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COSEWIC Assessment and Update Status Report on the Deepwater Sculpin (Western and Great Lakes-Western St. Lawrence Populations) in Canada

COSEWIC Status Report
on the
Deepwater Sculpin
Myoxocephalus thompsonii

Great Lakes-Western St. Lawrence populations
Western populations

in Canada

Species Information

Name and Classification

Genus and Species:
Myoxocephalus thompsonii (Girard 1851)
Common English name:
deepwater sculpin (Nelson et al. 2004)
Common French name:
chabot de profondeur (Coad et al. 1995)
Other common names:
kanayok (Inuktitut; McAllister et al. 1987).

The deepwater sculpin, Myoxocephalus thompsonii (Girard 1851), is a lake-dwelling sculpin with a North American distribution. It is closely related to the Arctic fourhorn sculpin, M. quadricornis (Linnaeus 1758). Much confusion and misinformation exists due to the lack of recognition of differences between three taxa: deepwater sculpin, freshwater forms of fourhorn sculpin, and marine fourhorn sculpin. This has resulted in misidentifications and muddled taxonomy. Scott and Crossman (1973) provided an extensive review of the papers that discuss the taxonomy of the deepwater sculpin. Girard (1851) first illustrated and described the bones of deepwater sculpin from Lake Ontario and listed them as Triglopsis thompsonii. This nomenclature was utilized by numerous subsequent authors, including Dymond (1926), and Hubbs and Lagler (1947). The genus Triglopsis was used in deepwater sculpin literature until the mid-1950s (McAllister 1961).

More recently, Walters (1955) referred to the deepwater sculpin as Myoxocephalus thompsoni. Based on a comparison of the morphological characteristics, distribution, and ecology of deepwater and fourhorn sculpin (M. quadricornis), McAllister and co-workers (McAllister 1961; McAllister and Aniskowicz 1976) agreed with this proposed nomenclature, and considered M. thompsoni and M. quadricornis to be distinct, but closely related, species. McAllister (1961) considered M. quadricornis to be the "ancestral" species from which M. thompsonii was derived.

Based on close morphological similarity, Hubbs and Lagler (1958) proposed that deepwater sculpin should be considered a subspecies (M. quadricornis thompsonii) of the fourhorn sculpin (M. quadricornis quadricornis). This nomenclature gained some acceptance (McPhail and Lindsey 1970). McAllister and Ward (1972) further accepted this subspecific designation of deepwater sculpin and reported the species as such when it was discovered in Upper Waterton Lake in Alberta, Canada. Scott and Crossman (1973) designated both the freshwater and marine forms as Myoxocephalus quadricornis, while Parker (1988) reported on the status of the deepwater sculpin in Canada, and referred to it as Myoxocephalus thompsonii. Using mitochondrial DNA (mtDNA) sequence data of eight individuals from two continental North American sites (Lake Michigan and Upper Waterton Lake), Kontula and Vainola (2003) supported the subspecific designations of North American deepwater and fourhorn sculpin proposed by Hubbs and Lagler (1958), and McPhail and Lindsey (1970), respectively. However, in a recent study based on the genetics and ecology of deepwater sculpin throughout their entire North American range, full specific rank for deepwater sculpin as M. thompsonii is supported (T. Sheldon, unpubl. data). Furthermore, freshwater populations of fourhorn sculpin found throughout northern Canada are phylogenetically nested within the marine fourhorn sculpin, and these are both clearly distinguishable from deepwater sculpin (T. Sheldon, unpubl. data).

Morphological Description

The deepwater sculpin (Fig. 1) has an elongate body and reaches an average length of 51-76 mm and a maximum length of 235 mm (Scott and Crossman 1973). It is both dorsoventrally flattened and stout anteriorly with its greatest width at the uppermost preopercular spine, equal body depth and width at the first dorsal fin, and slender caudal peduncle (Scott and Crossman 1973). It has a large mouth with small teeth on the upper and lower jaws, palatines, vomer and tongue (Scott and Crossman 1973; McPhail and Lindsey 1970). The eyes rest on top of the head. Preoperculomandibular pores are absent, but four preopercular spines are present. The upper two spines are large, pointing upward and posteriorly, while the lower two are reduced and point downward (Scott and Crossman 1973). Frontal and parietal spines, present in fourhorn sculpin, are absent in deepwater sculpin. Two dorsal fins are present, the first is small with 7 to 10 spines, the second is larger with a long base and 11 to 16 soft rays. The second dorsal fin can be enlarged in males (Scott and Crossman 1973). The pectoral fins are large with 15 to 18 soft rays, the pelvic fins are reduced with one spine and three (rarely four) rays, the anal fin has a long base with 11 to 16 rays, and the caudal fin is square or truncated. The overall coloration is dark grey to brown, with the grey-brown back gradually lightening along the sides and further lightening ventrally. The back is further marked with several dark saddles while the sides have mild speckling. Three dark, diffuse bands are present on the pectoral fins. The pelvic fins have light spotting, while the dorsal and anal fins show faint blotches (Scott and Crossman 1973; McPhail and Lindsey 1970).

Figure 1: The Deepwater Sculpin, Myoxocephalus thompsonii

Figure 1: The deepwater sculpin, Myoxocephalus thompsonii.

Drawing from Scott and Crossman 1973, used with permission of the authors.

True scales are absent in deepwater sculpin. Tubercles (typically less than 30) are present only above the lateral line, which is generally complete. There are typically 40 vertebrae (Scott and Crossman 1973).

The deepwater sculpin can be distinguished from species in the genus Cottus based on the presence of disklike tubercles on the upper sides along the body length, and distinct separation between the two dorsal fins. The deepwater sculpin also has a gill membrane that is free from the isthmus (McPhail and Lindsey 1970). The deepwater sculpin and fourhorn sculpin are very similar morphologically, but differ based on the absence of four cephalic horns on top of the head, which are present only in fourhorn sculpin (Stewart and Watkinson 2004).

Genetic Description

Kontula (2003) examined mtDNA sequence data of cytochrome b and ATPase6,8 from eight individuals in Upper Waterton Lake and Lake Michigan and suggested a single phylogeographical split separating deepwater sculpin from fourhorn sculpin. They proposed only subspecific designation for deepwater sculpin (M. q. thompsonii), based on low sequence divergence (0.9%) between deepwater and fourhorn sculpins. They report extremely low haplotype diversity of only 1-3 nucleotide differences out of 1976 bp (0.05-0.15%) within deepwater sculpin. However, their sample size (n=8) was too small to determine any phylogeographical detail within deepwater sculpin (Kontula and Vainola 2003).

Sheldon (unpubl. data) has also analyzed deepwater and fourhorn sculpin populations (including fourhorn sculpin from freshwaters in the Arctic) across their entire ranges. To gain further understanding of the relationship between fourhorn and deepwater sculpins, and to describe regional diversity within deepwater sculpin, he used mtDNA sequence data from the control region and ATPase6, 8 genes from a larger number of samples (approximately 300) from across Canada representing over 25 locations. Like Kontula and Vainola (2003), he found one major split between deepwater and fourhorn sculpins (both marine and freshwater forms). However, the use of a larger dataset resulted in sequence divergence estimates of 1.30% and 2.48% between fourhorn and deepwater sculpins for ATPase6,8 and the control region, respectively. These molecular data suggest that inland incursion and subsequent species formation occurred in the early Pleistocene. Regional diversity within deepwater sculpin was also evident and most likely corresponds to the refugial origins of these different lineages (Table 1; Figure 2). Three separate clades were present, one of which was common throughout the entire species range. The remaining clades were only locally distributed in Fairbank Lake, near Sudbury, and Upper Waterton Lake in southwestern Alberta (Fig. 3). The population in Upper Waterton Lake is particularly interesting, suggesting that deepwater sculpin may have invaded the area on at least two separate occasions; once during the early to mid-Pleistocene and once following the Wisconsin glaciation via glacial lakes. Based on these genetic data (in combination with ecological data), Sheldon et al. (unpubl. data.) propose full species-level designation for the continental deepwater sculpin of North America; thus M. thompsonii should be retained as a full specific taxon.

Table 1: Sequence divergence (%) between the clades of the deepwater sculpin from ATPase6, 8 (below diagonal) and the control region (above diagonal)
 Mississippi cladeSouthwest cladeFairbank clade
Mississippi clade 1.851.57
Southwest clade0.62 1.35
Fairbank clade0.530.6 

Modified from Sheldon et al. unpubl. data.

Figure 2: The Phylogeographic Structure of the Deepwater Sculpin Throughout its Range Using ATPase 6,8 and the Control Region

Figure 2: The phylogeographic structure of the deepwater sculpin throughout its range using ATPase 6,8 and the control region.

Figure 3: Mitochondrial Lineages of the Deepwater Sculpin Throughout its Range

Figure 3: Mitochondrial lineages of the deepwater sculpin throughout its range.

Black circle -- Fairbank clade, grey circles -- Mississippi clade, open -- southwest clade)

Designatable Units

Data indicate that most populations of deepwater sculpin belong to a single mtDNA lineage (T. Sheldon, unpubl. data; Fig. 2; Fig. 4). However, populations in Upper Waterton Lake and Fairbank Lake appear to exhibit distinct mitochondrial lineages; the Waterton Lake unit is interesting in the presence of two clades (Figure 3), but due to the small sample size it is recommended they not be considered as designatable units.

Deepwater sculpin have a somewhat disjunct distribution and populations appear to be isolated within 4 of the 14 Freshwater Aquatic Ecozones of Canada (see COSEWIC 2004, Figure 2). Locations in Quebec and eastern Ontario (Figure 4) are within Aquatic Ecozone 10 -- Great Lakes -- Western St. Lawrence; those in western Ontario, Manitoba and central Saskatchewan, as well as the disjunct Waterton Lake population are in Aquatic Ecozone 4 -- Saskatchewan-Nelson; northeastern Saskatchewan populations are in Aquatic Ecozone 5 -- Western Hudson Bay, and locations in northern Saskatchewan and the Northwest Territories are within Aquatic Ecozone 13 -- Western Arctic. Each of these could be considered a Designatable Unit (COSEWIC 2004). However, except for the Great Lakes/Upper St. Lawrence populations there is insufficient abundance and/or population size and trend information to individually assess the status of these populations, which are widespread and apparently not subject to any immediate threat. Therefore, we recommend assessment of the populations of Ecozones 4 (Saskatchewan -- Nelson), 13 (Western Arctic), as one unit, i.e., Western Populations, and those of the Great Lakes - Western St. Lawrence as a second unit, as most representative of the biological considerations for this species.

Figure 4: The Distribution of the Deepwater Sculpin Across Canada with the Generalized Extent of Former Glacial Lakes Mapped

Figure 4: The distribution of the deepwater sculpin across Canada with the generalized extent of former glacial lakes mapped.