Leeches in the extreme: Morphological, physiological, and behavioral adaptations to inhospitable habitats

Abstract

With more than 700 described species, leeches include morphological, physiological, and behavioral diversity and occur in terrestrial and aquatic habitats, including freshwater, estuarine, and marine ecosystems. Leeches inhabit a number of extreme environments, including extremes in temperature, moisture, salinity, pressure, light, and pollution. In some cases, leeches in extreme environments have specialized morphological, physiological, or behavioral adaptations to survive these conditions, yet unique adaptations are not apparent in some species. Leeches that inhabit inhospitable habitats occur in more than one branch or family of leech phylogeny suggesting that there have been independent invasions of environments with extreme conditions. Herein, we review examples of leeches that live in extreme conditions and the exceptional biology that has contributed to leeches being the most extreme annelids.

Keywords: Blutegel; Hirudinea; Hirudinida; Sangsue; Sanguessuga; Sanguijuela.

Graphical abstract

Fig. 1

Terrestrial leeches. A) Orobdella sp. out of water after a rainstorm in the Philippines. Leech is estimated to be more than 25 cm in length. Image credit: Will Reeves. B) Haemadipsa zeylanica pursuing the photographer as a host on the Vietnamese forest floor. Leech size approximately 4 cm in length. C) SEM image of the head of a haemadipsid leech. The inset depicts an outline of the same image with the eye spots marked by black dots. D) Caudal sucker of a haemadipsid leech with friction rays on the sucker surface. White arrows indicate the two flaps of the auricle. Scale bars in C and D = 0.5 mm.

Fig. 2

Leeches in polar regions. A) Channichthyid fish with several leeches (Trulliobdella bacilliformis) attached to the head region. B) View of the interior upper jaw of a channichthyid fish with leeches (Nototheniobdella sawyeri) attached. Image credits: Alex Dornburg.

Fig. 3

Extreme feeding. A) Hirudo verbana, a commercially important and frequently traded species of European medicinal leech. B) Several individuals of Hirudo verbana feeding on blood inside a nitrile rubber glove.

Fig. 4

Leech parental care. A) Light microscopy image of a glossiphoniid leech with pink circular eggs gathered on its ventral side for protection. B) Light microscopy image of a glossiphoniid leech with leech hatchlings gathered on the ventral side of the parent leech. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)