Small-mouthed salamander (Ambystoma texanum) COSEWIC assessment and status report: chapter 4

Distribution

Recent information on the distribution and the habitat association of A. texanum can be found in Petranka (1998). The species ranges from the coastal plain in eastern Texas, Louisiana, Mississippi, and western Alabama north to extreme southeastern Michigan, northern Ohio and Pelee Island in Ontario. Populations outside the coastal plain usually inhabit upland hardwood forests surrounding vernal ponds but some populations contain gilled, paedomorphic, adults that live in permanent or semipermanent ponds. Canadian A. texanum would represent a very small fraction of the range (less than 1% of the global range) and are only associated with vernal ponds and flooded wetlands.

Pelee Island

Specimens of Ambystoma were collected from various locations on Pelee Island from 1983 to 1995 (Fig. 1; Appendix 1). These specimens were used to test various hypotheses that related to genetics or to variation in fitness components that compared polyploids and diploids. Several papers have been published from these experiments but the actual distribution of Ambystoma texanum was not considered in these investigations. Because diploid and polyploid hybrids are much more common than either A. texanum or A. laterale on Pelee Island, a large sample size was necessary to estimate both the occurrence and the frequency of A. texanum in Island populations. Initially, specimens were collected as sexually mature adults but in the later collections, eggs and larvae were collected. Larvae that hatched from the eggs as well as wild caught larvae were raised in the laboratory through metamorphosis so that the individuals’ sex and genotype could be determined. Detailed methods for raising larvae, euthanizing, and processing transformed salamanders for the determination of genotype and ploidy are included in previous papers (Bogart 1982; Bogart et al. 1985; 1987; Bogart and Licht 1986).

Data were obtained (1984-1991) from more than 1200 larvae from the six breeding sites determined from our previous collections. Tissue samples from all individuals were analyzed for isozymes using starch-gel electrophoresis. Ploidy was determined by measuring the areas of erythrocytes and observations of allozyme band densities on the gels; if these other methods were inconclusive, karyotypes were examined. Only six breeding sites for Ambystoma have been found on Pelee Island and Ambystoma texanum was previously found in five of the six sites. The genotype frequencies were computed for comparisons (Table 1).

Table 1.  Frequency and Occurrence of Genotypes of Ambystoma texanum, A. laterale, and Hybrids from Six Sites on PeleeIsland (1984 to 1991).
Locality Nuclear Genome 2n
LL
Nuclear Genome 2n
TT
Nuclear Genome 2n
LT
Nuclear Genome 3n
LLT
Nuclear Genome 3n
LTT
Nuclear Genome 3n
LLLT
Nuclear Genome 4n
LLTT
Nuclear Genome 4n
LTTT
Nuclear Genome 4n
N
Quarry
81
0
64
335
2
13
3
0
498
(Males)
(39)
 
(1)
(13)
 
(2)
 
 
 
North End
Woods
0
15
2
1
1
0
0
0
19
(MalesFootnotea)
 
(1)
 
 
 
 
 
 
 
Pond
1
5
98
109
108
9
14
7
351
(MalesFootnotea)
 
(3)
(7)
 
(4)
 
 
 
 
Stone Road
1
56
20
4
15
0
0
0
96
(No males!)
 
 
 
 
 
 
 
 
 
Girl Guide
Pond
0
38
5
2
4
0
0
0
49
(Males)
 
(1)
 
 
 
 
 
 
 
Mosquito
Point
0
83
23
79
0
0
12
274
(Males)
 
(24)
 
(2)
(2)
 
 
 
 
Total
83
191
272
474
209
22
17
19
1287
Percentage
6.45
14.84
21.13
36.83
16.24
1.71
1.32
1.48
 

Hybrids made up 78 percent of the salamanders on Pelee Island and, of the pure species, Ambystoma texanum was found to be more common and more numerous than A. laterale. These data (Table 1) also show that each of the populations was distinctive with respect to the frequency of pure species compared with the frequencies of the various genomic contributions found in hybrids. Ambystoma laterale was almost completely restricted to the Quarry area in the north of Pelee Island and the highest density of A. texanum occurred at the opposite end of the Island in the Mosquito Point woods (Fig. 1). In both these populations, hybrids outnumbered either species. But, in populations on the east side of the Island (Girl Guide Pond, Stone Road, and North End Woods), A. texanum outnumbered the hybrids. The Pond locality was the only locality where both A. laterale and A. texanum as well as all combinations of hybrids were found to coexist.

Other islands in Lake Erie

Ambystoma texanum coexists with A. tigrinum, A. opacum and a suite of unisexual nuclear hybrids in Ohio on Kelleys Island, south of Pelee Island in Lake Erie (Bogart et al. 1987). Only unisexual nuclear hybrids have been found on the Bass Islands (also in Lake Erie) (Downs 1978). Downs assumed that the salamanders on those Islands reproduce by parthenogenesis (no males requied). However, a sperm cell was recovered from the cloaca of a Middle Bass Island, Ohio, female unisexual (Bogart 2003). The identity of the male is not known but, based on the fact that the female was a LTT unisexual, A. texanum was probably used as a sperm donor.

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