Geographic range and cold hardiness of the earthworm Drawida ghilarovi (Oligochaeta, Moniligastridae)

The earthworm Drawida ghilarovi Gates 1969 is a typical representative of the Amur fauna and the only species of the tropic family Moniligastridae on the territory of Russia. The northern boundary of its range passes from the Khingan (Hinggan) River on the west, along the mountain framing of the Amur plains, to Evoron Lake (or, probably, to the Amgun River valley) on the east. Drawida ghilarovi is widespread in the Sikhote Alin, but the northern boundary of its distribution in the northern part of this mountain range has not been delimited; the same applies to the left bank of the Amur downstream of Komsomolsk-on-Amur and its lower reaches. These earthworms lay cocoons in summer in the surface soil horizon (0-20 cm) and overwinter also at a depth of only 15-20 cm, although D. ghilarovi is classified as an anecic (deep burrower) species (Vsevolodova-Perel, 1997). The median lethal temperature (LT₅₀%) is about −15°C for cocoons and −12°C for worms; the minimum tolerable temperature, about −20 and −16°C, respectively. The mechanism of protection against freezing in cocoons involves a decrease in water content from an average of 71.1 ± 0.8% to a minimum of 39.8%; this decrease in worms is less significant: from 85.5 ± 0.8% (feeding worms) to 75.3 ± 0.7% (wintering worms). Since the development of juveniles in cocoons is completed by autumn, the critical factor is the minimum temperature to which the worms are exposed. In woodless areas near Khabarovsk, the average soil temperature at a depth of 40 cm during the coldest month is only 2-3°C higher than LT₅₀% (the difference is greater in forest habitats), and the minimal temperature should be still lower. Under current climatic conditions, D. ghilarovi could have inhabited the area extending over the Zeya River basin and, in the north, to the sources of left-bank tributaries of the middle and lower reaches of the Amur (not everywhere, but in the warmest habitats). The present-day boundaries of the D. ghilarovi appear to reflect past changes in climatic and soil conditions.