Pacificabathynella ruthae, Camacho & Newell & Reid, 2009

Camacho, A. I., Newell, R. L. & Reid, B., 2009, New records of Bathynellacea (Syncarida, Bathynellidae) in North America: three new species of the genus Pacificabathynella from Montana, USA, Journal of Natural History 43 (29 - 30), pp. 1805-1834 : 1825-1831

Pacificabathynella ruthae sp. nov.

( Figures 6 to 8)

Material examined

Type locality. Flathead County, Middle Fork Flathead River , 2 February 2004, Wally ER, Chris B, HA-12, Wally C wells (six males and two females), Montana, USA; other locality: Flathead County, Great Bear well, 9 November, 2003 (one male and four females), Montana, USA (see Table 1). The details of the new description are based on all 13 specimens. The holotype is a male and the allotype is a female and the type series contains 11 additional specimens (six males and five females). (Holotype MNCN20.04 /8096, allotype MNCN20.04 /8097 and type series MNCN20.04 /8098.)

Description

Body. Total length of holotype (male) 1.94 mm and allotype (female) 1.68 mm. Largest male total length 2.07 mm, smallest 1.57 mm; largest female total length 1.78 mm, smallest 1.45 mm (see Table 4). Body form not very elongated, almost cylindrical, approximately 13 times long as wide. Pleotelson with one barbed dorsal seta on either side, shorter than furcal rami.

Antennule ( Figure 6A ). Seven segments; length of first three segments similar to the other four segments; segments five and six similar in size and smaller than other segments; setulation as in Figure 6A ; three aesthetascs on segments six and seven. A.I almost equal in length to A.II.

Antenna ( Figure 6B ). First four segments longer than terminal three; three terminal segments of similar length; setal formula: 0/2+exp/2+0/2+0/0+0/2+2/5; ventromedial seta of exopod absent, two terminal setae, one a bifurcated sensory seta.

Labrum ( Figure 6C ). With median protusion.

Paragnath ( Figure 6D ). Large, subrectangular, with one tooth on distal part and with very thick setulation on entire surface of distal half.

Mandible ( Figure 6E–G ). Palp with three segments, terminal segment with two barbed similar claws, second segment more or less rectangular. Incisor process (pars incisiva) with two teeth; processus incisivus accessorius with one tooth, and one long seta-like tooth; pars molaris with two dentated structures ( Figure 6F,G ) parallel to main axis of teeth, first with four similar teeth, other with small similar teeth arranged in two rows, crown-like, most distal tooth is strong and longer than others.

Maxillule ( Figure 6H ). Proximal endite with four setae; distal endite with six teeth, four with spines (denticles), other two setae-like, with three plumose setae.

Maxilla ( Figure 6I ). Four segments; setal formula 7, 4, 7, 5.

Thoracopods I to VII ( Figure 7A–H ). Sexual dimorphism Th. VI, ( Figure 7F,H ). Th. I ( Figure 7A ) lightly smaller than others; Th. II ( Figure 7B ) to V ( Figure 8B ) similar in size to Th. VII ( Figure 7G ) and longer than others. Th. I and II without epipod; coxa of Th. I with long, strong plumose seta, basipod with three setae, one of these plumose. Exopod one- segment for all thoracopods, shorter than endopod in all cases; with six barbed setae, two terminal, one dorsal and three ventral. Endopod with four segments in all thoracopods, setal formulae (number of setae of basipod in brackets):

Th. I: (3) 6+0/5+1/5+0/5

Th. II: (4) 4+0/4+1/4+0/5

Th. III: (4) 3+0/3+1/2+0/5

Th. IV: (3) 2+0/3+1/2+0/4

Th. V: (1) 2+0/2+1/2+0/4

Th. VI: (1) 1+0/0+1/2(1)

Th. VII: (1) 1+0/0+1/0+0/2(1)

Thoracopod VI ( Figure 7F ) with only three segments in endopod: first segment broader than usual, second segment broad-dilated, bearing on its outer margin strong medially curved seta and third segment small with two setae, one long and another short. Basipods of Th. I and Th. VII have tufts of long and fine setules.

Male thoracopod VIII ( Figure 8A–C ). Longitudinal axis of coxa and basipod form angle of 25°. Penial region with frontal lobe, inner lobe, outer lobe. Frontal lobe with two very well-developed lobules that cover outer lobe and partially cover inner lobe. Well-developed inner lobe has distal region elongated with very large bidentated projection on internal side. Outer lobe similar to frontal lobe and with two almost square lobules. Basipod very large, with distal crest-like protuberance, three rows of setules distally on inner side, two smooth setae, one distal-lateral and another on external side. Endopod one-segmented, small, 33% of the exopod, with only one barbed distal seta. Exopod well developed with six setae.

First pleopods ( Figure 8D ). Two segments, first segment with one very long seta; second segment with six setae.

Female thoracopod VIII allotype ( Figure 8G ). Coxa with one, small, barbed lateral seta; very large epipod, twice length of basipod; basipod almost square, longer than endopod, with smooth seta; endopod one-segmented, with one smooth seta; exopod a little longer than endopod and with two apical smooth setae of similar length.

Female thoracopod VI allotype ( Figure 7H ). Exopod one segmented and with six setae, as in other thoracopods; endopod four-segmented, setal formula 1+0/0+1/0+0/ 2(1). Number of setae on segments of endopod and on basipod of thoracopods is different on female. Setal formulae of allotype (number of setae of basipod in brackets):

Th. I: (3) 6+0/6+1/6+0/5

Th. II: (5) 5+0/5+1/4+0/5

Th. III: (4) 3+0/3+1/3+0/5

Th. IV: (4) 3+0/3+1/3+0/5

Th. V: (4) 3+0/3+1/3+0/5

Th. VI: (1) 1+0/0+1/0+0/2(1)

Th. VII: (1) 1+0/0+1/0+0/2(1)

Uropods ( Figure 8E ). Sympod a little longer than endopod, almost twice as long as wide, with six equal spines on distal end; endopod 10% longer than exopod, with six strong claws, two most distal longest, twice length of penultimate, three times length of two most basal, with two very long terminal setae and with two shorter ones located dorsolaterally, all of which barbed; exopod with nine setae, two terminal, four medial, three basal.

Furcal rami ( Figure 8F ). Almost square, bearing five spines; a long dorsal spine, almost half length of second spine, second spine 30% longer than two equal length medial spines, medial spines 25% times longer than fifth.

Variability

The variability is very low between individuals and populations.

The observed variability only affects the numbers of setae on the different segments of the endopods of thoracopods I to V in males and females (see Table 5), which differ from the holotype and allotype as follows :

Males:

Th. I: 6+0/5–6+1/5–6+0/5

Th. II: 4+0/4+1/4+0/5

Th. III: 3+0/3+1/2–3+0/5

Th. IV: 2+0/2–3+1/2–3+0/4–5

Th. V: 2+0/2–3+1/2+0/4

Females:

Th. I: (3) 5–6+0/5–6+1/5–6+0/5

Th. II: (5) 4–5+0/4–5+1/3–4+0/5

Th. III: (4) 3+0/3+1/3+0/5

Th. IV: (4) 3+0/3+1/3+0/5

Th. V: (4) 2–3+0/2–3+1/2–3+0/5

Etymology

The species name is derived from Ruth Dalimata, wife of John Dalimata, who owns the land surrounding the Nyack well sites and has long been supportive of the research occurring on this large floodplain.

Remarks

Pacificabathynella ruthae sp. nov. is the largest known species of the genus (see Table 4); the largest specimens reach 2 mm. This is the only species of the genus in which the antennule is similar in length to the antenna. The female thoracopod VIII has the smallest epipod within the genus. Despite being the largest species of the genus it does not have more setae on thoracopods (see Table 5) and uropods (see Table 3) than other species. The endopod of the uropod has six spines (the others species have four, five or eight), the exopod has nine setae, while other species have only seven or eight setae.

The species that share the greatest number of similar characters are P. stanfordi .sp. nov. and P. sequoiae . Pacificabathynella stanfordi sp. nov. differs principally in the number of setae on the exopod of the thoracopods, having six (see Table 5), which is the norm in the genus, and the male thoracopod VIII is very different from that of the other species.

Figures

Figure 6. Pacificabathynella ruthae sp. nov., male holotype. (A) Antennule (dorsal view); (B) antenna (ventral view); (C) labrum; (D) paragnath (E) mandible; (F, G) mandible masticatory part; (H) maxillule (dorsal view) and (I) maxilla (dorsal view). Scale bar in mm.

Figure 7. Pacificabathynella ruthae sp. nov., male holotype. (A) Thoracopod I; (B) thoracopod II; (C) thoracopod III; (D) thoracopod IV; (E) thoracopod V; (F) thoracopod VI; (G) thoracopod VII and (H) thoracopod VI female allotype. Scale bar in mm.

Figure 8. Pacificabathynella ruthae sp. nov., male holotype. (A) thoracopod VIII (laterointernal view); (B) thoracopod VIII (frontal view); (C) thoracopod VIII (latero-external view); (D) first pleopod; (E) uropod (latero-external view); (F) furcal rami (drsal view) and (G) thoracopod VIII female (frontal view). Scale bar in mm. Abbreviations: F. lb, frontal lobe; I. lb, inner lobe; O. lb, outer lobe; Prj, projection of inner lobe; Bsp, basipod; Crt. bsp, crest-like protruberance of basipod; Endp, endopod; Exp, exopod.