Macrobdella decora

Macrobdella decora
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Annelida
Clade:	Pleistoannelida
Clade:	Sedentaria
Class:	Clitellata
Subclass:	Hirudinea
Order:	Arhynchobdellida
Family:	Macrobdellidae
Genus:	Macrobdella
Species:	M. decora
Binomial name
Macrobdella decora (Say, 1824)
Synonyms
Hirudo decora Say, 1824 (basonym) Hirudo ornata Ebard, 1857

Macrobdella decora, also known as the North American medicinal leech, is a species of freshwater leech. It is found in much of eastern North America in freshwater habitats, although there is one disjunct population in northern Mexico. M. decora is both a parasite of vertebrates, including humans, and an aquatic predator of eggs, larvae, and other invertebrates.

Macrobdella decora is a medium-sized leech with a spotted greenish-brown back and a reddish underbelly. It has ten ocelli, or simple eyes, arranged in a horseshoe shape and three long jaws. Internally, a pharynx takes up a tenth of its digestive tract; a stomach, the majority of its body length. The stomach connects to an intestine, followed by a colon, a rectum, and finally an anus located on the leech's back. M. decora, like all leeches, is hermaphrodite, and has ten testisacs and two ovisacs, in addition to male and female gonopores.

First described by Thomas Say in 1824, the species is now placed in the genus Macrobdella. Its sister taxon is believed to be the species Macrobdella diplotertia.

Macrobdella decora is a medium-sized leech, growing between 5 and 8.5 cm (2.0 and 3.3 in) long, and weighing from 1.48 to 3.69 grams (0.052 to 0.130 oz).^{[1]: 67 [2]: 155} It has a dark green, brown or olive-green back with a line of 20 or so small orange or red dots down the middle, and two corresponding sets of black dots on its sides. Its underbelly is reddish with black spots dispersed irregularly across it.^{[1]: 67 [3][4][5]: 160} Its back is rounded but its belly is flattened.^{[5]: 160 [6]: 230} All leeches have 32 segments, but they are also covered with annuli; the body of M. decora has between 90 and 94 annuli total.^[7]

The head of M. decora is rounded and has ten ocelli on the front of its body: one pair between segments two and three; a second pair on segment three; a third on four; a fourth on six; and a fifth and final pair on segment nine.^[8]: 138 They are arranged in the shape of a horseshoe.^{[5]: 160 [6]} A jawed leech, Macrobdella decora has three long jaws which are semicircular and laterally compressed, each with one row of about sixty-five teeth.^[6][9]

M. decora has a large muscular pharynx which accounts for the first tenth or so of the leech's digestive tract. The stomach, a large pouch composed of smaller sacs, is not nearly as muscular as the pharynx, but it occupies about five sixths of the leech's whole body and is subdivided into eleven chambers. The intestine extends from behind the stomach and narrows towards the anus. The last part of the intestine is the colon, followed finally by a small rectum.^{[5]: 163–5} The anus is located on the leech's back, above of its circular acetabulum.^[6]

All leeches are hermaphrodites whose male reproductive organs mature first and the female ones later.^[10] M. decora has ten pairs of testisacs located from segments thirteen to twenty-three, with large much compact and much-coiled epididymes, and crescent-shaped or globular ovisacs which are located in the thirteenth segment. A 2023 paper described the leech's oviducts as "thin" and "torturously folded".^{[2]: 155 [9]: 510} The male and female gonopores are usually separated by five annuli, or external rings.^[1]: 67 The male one, when withdrawn, appears as nothing more than a hole in between segments eleven and twelve; however, when the male gonopore and its surrounding parts are everted, they appear as a small cone, with the organ at the tip and having deeply furrowed sides. The leech's four copulatory glands are arrayed in a square in an area of rough skin on segments thirteen and fourteen.^[9]: 509

The most widely distributed Macrobdella species, M. decora is found in North America east of the Rocky Mountains in southern Canada and the neighbouring United States. There is, however, one isolated population in Mexico, in the state of Nuevo León.^[11]: 587Leeches of the species have been found as far west as (from north to south): Alberta, North Dakota, Nebraska, Colorado, and New Mexico.^[12]: 11

Macrobdella decora is a freshwater species that is found in still or slow-moving water bodies such as streams, temporary ponds, ditches, and wetlands.^[1][13][14] In Ontario, NatureServe lists the species as "Secure".^[15]

Macrobdella decora is both parasitic and predaceous. It sucks the blood of many vertebrates, using its teeth to pierce the host's skin, including humans but also amphbians, fish, turtles, wading birds, and cattle. It also hunts voraciously, and eats oligochaete worms, snails, amphibian eggs, the larvae of insects, and even other individuals of its own species. In the spring, the leech's aggressive predation of American toad eggs may lead to up to 80% mortality.^{[1]: 67–8}

Macrobdella decora does parasitize humans and is often found by people swimming in Canada and the northern United States. Sometimes swimming areas have had to be restricted or even closed due to the leech's presence.^[1]: 67

The leeches engorge themselves with blood before mating. One or two months after feeding, the leeches produce spongy cocoons, which are pale yellow and elliptical in shape. About another month later, the young, only 20 to 22 millimetres (0.79 to 0.87 in) long, emerge.^[1]: 68 They take several years to become fully mature.^[16]

Macrobdella decora was originally placed in the genus Hirudo by Thomas Say, who described it in 1824 in an appendix to a book about an expedition up the Minnesota River.^[3] When Addison Emery Verrill erected the genus Macrobdella in 1872, he transferred Say's species into his new genus Macrobdella.^[8] A common name for the species is the North American medicinal leech.^[17]

Macrobdella decora is the sister taxon to Macrobdella diplotertia; the genus Macrobdella is believed to be a monophyletic grouping. Macrobdella ditreta was previously believed to be sister to the decora / diplotertia clade, but a new species, Macrobdella mimicus was discovered in 2023 and placed as the sister taxon to said clade.^{[11]: 587 [18]: 563}

• Media related to Macrobdella decora at Wikimedia Commons