Abstract
A new leech species Helobdella blinnisp. n., is described from Montezuma Well, an isolated travertine spring mound located in central Arizona, USA. In its native habitat, Helobdella blinni had been previously identified as Helobdella stagnalis (Linnaeus, 1758), which was later reclassified to Helobdella modesta (Verrill, 1872). Similar to the European Helobdella stagnalis and North American Helobdella modesta, Helobdella blinni has six pairs of testisacs, five pairs of smooth crop caecae, one lobed pair of posteriorly-directed crop caecae, one pair of eyes, a nuchal scute, and diffuse salivary glands. However, the pigmentation of this new species ranges from light to dark brown, unlike Helobdella modesta which tends to be light grey in color. Also, Helobdella modesta produces a clutch of 12--35 pink eggs, whereas Helobdella blinni produces smaller clutches of white eggs (7-14, 0.5 ± 0.15 mm, N = 7) and consequently broods fewer young (1-14, 7 ± 3.3 mm, N = 97). Helobdella blinni are also able to breed year-round due to the constant warm water conditions in Montezuma Well. Their breeding season is not restricted by seasonal temperatures. These species are morphologically similar, however, comparing the COI mtDNA sequences of Helobdella blinni with sequences from nearby populations of Helobdella modesta and other Helobdella species from GenBank indicate that Helobdella blinni is genetically distinct from these other Helobdella populations.