Pacific Giant Salamander, Coastal Giant Salamander
© 2007 Bradford Norman (1 of 54)
Can you confirm these amateur observations of Dicamptodon tenebrosus?
Dicamptodon tenebrosus (Baird and Girard, 1852[b])
Lawrence L.C. Jones1
1. Historical versus Current Distribution. Coastal giant salamanders (Dicamptodon tenebrosus) occur from Mendocino County, California, north to extreme southwestern British Columbia, Canada, along the Coast and Cascade Ranges, exclusive of the Olympic Peninsula (Nussbaum, 1976; Good, 1989; McAllister, 1995). They are found on one marine island, Long Island, Washington (Jones and Atkinson, 1989). Coastal giant salamanders are widely distributed throughout most of their range; however, in British Columbia, they have a limited range (Cook, 1970) and are listed as Endangered. There have been no indications of declines or increases in historical distribution, but there has probably been some fragmentation within their range resulting from habitat alterations, mostly due to forestry practices.
2. Historical versus Current Abundance. Throughout most of their range, coastal giant salamanders are one of the most abundant stream vertebrates. Larvae and paedomorphic animals may reach high densities and are often the dominant vertebrate predators because of a relatively large biomass (Hall et al., 1978; Murphy and Hall, 1981; Hawkins et al., 1983; Corn and Bury, 1989b). Numerous studies have investigated the relationships of the aquatic life stages of coastal giant salamanders to timber harvesting (Hall et al., 1978; Murphy and Hall, 1981; Murphy et al., 1981; Hawkins et al., 1983; Bury and Corn, 1988a; Corn and Bury, 1989a; Bury et al., 1991; Welsh and Lind, 1991; Welsh, 1993; Kelsey, 1995). These studies have produced variable results, with aquatic coastal giant salamanders usually found in higher densities in streams in unlogged mature and old‑growth forests; in several studies, higher densities were reported in streams crossing recently logged sites, presumably the result of an increase in primary productivity. Welsh and Ollivier (1998) studied the effects of increased sedimentation and found that coastal giant salamanders occurred in higher densities in streams unimpacted by fine sediments. Increased siltation negatively affects these animals by filling rocky interstices under cover objects, particularly in low‑gradient streams that do not adequately flush sediments. Despite the negative effects of habitat alteration on coastal giant salamanders, they seem to be more resilient than other stream‑associated amphibians in the coastal Northwest and are likely to persist in streams that are moderately disturbed.
Metamorphosed coastal giant salamanders are encountered less frequently in most field investigations (e.g., Bury, 1983; Bury and Corn, 1988b; Raphael, 1988; Welsh and Lind, 1988, 1991; McComb et al., 1993), even though they are probably locally abundant in inaccessible subterranean retreats. Consequently, stand-scale habitat relationships for the terrestrial life stage are poorly known. Habitat use patterns from a telemetry study of terrestrial animals (Johnston, 1998) were somewhat nebulous, but it appeared that good-quality, near-stream habitats (e.g., old forests or younger forest stands with wide stream buffers) were selected.
3. Life History Features.
A. Breeding. Reproduction is aquatic.
i. Breeding migrations. Adult females migrate from upland habitats to streams to oviposit.
ii. Breeding habitat. Pools or slow-moving portions of streams (Nussbaum, 1969a; Jones et al., 1990).
i. Egg deposition sites. Courtship sites are unknown. Eggs are attached singly to the undersurface of rock or wood, and females attend their eggs. Three nest sites have been reported from the Oregon Coast Range. Nussbaum (1969a) reported two subterranean nests in shallow, slow-moving portions of two streams; Jones et al. (1990) described the nest of a paedomorphic female in a small pool of a third stream. No nests are known from lentic habitats.
ii. Clutch size. Nussbaum (1969a) reported 83 and 146 eggs from the two subterranean nests he discovered, and Jones et al. (1990) reported 129 eggs from the paedomorphic female he found.
i. Length of larval stage. Larvae typically metamorphose 18–24 mo after hatching (Nussbaum et al., 1983). Some individuals may overwinter and transform in the third year (Nussbaum and Clothier, 1973). Metamorphosis occurs from 92–166 mm TL (Nussbaum and Clothier, 1973).
ii. Larval requirements.
a. Food. Larval coastal giant salamanders are known to feed on a variety of invertebrates, mostly benthic, including insect larvae and adults, amphipods, ostracods, trematodes, mollusks, and crayfish, as well as some vertebrates such as salmonids (Salmonidae), sculpins (Cottidae), northwestern salamanders (Ambystoma gracile), and congeners (Fitch, 1936; Schonberger, 1944; Johnson and Schreck, 1969; Antonelli et al., 1972; Parker, 1993a,b, 1994; Esselstyn and Wildman, 1997).
b. Cover. Larvae and paedomorphic animals are associated with coarse substrates of streams, rivers, and mountain lakes (Nussbaum et al., 1983; Leonard et al., 1993). Welsh (1993) found coastal giant salamanders most abundant in late-successional forests, but also noted that within-stream conditions were better predictors of presence and abundance than surrounding forest conditions. Parker (1991) reported that salamander density increased with density of large stones (> 7.5 cm). Welsh (1993) found salamanders most abundant in reaches of intermediate gradient due to a combination of microhabitat diversity (i.e., pools, riffles, runs) and sediment flushing. Several studies have concluded that fine sediments fill and eliminate critical cover interstices within the streambed matrix (e.g., Hall et al., 1978; Hawkins et al., 1983; Corn and Bury, 1989a; Welsh and Ollivier, 1998). Welsh (1986) reported a paedomorphic individual in a pitfall trap 3 m from a small stream. Larvae increase cover use in the presence of chemical cues from cutthroat trout (Onchorhynchus clarki; Rundio and Olson, 2003).
iii. Larval polymorphisms. None.
iv. Features of metamorphosis. Paedomorphosis is typical in perennially aquatic sites (large streams, rivers, lakes, and ponds), while small streams that may dry up tend to harbor predominantly metamorphic populations (Nussbaum and Clothier, 1973).
v. Post‑metamorphic migrations. Newly metamorphosed animals may venture into upland habitats during rainy periods, ≤ 400 m or more from water (McComb et al., 1993).
vi. Neoteny. Coastal giant salamanders exhibit facultative paedomorphosis (Nussbaum, 1976).
D. Juvenile Habitat. Similar to adults and larvae.
E. Adult Habitat. Terrestrial adults are more abundant in forested habitats than in pre-canopy sites, where they are found under rocks and in logs, root channels, and burrows (Johnston, 1998). Paedomorphic animals require perennial water, where they seek cover in coarse substrates. They are found primarily in lotic waters, from headwaters to rivers, but may also be found in some lentic habitats (Nussbaum et al., 1983; Leonard et al., 1993). Paedomorphic animals have habitat affinities similar to large larvae.
F. Home Range Size. Johnston (1998) found an average home range of 3,047–5,196 m2 (modified convex polygon and 95% adaptive kernal methods, respectively) for 20 radio-telemetered, metamorphosed animals in British Columbia.
G. Territories. Not documented.
H. Aestivation/Avoiding Dessication. Unknown; during the summer, larvae and paedomorphic animals are active in aquatic habitats (personal observations).
I. Seasonal Migrations. Newly metamorphosed (Nussbaum et al., 1983; Johnston, 1998), and sometimes paedomorphic (Welsh, 1986) animals move out of streams to the surrounding habitat during rainy and wet periods. Some metamorphosed individuals remain in the vicinity of the stream, others prefer upland areas (Johnston, 1998).
J. Torpor (Hibernation). Larvae (Antonelli et al., 1972) and adults probably seek refuge from temperature extremes in the winter, at least in areas with freezing temperatures.
K. Interspecific Associations/Exclusions. In a few streams of southern Mendocino County, coastal giant salamanders are known to hybridize with California giant salamanders (D. ensatus; Good, 1989). Coastal giant salamanders are sympatric with Cope's giant salamanders (D. copei) across the range of the latter, except for the Olympic Peninsula, and may occur in the same stream (Nussbaum and Clothier, 1973; Nussbaum, 1976; Daugherty et al., 1983). The extent and mechanisms of sympatry are not well known, as small larvae are phenotypically similar and are often grouped together as Dicamptodon spp. in scientific investigations (e.g., Antonelli et al., 1972; Wilkins and Peterson, 2000). The two species are not known to hybridize (Nussbaum, 1976; Good, 1989). They are often sympatric with torrent salamanders (Rhyacotriton spp.) and tailed frogs (Ascaphus truei; Bury et al., 1991). Welsh (1993) found larval coastal giant salamander numbers higher in stream reaches with larval tailed frogs and southern torrent salamanders present, both of which are known to be prey of coastal giant salamanders. They are sometimes found in streams with sculpins or salmonids, with which they may compete for food or function as predator or prey (Antonelli et al., 1972; Parker, 1993a,b).
L. Age/Size at Reproductive Maturity. Except for paedomorphic animals, larvae mature after metamorphosis, generally when they are 115 mm SVL or longer (Nussbaum, 1976).
M. Longevity. Nothing is known about longevity in coastal giant salamanders, but other large aquatic salamanders can be long lived (Hairston, 1987).
N. Feeding Behavior. Coastal giant salamanders are sit-and-wait predators, lunging short distances with surprising speed to procure live prey (Bury, 1972). Adults may consume small mammals and other vertebrates and have even been seen with snakes in their jaws (Diller, 1907; Graf, 1949; Wilson, 1970; Bury, 1972).
O. Predators. Weasels and river otters (Mustelidae), water shrews (Soricidae), garter snakes (Thamnophis sp.), salmonids, and conspecifics (Fitch, 1941; Nussbaum and Maser, 1969; Nussbaum et al., 1983; Lind and Welsh, 1990, 1994; Parker, 1993a).
P. Anti‑Predator Mechanisms. A variety of defenses may be employed, including biting, arching, tail‑lashing, and exudation of noxious skin secretions (Brodie, 1977; Nussbaum et al., 1983).
Q. Diseases. Unknown.
R. Parasites. Known parasites include helminths (Lynch, 1936; Lehmann, 1954; Anderson et al., 1966).
4. Conservation. There have been no indications of declines or increases in the historical distribution of coastal giant salamanders, but there has been some fragmentation within their range resulting from habitat alterations, mostly due to forestry practices. Coastal giant salamanders seem to be more resilient than other stream‑associated amphibians in the coastal Northwest and are likely to persist in streams that are moderately disturbed.
1Lawrence L.C. Jones
2Hartwell H. Welsh Jr.
Literature references for Amphibian Declines: The Conservation Status of United States Species, edited by Michael Lannoo, are here.
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