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Montana Field Guide

Montana Field Guides

A Subterranean Amphipod - Stygobromus puteanus

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. puteanus: [From Holsinger 1974] "Antenna 1, 45 to 50 percent as long as body, about 40 percent longer than antenna 2; primary flagellum with 19 or 20 segments. Antenna 2; flagellum with 5 segments. Mouthparts similar to those of S. hubbsi. Gnathopod 1: propod palm slightly convex, armed with double row of about 5 small spine teeth; posterior angle distinct, with 1 long and 2 short spine teeth on outside, 5 short spine teeth on inside; posterior margin without setae; medial setae few in number, singly inserted; dactyl nail rather long. Coxal plate of gnathopod 1 about twice as long as broad, with 3 marginal setae. Gnathopod 2: propod up to one-third larger than 1st propod; palm oblique, nearly straight, armed with double row of 8 or 9 spine teeth; posterior angle defined by 1 long, curved spine tooth and 3 long setae on outside, 1 or 2 small spine teeth on inside; posterior margin convex, with 2 sets of long setae; inferior medial setae singly inserted, superior medial setae slightly and doubly inserted; dactyl rather long, slightly curved, nail moderately long. Coxal plates of gnathopod 2 and pereopod 3 rather shallow, about as broad as long; plate 2 with 3 marginal setae, plate 3 with 6 marginal setae. Coxal plate of pereopod 4 broader than long, with 8 marginal setae" (p. 11). 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 with 52 spines, uropod 2 with 34 spines. Largest female 6.5 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), with setose brood plates and apparently sexually mature, were collected in February.

Diagnostic Characteristics
A medium-sized subterranean species similar to S. tritus but distinguished from that species by the straight to slightly convex palms of gnathopodal propods, broader bases of pereopods 5-7; having more apical spines on uropods 1 and 2, longer apical spines on uropod 3; proportionately longer telson with 16 apical spines, 4 of which are very long and slender (From Holsinger 1974; Wang and Holsinger 2001).

Species Range
Montana Range

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Range Comments
Montana endemic; known only from type locality, a water well near Three Forks, Gallatin 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 reported to be 24.4 m deep (Holsinger 1974).

Food Habits
Not described.

Reproductive Characteristics
Little information. Females 6.0-6.5 mm body length with setose brood plates and apparently sexually mature (Holsinger 1974).

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