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Montana Field Guides

A Subterranean Amphipod - Stygobromus tritus

Species of Concern

Global Rank: G1G2
State Rank: S1S2
* (see State Rank Reason below)

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State Rank Reason (see State Rank above)
This Subterranean Amphipod is currently listed as "S1S2" in MT due to extremely limited and/or rapidly declining population numbers, range and/or habitat, making it highly vulnerable to global extinction or extirpation in the state. These subterranean amphipods are generally endemic to a few locations.
 
General Description
Stygobromus: without eyes and pigment, subterranean; sexually mature animals range from 2.5-12.0 mm; antenna 1 longer than antenna 2; propod of gnathopod 1 usually longer than second propod; pereopod 6 a little longer than pereopod 7, always longer than pereopod 5; uronites free, not fused, dorsal margins without spines; telson usually longer than broad, apical margin entire or emarginate (but not deeply cleft), armed with spines. The principle diagnostic character of the genus is the uniramus third uropod (U3), in which the ramus is 1-segmented, shorter than the peduncle (sometimes vestigial or absent), and when present is armed with 1 to several short spines (Holsinger 1974; Wang and Holsinger 2001).

S. tritus: [From Holsinger 1974] "Antenna 1, 60 percent as long as body, 50 percent longer than antenna 2, with up to 14 segments in primary flagellum. Antenna 2, flagellum with 6 segments. Mandibles similar to those of S. hubbsi. Maxilla 1: inner plate with 6 apical, plumose setae; palp with 4 short spines and 2 setae apically. Maxilla 2, inner plate with oblique row of 7 or 8 plumose setae on inner margin. maxilliped: inner plate with 3 serrate spines, 3 plumose spines and 2 stiff setae apically; outer plate with 1 small spine and several stiff setae apically to subapically on inner margin. Lower lip, inner lobes indistinct, vestigial or absent" (p. 8). Sternal processes absent; coxal plate of pereopod 4 reaching up to half the length of segment 2 but usually less; bases of pereopods 5-7 not broadly expanded, usually rather narrow, distoposterior lobes small or lacking; telson not twice as long as broad, not much broader proximally than distally; inner plate of maxilla 1 with 6-9 apical, plumose setae; inner plate of maxilla 2 with oblique row of 13-15 plumose setae on inner margin; inner plate of maxilliped with 8 to 13 apical spines; bases of pereopods 5-7 often not much broader proximally than distally; apical margin of telson usually with shallow notch and 16 apical spines, 4 of which are up to twice the length of the others; uropod 1 usually with < 38 spines, uropod 2 usualy with < 27 spines, apical spines of telson mostly subequal in length; outer plate of maxilla with 7 serrate spines apically; sexually mature females < 7.0 mm body length. Largest female 5.0 mm, largest male 4.5 mm. See Diagnostic Characteristics and Holsinger (1974) for description of additional body regions.

Phenology
Not described. Presumably active throughout the year. Type specimens (males and females) collected 11 and 19 October; additional individuals (males, females, juvenile) from Missoula County collected in May (Wang and Holsinger 2001).

Diagnostic Characteristics
"A rather small subterranean species distinguished by the palms of the gnathopoda propods, which are slightly concave and have short spine rows; comparatively narrow propod of gnathopod 2; slender bases of pereopods 5-7 without distoposterior lobes; rather long marginal spines on inner ramus of uropod 2; short telson (borader than long); proportionately short ramus of uropod 3" (Holsinger 1974:8). Similar to S. puteanus but differs by having concave palms of gnathopodal propods, narrower bases of pereopods 5-7, fewer apical spines on uropod 3 and telson, smaller body length of mature females (Wang and Holsinger 2001).

Species Range
Montana Range

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Range Comments
Montana endemic; known only from type locality, a water well at Victor Crossing, Ravalli County, and shallow water wells in adjacent Missoula County (Holsinger 1974; Wang and Holsinger 2001).

Summary of Observations Submitted for Montana
Number of Observations: 2

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Relative Density

Recency

 

(Observations spanning multiple months or years are excluded from time charts)



Migration
Not described. Likely moves through hyporheic groundwater system.

Habitat
Limited information. Water wells, hyporheic groundwater system; well at type locality 6.1 m deep (Holsinger 1974), Missoula County wells 2.6-6.6 m deep (Wang and Holsinger 2001).

Food Habits
Not described.

Reproductive Characteristics
Limited information. Females outnumbered males 3:1 in October samples, 3.5:1 in May samples. Females 4.5-5.0 mm body length sexually mature with setose brood plates, but none were ovigerous. Juveniles present in May.

References
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Citation for data on this website:
A Subterranean Amphipod — Stygobromus tritus.  Montana Field Guide.  .  Retrieved on , from