Leeches in Lake Kinneret?
Dr. Gideon Gal and Dr. James Easton
The Yigal Allon Kinneret Limnological Laboratory
In July of this year, we received reports on large numbers of leeches at a swimming beach on the western shore of Lake Kinneret. An examination of the location soon revealed large numbers of leeches inhabiting a limited area of the beach. The leeches were confined to the fine gravel sediment at a water depth of 0.5- 1 m. Standing in this confined region for just a brief time (1-2 minutes) suffices for tens to hundreds of small leeches to cover any bare skin. The leeches, no longer than approximately 2 cm, attach easily to the skin with their ventral suckers. Exposing the leeches to air and sun eases their removal. It should be noted that Lake Kinneret's leeches have been reported in the past (Serruya 1978). The reports go back as far the start of the 20th
century and include four species. More recently, there have been further reports on existence of leeches in the lake although not at the density of the past summer.
The leeches attach easily to the skin with their ventral suckers
Samples of the leeches were sent for identification to Dr. Hanan Dimantman of the Hebrew University. He identified them as Helobdella stagnalis
. This species has been known from other water sources in northern Israel, including parts of the Lake Kinneret watershed, however they have never been reported in the lake itself. Based on the literature, it is believed that they feed mainly on snails, worms and other invertebrates but not on vertebrates, mammals or humans.
The hotel management of the beach in question decided to deal with the problem by covering the leech infested area with fine gravel. This solution, which, at first seemed to solve the problem, turned out to be short lived. Within weeks, leeches reappeared in the same confined area. We have since received reports on leeches from other beaches around the lake although not as dense.
We have no definite explanation for the phenomena described here though we offer a hypothesis that would require thorough examination; Substantial reduction in the numbers of snails along the lake’s shoreline in the past year, attributed to the rapid changes in lake’s water level, resulted in a marked decrease in predation on leech eggs by the snails. Consequently, there was a higher survival rate of eggs leading to larger population of leeches. Similar events have been reported from other locations worldwide.
A team of scientists from the Kinneret Laboratory will continue to monitor this unusual event.