Northern brook lamprey (Ichthyomyzon fossor) COSEWIC assessment and status report: chapter 3

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Species Information

Name and classification

The northern brook lamprey is one of six species in the genus Ichthyomyzon. The six species are composed of pairs of a closely related parasitic (stem) and a non-parasitic (satellite) species (Hubbs and Potter 1971). The northern brook lamprey is considered a dwarfed relative of the larger, parasitic silver lamprey (I. unicuspis) (Potter 1980a). Two species in this genus, the parasitic chestnut lamprey (I.castaneus) and the silver lamprey have overlapping ranges with northern brook lamprey (Vladykov and Kott 1979).

There have been several studies investigating the genetics of silver lamprey (Mandrak et al. 2004; Docker et al., 2005; Filcek et al., 2005) and its relationship to the northern brook lamprey (see Genetic Description). Nelson et al. (2004) recognized the northern brook lamprey as a valid species.

A history of nomenclature, according to Scott and Crossman (1973) includes:

• Ichthyomyzon fossor – Reighard and Cummins 1916
• Ammocoetes unicolor – DeKay 1842
• Ammocoetes borealis – Agassiz 1850
• Ichthyomyzon (Reighardina) unicolor DeKay – Creaser and Hubbs 1922
• Reighardina unicolor (DeKay) – Jordan et al.1930

The confusing history of the nomenclature of this species is likely due to the difficulty in identifying larvae, which are very similar among species (Scott and Crossman 1973).

Morphological description

The adult northern brook lamprey has small eyes and seven pairs of gill openings (Figure 1). They can be distinguished from other lamprey species due to their small size, dentition, and single, continuous dorsal fin. Adult length ranges have been documented as 86 – 166 mm (Morman 1979) and 105 – 162 mm (Becker 1983). Average length of 67 adult northern brook lamprey collected from Great Lakes tributaries by the Department of Fisheries and Oceans (DFO), Sea Lamprey Control Centre (SLCC) between 1996 and 1999 was 127 mm (range 104 – 154 mm). The teeth of the adult northern brook lamprey are small, dull, and knob-like in contrast to the sharp, longer teeth of the parasitic species. All endolateral teeth are unicuspid.

Their skin is smooth and scale-less. The adult is dark grayish brown on its back and sides, and pale gray or silvery white on its belly (Vladykov 1949). The sense organs of the lateral line system are of the same colour as its trunk, which separates it from its parasitic stem species, the silver lamprey. In contrast, the silver lamprey appears slightly spotted as an adult due to the dark colouration of the lateral line organs (Becker 1983). After spawning, northern brook lamprey become slate-blue to black on their back and sides, and white or white-gray on their ventral surface (Becker 1983). Prior to spawning, the female can have an orange-tinted ventral surface, where the eggs show though the body wall (Vladykov 1949).

The Canadian distribution of northern brook lamprey overlaps the distributions of four other lamprey species (Page and Burr 1991). Adult northern brook lamprey can be distinguished from the American brook lamprey (Lampetra appendix) and sea lamprey (Petromyzon marinus) by the two dorsal fins present in these species (Scott and Crossman 1998). It can be distinguished from the chestnut lamprey and silver lamprey by the sharp, long teeth present in these species (Scott and Crossman 1998).

The larvae, known as ammocoetes, vary little within the Ichthyomyzon genus. They lack eyes and teeth, and possess an oral hood in contrast to the sucker mouth of the adult (Scott and Crossman 1973). Larval silver and northern brook lampreys have been differentiated using differences in pigmentation patterns in the branchial region (Lanteigne 1981, Lanteigne 1988, Stewart and Watkinson 2004) and tail (Vladykov and Kott 1980, Fuiman 1982). Other authors have found no reliable differences between ammocoetes of these two species (Purvis 1970, Morman 1979, Becker 1983, Neave 2004). Large ammocoetes (>105 mm) of the chestnut lamprey develop pigmented lateral line organs that appear as spots; however, before reaching this size this pigmentation character is not reliable (Neave 2004). All other features of larval chestnut lamprey are very similar to northern brook lamprey ammocoetes (Neave 2004).

Genetic description

No consistent differences between the paired northern brook lamprey and silver lamprey were detected in a study by Mandrak et al. (2004).They analyzed seven adult northern brook lamprey and five adult silver lamprey from different regions around the Great Lakes. Although they found intraspecific differences likely due to geographic variation, they found no species-specific differences between the two species in the 10,255 base pairs of the mitochondrial genome and the 523 base pairs from the nuclear genome that have been sequenced. This raises questions regarding the separate species status of the silver and northern brook lamprey. Subsequent genetic analysis showed that individuals from the two species could be distinguished using microsatellite analysis (Filcek et al., 2005). Using one microsatellite locus, Filcek et al. (2005) had high success rates in differentiating between silver and northern brook lampreys from tributaries to Lake Michigan and Lake Superior, respectively. However, a follow-up study using the same microsatellite markers and individuals from a greater geographic range and from areas where they occur sympatrically found substantially different results: Docker et al. (2005) suggested that northern brook and silver lampreys are different feeding types of the same species. This was supported by low F_ST values, which indicates contemporary gene flow between northern brook and silver lampreys. Interspecific variation was found to be less than intraspecific variation, which indicated the two species may not be distinct (Docker et al. 2005). The debate over whether the northern brook lamprey represents a distinct species will not be resolved in the near future. In the meantime, the northern brook lamprey is considered to be a distinct species through the invoking of the precautionary principle.

Several studies have examined the genetic relationship between other paired lamprey species, such as the river lamprey (Lampetra ayresii) and the western brook lamprey (L. richardsoni) (Docker et al. 1999, Meeuwig et al. 2002); and the European brook lamprey (L. planeri) and the European river lamprey (L. fluviatilis) (P. Raposo de Almeida, pers. comm. 2004) and found no clear genetic differences. Schreiber and Engelhorn (1998) concluded that there must be some degree of gene flow between the European brook lamprey and the European river lamprey due to a lack of allozyme differentiation between the two species. These studies showed the genetic similarity of many of these paired species, and indicated that some paired species have either separated very recently, or are capable of hybridizing.

Successful hybridization experiments have been performed between both northern brook and silver lampreys, and northern brook and chestnut lampreys (Piavis et al. 1970); however, the offspring were not raised beyond several weeks, and the reproductive capacity of the offspring of these crosses is unknown.

Being largely non-migratory, northern brook lamprey populations are likely to have limited gene flow between streams, but gene flow may be mitigated by potential crosses with migratory silver lamprey. Barriers interfering with movement in streams may impede within-stream gene flow (Schreiber and Engelhorn 1998).

Designatable units

There were two distinct dispersal routes for this species following the Wisconsian glacial period (Mandrak and Crossman 1992). The source of both routes was the Mississippian Refugium, with subsequent colonization via the Warren or Brule-Portage routes (Mandrak and Crossman (1992). This has resulted in Canadian populations in two national freshwater biogeographic areas used by COSEWIC: Great Lakes-Upper St. Lawrence; and Saskatchewan-Nelson. Therefore, the populations in these two biogeographic areas are separate designatable units.