Branchinella erosa, Timms, Brian V, 2012

Timms, Brian V, 2012, Further studies on the fairy shrimp genus Branchinella (Crustacea: Anostraca: Thamnocephalidae) in Australia, with descriptions of five new species, Zootaxa 3595, pp. 35-60 : 42-46

B. erosa sp. nov.

Figs. 1 E, 5C, 6.

Etymology. This species is named after the Greek God of love, Eros. The various tumidities on the thorax and genital segments of the female suggest touch is more obvious for mate recognition and possible stimulation in this species than in almost all other species of Branchinella .

Type locality. 20 km N of Moora, Coomberdale Rd West, hyposaline lake just north of road, 30o 28’ 02”S, 115o 59’ 18”E, collected 6 September 2009 by BVT.

Holotype: male deposited in the Western Australian Museum. Total length 18.5 mm Accession number: WAM C49891.

Allotype female, same data as holotype. Total length 18.3 mm. Accession number: WAM C49892. Paratypes 6 males, 4 females, same data as above, accession number: WAM C49893. Other Material. 2 males, 2 females, same locality as above, 7 September, 2011, BVT, WAM C49894; 5 males,

10 females, Coorow-Green Head Road, roadside pool, 5 September, 2009, BVT, WAM C49895.

Diagnosis. Males are indistinguishable from B. affinis . Females have paired sac-like tumidities dorsally on one or all of thoracic segments 6, 8 and 10 and a lateral triangular tumidity on genital segment 1, and ventral hooks on the brood chamber.

Description. Male. Eyes freely projecting on peduncles slightly shorter than eye diameter.

First antennae filiform, about 1.5 times longer than second antenna proximal segment, and terminating in 3–5 sensory setae.

Second antennae. Distal antennomere slightly longer than proximal segment. Each proximal antennomere cylindrical, fused basomedially and with a bulbous tumidity distomedially. Tumidities covered with denticles. Distal antennomeres evenly curved, hardly narrowing distally, but with a slightly expanded apex. These claspers with weakly developed medial transverse ridges.

Frontal appendage long and when extended reaching about the eight thoracomere. Frontal appendage strongly pseudosegmented, basal part 2/3 to 3/4 of its total length, and wide at 1/2 to 3/4 width of basal segment, though a little narrower basally and apically. The two distal branches less strongly pseudosegmented, about half the width of the basal trunk and terminating in a broad rounded apex. All pseudosegments papillate ventrally with digitiform outgrowths, most prominent along the distal two-thirds of the trunk.

Fifth thoracopod with endite 1+2 and 3 evenly curved, the former about three times the size of the later. Anterior setae of endite 1 naked, about 5/8ths length of adjacent posterior setae. Anterior setae of endite 2 small, about half the length of endite 1 anterior setae, bearing a one-sided pecten of spines and attended at its base by a small smooth spine. Endite 3 anterior seta about twice the length of endite 2 anterior seta and also bearing a onesided pecten of spines Both anterior setae of endites 2 and 3 attended by a small spine at their base. Endites 4–6 asymmetrical and covered with small spines. Endites 4 and 5 each with two anterior setae and endite 6 with one anterior seta, representing two types. The first on all three endites subequal to the length of endite 2 anterior seta and with feathered setae at their base and a one sided pecten along it. Second type on endites 4 and 5 about half the length of the anterior seta on endites 3,4, and 5 and naked. Posterior setae of all endites long and numbering about 90 on endites 1+2, 22 on endite 3, then 3, 2, 2 respectively on endites 4–6. Endopod broadly rounded and bearing about 36 spaced posterior setae (ca 10–12 medially before the slight dorsal notch and ca 25 laterally) and generally not as long as the posterior setae of the endites. The medial setae inserted on mounds and with 3–5 small basal setae. Exopod oval bearing about 45 posterior setae closely spaced, generally even shorter than the endopodal setae Epipodite elongate oval and unadorned. Praeepipodite large and broad, about one and a half the size of endite 1+2, and with a serrated margin, with most serrations asymmetrical and with an elongated bent point.

Genital segments similar, slightly enlarged mainly ventrally. Gonopods short, slightly longer than the first abdominal segment and with lateral basal swellings. No gonopod in type material extended for study.

Cercopods of typical structure for Branchinella .

Female. Eyes mounted on a similarly sized peduncle and whole structure about same length as the second antenna.

First antenna filiform and about 50% longer than second antenna.

Second antenna with a wide base and narrowing to a sharp apex and about the same length as the labrum.

Fifth thoracopod and cercopods as in male.

Paired dorsal sack-like tumidities on thoracic segments 6, 8 and 10 and also a minor pair of tumidities on the posterior dorsal surface of segment 5.

First genital segment with a triangular outgrowth laterally and brood chamber with a pair of hooks ventrally.

Brood pouch extending posteriorly as a tube to about abdominal segment 5.

Resting eggs have been described by Timms and Lindsay (2011). These eggs average 224 µm in diameter and mean depression number is 41.5. These depressions somewhat polygonal, but generally constricted and linear. Walls of depressions thick with ridge crests lumpy; floors concave and strongly dimpled. Depressions moderately deep (wall height: depression width 0.3–0.5).

Variability. In a study of other material, some variability in the frontal appendage of males and tumidities of females was noticed. In males the frontal appendage need not be so wide, nor the medial tumidities on the proximal antennomere so well developed, while some females lack the dorsal sac-like tumidities on thoracic segments 6 and sometimes 8 or 10 as well and the hook on the brood chamber can be small and indistinctive.

Differential diagnosis. Branchinella erosa sp.nov. is separated from B. affinis by a 9.4% difference in their 16SmtDNA ( Pinceel et al., 2012), but morphologically the two species are difficult to separate. In males, features of the frontal appendage, second antennae and genital segments lie within the range known for B. affinis . Though the trunk of the frontal appendage is unusually wide, the marginal sensory papillae more numerous and lining almost the whole length of the frontal appendage, and the tumidity on the medial surfaces of the proximal antennomere well developed. These features considered alone or in concert are not enough to be sure of distinctiveness of this taxon. Not enough is known on variation in thoracopods between populations of B. affinis / B. erosa sp. nov. and similar species to comment on apparent distinctiveness of the 5th thoracopod of B. erosa sp. nov. with its naked shorter anterior setae of endites 4 and 5, unusual serrated praeepipodite and medial endopodial setae bearing small basal setae. In an unusual case for Branchinella , it is the female which is distinctive (see Geddes, 1981, Timms 2004). No other described Branchinella has paired dorsal sac-like tumidities on thoracic 6, 8 and 10, or maybe just on 8 or 10 (see later), triangular lateral tumidities on the genital segment 1 and ventral hooks on the brood chamber. These tumidities, on male proximal antennomere and female genital segments, may mean this species ultilizes ‘lock and key’ amplexus ( Rogers, 2002). Certainly the female dorsal tumidities suggest a mate recognition role by the male frontal appendage being stimulated by these tumidities.

The resting eggs of B. affinis and B.erosa sp. nov. are too similar for reliable differentiation ( Figs. 1 A,E) as are those of many allied species (including B. anatinorhyncha sp. nov., B. basispina Geddes, B. denticulata Linder , and B. halsei Timms ) (Timms and Lindsay, 2011).

At least one other population of shrimps related to B. affinis—B. erosa sp. nov. from pools on the top of Uluru, Northern Territory, has various tumidities on the dorsal surface of posterior thoracic segments and on genital segment 1, but they are on different segments or different shapes to those in B. erosa sp. nov. These Uluru specimens could be of a different species or perhaps variation in B. erosa sp. nov., though this seems unlikely given the vastly different habitat and different structures. Detailed analysis must await further collections from Uluru.

Distribution and Ecology. B. erosa sp. nov. occurs in ponds and shallow lakes in the coastal area north of Perth, W.A. from about Moora to Coorow. Some of these were hyposaline up to 12 g /L when specimens were collected.

Figures

FIGURE 1. Resting eggs of selected species of Branchinella. A, B. affinis; B, B. clandestina n. syn.; C, B. anatinorhyncha; D, B. compacta; E, B. erosa sp. nov.; D, B. compacta; F, B. hearnii sp. nov.; G, B. herrodi sp. nov.; H, B. simplex; J, B. wellardi. Scale bars 50 µm. Images modified from Timms and Lindsay, 2011.