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Montana Field Guide

Montana Field Guides

A Subterranean Amphipod - Stygobromus obscurus

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. obscurus:[From Holsinger 1974] "Antenna 1: about 50 percent as long as body, 35 percent longer than antenna 2; primary flagellum with 15 segments. Antenna 2; peduncular segments 4 and 5 with a number of slender spines and stiff setae; flagellum with 5 segments. Mandibles subequal; spine row with 5 spines; palpal segment 2 with 8 long, inner marginal setae; palpal segment 3 with row of short setae on inner margin and 5 long setae near apex. Maxilla 1: inner plate with 5 apical, plumose setae; outer plate with 4 spines and 2 setae apically. Maxilla 2: inner plate with oblique row of 6 plumose setae on inner margin; outer plate with 8 to 10 stiff setae. Maxilliped: inner plate with 4 thick spines and 1 stiff seta apically; outer plate reaching only about one-fourth the length of palpal segment 2, apex with 2 spines and 1 seta, inner margin with 4 or 5 setae. Lower lip with prominent outer lobes and small but distinct inner lobes" (p. 55). Simple lateral sternal processes present on pereonites 6 and 7, gnathopodal propods subequal in size (1st perhaps slightly larger than 2nd); bases of pereopods 5-7 narrow, not expanded proximally; telson incised nearly one-fourth the distance to base. Largest female 7.0 mm, male unknown. See Diagnostic Characteristics and Holsinger (1974) for description of additional body regions.

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: relatively slender propods of gnathopods; shallow coxal plates of gnathopod 2 and pereopods 3 and 4; narrow bases of pereopods 5-7 which lack distoposterior lobes; 2 pairs of simple, lateral sternal gills on pereonites 6 and 7; absence of ventral spines on pleonal plates; heavily spinose uropods and telson; and relatively long spines on uropods 3 and telson" (Wang and Holsinger 2001:121). "This species differs from members of the hubbsi group by having gnathopodal propod 1 equal to or slightly larger than propod 2, possession of setae on the posterior margin of gnathopodal propod 1, possession of lateral sternal processes, equal length of pereopods 6 and 7, and heavily spinose uropods. With S. montanensis, it shares the possession of lateral sternal processes and setae on the posterior margin of gnathopodal propod 1, but in other characters it does not appear to be closely related. Were it not for the free uronites and its geographic distribution, S. obscurus could be assigned to the genus Stygonectes" (Holsinger 1974:58).

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



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

Not described. Likely moves through hyporheic groundwater system.

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.

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