In Israel, the former two are limited in their distribution to the Kinneret. Thus, the distribution of cryptic variants of the snail is positively correlated to the distribution of some of its predators.Beyond the Kinneret, shells are black probably as a result of selection by the injurious effects of ultraviolet radiation. Within the Kinneret, snails can manage without an anti-radiation shield probably because the penetration of UV light into the water is considerably reduced, due to the daily summer storms and to the seasonal bloom of unicellular algae.Apparently, then, outside the Kinneret selective forces for crypsis are partly relaxed and their effects overidden by selective pressures for solar radiation devices; within the Kinnertt, selective pressures for anti-radiation devices are partly reduced and their effects overridden by selective pressures for crypsis.
以色列淡水蜗牛约旦豆螺在壳色和壳纹方面表现出高度变异性。在以色列的大多数地区，无论背景颜色如何，超过80%的蜗牛是黑色的。然而，在加利利海内部，深色带纹壳的相对频率与黑色（玄武岩）背景的分布之间存在显著关联。约旦豆螺的捕食者包括长头无须魮、河鳚等鱼类以及蟹类巴勒斯坦潜穴蟹。在以色列，前两种鱼类的分布仅限于加利利海。因此，蜗牛的隐匿变体的分布与它的一些捕食者的分布呈正相关。在加利利海之外，壳是黑色的可能是由于紫外线辐射的有害影响所导致的选择结果。在加利利海内部，蜗牛可能无需防辐射保护，这可能是因为夏季的日常风暴和单细胞藻类的季节性大量繁殖，使得紫外线进入水中的量大幅减少。显然，在加利利海之外，隐匿的选择压力部分得到缓解，其影响被太阳辐射防护装置的选择压力所取代；在加利利海内部，防辐射装置的选择压力部分降低，其影响被隐匿的选择压力所取代。
