Species of Macrobdella Verrill, 1872, also known as North American medicinal leeches, are large hirudineans found in freshwater habitats of North America, including the USA, Canada and Mexico (Klemm 1982). Macrobdella is comprised of four species (Sawyer 1986): Macrobdella decora (Say, 1824), Macrobdella sestertia Whitman, 1886, Macrobdella ditetra Moore, 1953, and Macrobdella diplotertia Meyer, 1975. Macrobdella spp. are easily identified by the olive green dorsum sometimes with reddish-orange spots, and external morphological features of the ventral clitellar region (Klemm 1982; Sawyer 1972, 1986).

Macrobdella sestertia is one of the rarest hirudiniform leeches in North America. It is considered a poorly known species with few occurrence records and even fewer specimens in scientific collections (Whitman 1886; Sawyer 1972; Smith 1977; Klemm 1982; Smith & Hanlon 1997). It is listed under the State of Massachusetts Endangered Species Act as a Species of Special Concern (SSC) in the Massachusetts State Wildlife Action Plan (Massachusetts Division of Fisheries & Wildlife 2015). This species was only known from coastal areas of eastern Massachusetts and was an elusive species thought possibly to be extinct prior to its rediscovery in 1977 (Smith 1977). It was collected again from Sebec Lake, Maine in 1993 (Smith & Hanlon 1997). The geographic distribution of M. sestertia has been presumed to be restricted to coastal habitats of New England (Smith 1977; Smith & Hanlon 1997) and in the western extent of its distribution possibly to overlap with that of M. decora (Sawyer 1972), but the two species have yet to be found in sympatry.

In 2015, the holdings of Macrobdella specimens at the National Museum of Natural History (NMNH), Smithsonian Institution, were assessed and the identifications of four specimen lots were re-determined.

Materials and Methods

RSM and AJP critically examined diagnostic external morphological features of the gonopores and copulatory glands of specimens identified as Macrobdella species from the Department of Invertebrate Zoology annelid collection (NMNH, Smithsonian Institution, Washington, D.C.). Specimens were examined with the aid of a Leica^® M125 stereomicroscope. Photographs were taken with a Leica^® IC80 HD attached to the stereomicroscope and images were adjusted for levels to improve clarity in Adobe^® Photoshop^® CS6.

Results

A total of 117 specimen lots of Macrobdella were examined. A complete list of the holdings of Macrobdella specimen lots at the NMNH with specimen information is provided in Table 1. The identification of USNM 38898 (M. ditetra) could not be verified because the specimens were shriveled, as well as 3 of the 29 specimens of USNM 38912 (M. ditetra) because the specimens were creased on the ventral side in the gonopore region, an artifact of preservation. Of the specimen lots examined, the identity of 4 lots was re-determined as follows.

Table 1.—

Macrobdella specimens of the NMNH with identifications confirmed or redetermined in this study. USNM catalog numbers marked with a section sign (^§) indicate the identification of the specimens as it was redetermined in this study. USNM catalog numbers marked with an asterisk (*) indicates the designation of holotype and caret (^) indicates the designation of paratype. (^a) indicates the specimen lot is part of the collection of C.O. Whitman.

Remarks.—These leeches had two annuli between the gonopores and four accessory pores in each of two rows, with the first row of pores opening eight annuli posterior to the female gonopore. Based on these features the specimens were re-determined to be M. ditetra.

Macrobdella sestertia Whitman, 1886
Figs. 1–3

Fig. 1. 

USNM 1405211 Macrobdella sestertia. Top row: dorsal view. Bottom row: ventral view. Scale bar = 5 mm.

Fig. 2. 

USNM 1405211 Macrobdella sestertia. Ventral side with 2 + 1/2 annuli between the gonopores and 24 copulatory glands, diagnostic features of the species. Scale bar = 2 mm.

Fig. 3. 

USNM 1405211 Macrobdella sestertia. Copulatory glands with pores that occur in pairs on top of swollen tubercles in two groupings of two rows of three glands in each row. Scale bar = 1 mm.

Conclusions

This is the first report of M. sestertia from New Hampshire and represents the sixth time this poorly known species has ever been reported. The redetermination of the specimens of M. decora and M. ditetra are consistent with the geographic distributions of these common species.

Smith's (1977) suggestion that M. sestertia is distributed in Coastal Till Plains of New England is consistent with the previously known localities of this species (Fig. 4). The new locality in New Hampshire falls within the Northeastern Coastal Zone (map region 8.1.7) as defined by the Commission for Environmental Conservation (Wiken et al. 2011) along with the localities in Massachusetts. This region is characterized by warm summers and severe winters and is considered predominantly Dry Appalachian Oak forest (formerly Dry Appalachian Oak-Hickory forest) and Northeast Oak-Pine forests (Wiken et al. 2011). Sebec Lake, Maine is within the Maine/New Brunswick Plains and Hills (map region 8.1.8) with a climate that resembles the Northeastern Coastal Zone but predominantly Mixedwood forests. Smith and Hanlon (1977) reported finding, but not collecting, a sub-adult of M. sestertia from Harwich, Massachusetts, which falls within the Atlantic Coastal Pine Barrens (map region 8.5.4), a region with hot summers and cold winters and predominantly Oak-Pine forests (Wiken et al. 2011). These 3 ecoregions have similar climate of warm to hot summers and cold, sometimes severe winters, 110–230 frost free days per yr, mean annual temperatures between 4–11°C, mean annual precipitation between 890–1270 ml, and elevation below 500 masl, with abundant perennial streams, and deciduous forests with Oak (Quercus) and Pine (Pinus) species. Macrobdella sestertia likely has a patchy distribution throughout the coastal areas of New England predominantly within the Northeastern Coastal Zone but also within the Maine/New Brunswick Plains and Hills and the Atlantic Coastal Pine Barrens of New England (Fig. 4). This species likely prefers specialized microhabitats that are distributed within these map regions that may have changed with land use during the past 100 years.

Fig. 4. 

CEC ecoregions (level III) of New England with the type locality (star) and distribution records (dots) of Macrobdella sestertia. Modified from Wiken et al. (2011).

In the time since Whitman collected the specimens of M. sestertia in the late 1800s, New England has experienced a period of forest recovery as farmers abandoned farmland and forest has returned, although since 2005 forest coverage has decreased and been replaced by new residential and commercial development in suburban areas (Jeon et al. 2014). The highest rate of forest loss has been in southern New Hampshire and eastern Massachusetts, a region central to the known geographic distribution of M. sestertia. Habitat degradation that alters these microhabitats within lakes and ponds of this region through changes in land use poses an imminent threat to this rarely encountered species (Massachusetts 2015 State Wildlife Action Plan).

An anecdotal report of 9 individuals of M. sestertia collected in 2008 from Edgefield County, South Carolina (Kohlsaat et al. 2005; Poly 2005) has not been verified and there is no mention of these specimens being deposited in a scientific collection. If corroborated, this would represent a significant range expansion for M. sestertia.

This is the first report of M. decora and M. sestertia being collected during the same collecting event. Sawyer (1972) and Smith (1977) suggested that the western extent of the geographic distribution of M. sestertia overlaps with that of M. decora. This new locality in New Hampshire indicates that the distribution of M. decora extends eastward, overlapping with M. sestertia and that these 2 species do occur in sympatry.