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A Parabathynellid Crustacean - Pacificabathynella ruthae


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State Rank: SNR

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General Description
[From Camacho et al. 2009; Camacho et al. 2016] Genus Pacificabathynella: Male body length 0.53-2.07 mm, female body length 0.51-1.78 mm; antennule with 7 segments, antenna with 7-8 segments; mandible with 2 teeth on incisor process (pars incisiva), pars molaris with 2 bidentated structures; labrum with a small central protuberance at distal edge; epipod of thoracopod I absent, epipod of thoracopod II absent or present; exopod of thoracopods I-VII with 5-6 setae; thoracopods VI and VIII sexually dimorphic. Thoracopod VI of male with 3-segmented endopod, thoracopod VIII of male with 3 lobes around genital opening, basipod prominence present, exopod 1-segmented, endopod 1-segmented; thoracopod VI of female with 4-segmented endopod, thoracopod VIII of female with 1-2 plumose setae on coxa, 1-2 setae on basipod, epipod is present, exopod and endopod 1-segmented, 2 apical setae on exopod, 1-2 apical setae on endopod; sympod of uropod with 6-8 spines, endopod of uropod with 4-8 strong spines along inner margin; furcal rami with 5 spines.

P. ruthae: Male body length 1.57-2.07 mm, female body length 1.45-1.78 mm; body form not very elongated, almost cylindrical, approximately 13 times long as wide; pleotelson with 1 barbed dorsal seta on either side, and shorter than furcal rami. Antennule and antenna of similar length, each with 7 segments; epipod of thoracopods I-II absent, exopod of thoracopods I-VII with 6 setae; female thoracopod VIII with 1 plumose seta on coxa, 1 smooth seta on basipod, 2 smooth setae on exopod and endopod; male thoracopod VIII with 2 setae on basipod, 1 seta on endopod, crest of basipod pronounced; 1/7 setae on pleopod; endopod of uropod with 6 spines, exopod of uropod with 9 setae; first spines on furca 1.5 times longer than second and 2 times longer than dorsal spines, size of first spines>dorsal spines>second spines=third spines>fourth spines.

Phenology
Not described. Adult males and females collected in February and November (Camacho et al. 2009; Camacho et al. 2016). Adults probably active throughout the year.

Diagnostic Characteristics
The only species of the genus where the antennule and antenna are of similar length; female thoracopod VIII with the smallest epipod in genus; endopod of uropod with 6 spines (not 4, 5, or 8), exopod of uropod with 9 setae (not 7 or 8).

Range Comments
Montana endemic: reported from one locality (five wells) on the Nyack floodplain (Nyack Aquifer) in Flathead County (Camacho et al. 2009), at about 1030 m elevation. Specific well depths not reported but largely between 1 and 12 m.

Migration
Not described. Likely moves through hyporheic groundwater system.

Habitat
Hyporheic groundwater aquifers and water wells in alluvial flood plains, interstitial (Camacho et al. 2009).

Food Habits
Not described.

Reproductive Characteristics
Not described.

References
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Citation for data on this website:
A Parabathynellid Crustacean — Pacificabathynella ruthae.  Montana Field Guide.  .  Retrieved on , from