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Montana Field Guide

Montana Field Guides

A Subterranean Amphipod - Stygobromus montanensis

Species of Concern

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

Agency Status
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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. montanensis: [From Holsinger 1974] "Antenna 1, 50 percent as long as body, 40 percent longer than antenna 2; primary flagellum with 13 segments. Antenna 2, flagellum wit 6 segments. Mandibles subequal; spine row with 7 spines; segment2 of palp with 7 stiff setae on inner margin; segment 3 of palp with row of short setae on inner margin and 3 long setae on apex. Maxilla 1: inner plate with 7 plumose setae apically; outer platewith 3 apical spines and 3 or 4 setae subapically. Maxilla 2: inner plate with oblique row of 7 plumose setae; outer plate with about 15 long, stiff setae apically. Maxilliped: inner plate with 2 bladelike spines, 4 serrate spines and 3 setae apically; outer plate with 1 bladelike spine and 5 setae apically. Lower lip, inner lobes tiny or vestigial" (p. 52). Simple lateral sternal processes present on pereonites 6 and 7, gnathopodal propod 2 larger than 1; bases of pereopods 5-7 broader proximally than distally; telson with shallow notch. Largest female 5.5 mm, male unknown. See Diagnostic Characteristics and Holsinger (1974) for description of additional body regions.

Phenology
Not described. Presumably active throughout the year. Type specimens (all female) collected 11 October.

Diagnostic Characteristics
"A small to medium-sized subterranean species, distinguished by possession of few setae on posterior margin of gnathopod propod 1 and presence of 2 pairs of tiny, simple lateral sternal gills on pereonites 6 and 7; 4 relatively long spines on ramus of uropod 3" (Wang and Holsinger 2001:121). "The presence of setae on the posterior margin of gnathopodal propod 1 and possession of lateral sternal processes exclude this species from the hubbsi group. With the exception of these two characters, however, S. montanensis appears to be more closely allied with species of this group than with S. obscurus" (Holsinger 1974:55).

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 (Holsinger 1974; Wang and Holsinger 2001).

Summary of Observations Submitted for Montana
Number of Observations: 1

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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).

Food Habits
Not described.

Reproductive Characteristics
Not described.

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