Hexabathynella valdecasasi, Camacho, 2003

Camacho, Ana I., 2004, An overview of Hexabathynella (Crustacea, Syncarida, Parabathynellidae) with the description of a new species, Journal of Natural History 38 (10), pp. 1249-1261 : 1250-1254

Hexabathynella valdecasasi sp. n.

(figures 1, 2)

Material examined. Gravel bank on the Torcon stream in Las Ventas con Peña Aguilera ( Toledo Province , Spain), collected by Carolina Noreña and collaborators (14 April 1996) (two males, one female and one juvenile). All four specimens were studied. The holotype is a male and the type series contains two specimens (paratypes) ( MNCN 20.04 /4866) .

Diagnosis. This species can be distinguished by the male antennal organ; all segments of the antennule are long and narrow (the fifth segment being very long); the expopod of the uropod has four setae; the female thoracopod 8 is exclusive and the setation on the endopod of the uropod (two smooth setae of similar length and one plumose seta) is also exclusive.

Description. Male (holotype): total body length 1.00 mm (of other male, paratype, 0.95 mm). Female (paratype); total body length 1.03 mm. Body elongated, almost cylindrical, segments progressively widening and lengthening towards posterior end of body. Head is longer than wide. Antennule (figure 1A, B, C): with six thick, long segments; with sexual dimorphism on the second segment (antennal organ present in the male; figure 1B, B∞); length of first three segments slightly greater than the other three, segments five and six of similar length; setation similar in male and female in all segments except segment two (figure 1A, B) (segment five with three terminal aesthetascs and segment six with three subterminal aesthetascs); antennal organ in the male located on a long protuberance on the inner distal angle, and in the apical zone it bears two short, curved prolongations of equal length with a rounded top. Antenna (figure 1D): five-segmented, the distal two are the longest; fourth segment with a seta and last segment with two smooth and one plumose terminal seta; first three segments without setae. Labrum (figure 1E): with eight similar main teeth on ventral side, and one smaller lateral tooth at each end. Mandible (figure 1F, G): pars incisiva with four well-developed teeth (figure 1G); pars molaris with five teeth, one strong and distal and the other proximal, all with fine setae; mandibular palp not reaching below pars incisiva. Maxillule (figure 1H): proximal endite with four claws (spines of different sizes); distal endite with four claws, with spines concentrated on distal end and three smooth setae on outer distal margin. Maxilla (figure 1I): three-segmented, with two long setae on basal segment; segment two with two long setae and one shorter seta at distal end; segment three elongated with 13 setae. Thoracopods 1–6 (figure 2A–F): six pairs of well-developed thoracopods, length gradually increasing from pairs 1 to 4, last two pairs similar in size; well-developed epipodite on pairs 1–6, trilobular shape, not exceeding half length of basipod. Thoracopod 1 (figure 2A): basipod with one lateral terminal seta; exopod one-segmented with one terminal barbed seta, and with one group of ctenidia at base of seta; endopod four-segmented: first segment with one smooth seta, second with one plumose seta, third with one small, smooth seta, terminal segment with two claws of slightly different length. Thoracopods 2–6 (figure 2B–F): basipod without seta; exopod biarticulated, segment one with two terminal setae, one smooth, the other barbed, and segment two with a terminal barbed seta (ctenidia at base of seta); endopod four-segmented, first segment short and without setae, second with two groups of lateral ctenidia, and one plumose seta, third with one tiny seta and with two groups of lateral ctenidia and fourth segment with one strong claw. Thoracopod 8 male (figure 1K–M): compact, twice as long as wide; endopod long and slim with two setae; basal segment of the protopod massive and with very pronounced distal curvature (almost reaching end of basipod), free side with small teeth; basipod rectangular ending in a point, with one seta where endopod is inserted; small, triangular exopod. Thoracopod 8 female (figure 1J): small, almost cylindrical with two small terminal teeth. With no appreciable trace of pleopods on the pleon. Dorsal margin of the pleotelson (figure 2H, I): anal operculum very pronounced, longer in female (figure 2I) than in the male (figure 2H), almost as long as furca; one plumose seta on lateral side. Uropod (figure 2G): sympod almost three times length of the endopod and six times as long as wide, with seven spines, the most distal one almost three times as long as the others, which are of equal length and located from almost the middle towards distal end; endopod of similar length as exopod, with a rim of ctenidia and with three outer dorsal setae, one long and plumose, the other two smooth and similar in length (they do not reach tip of the endopod); exopod with three terminal barbed setae of different lengths and one subterminal barbed seta. Furca (figure 2H, I): with three similar smooth spines and one long dorsal plumose seta.

Etymology. This species is dedicated to Dr Antonio G. Valdecasas, colleague and friend, who started me off in scientific research.

Taxonomic discussion. The new species presents a series of unique characters within the genus. The antennal organ is exclusive; although the size of the protuberance is comparable with that of H. breviappendiculata (figure 3N) and H. longiappendiculata (figure 3I) (European species) the two structures which form the antennal organ (figure 1A, B, B∞) are very different to those of these two species. The third segment of the antennule in the male is modified which does not occur in any other species. In the new species all segments of the antennule are long and narrow (the fifth segment being very long); in all other species of the genus these segments are short and wide, particularly the fifth. The expopod of the uropod has four setae, the rest of the species in the genus have between one and three. The anal operculum of the new species seems to be longer than in the rest of the species of the genus. The appearance of the female thoracopod 8 is exclusive to the new species. The setation of the endopod of the uropod (two smooth setae of similar length and one plumose seta) is also exclusive.

The thoracopod 8 male is, relatively, very large for the medium size of the species (as is also the case in H. nicoleiana ); the relative proportions of the lobes resemble only H. nicoleiana , in which the basipod is smaller and about the same size as the endopod, but H. nicoleiana is a small species with a small and squarer male thoracopod 8.

The respiratory epipodites of thoracopods 2–6 are three-lobed (figure 2A–F), an unusual shape within the group.

The remaining characters occur in unique combination although individually they are found in other species of the genus Hexabathynella . The labrum is similar to that found in the two other Spanish species ( H. minuta and H. nicoleiana ). The mandible is similar to that of H. nicoleiana . The pars distalis of the maxillule has four teeth as in H. nicoleiana , H. minuta , H. szidati and H. otayana (figure 3V). The thoracopod 1 (figure 2A) is similar to the Spanish species, H. nicoleiana (figure 3E) and H. minuta , and to the geographically closest European species ( H. knoepffleri , from Corsica). The thoracopod 2 (figure 2B) is similar in H. nicoleiana , H. knoepffleri and H. otayana . The absence of setae on the basipod of thoracopods 3–6 is also seen in H. nicoleiana , H. minuta , H. knoepffleri , H. otayana , H. pauliani , H. decora , H. aotearoae and H. halophila . Only the new species and H. nicoleiana lack a pleopod. The female thoracopod 8 is not significantly reduced in size, as occurs in many other species ( H. nicoleiana , H. minuta , H. knoepffleri , H. paranaensis , H. tenera (figure 3M), H. longiappendiculata , H. otayana , H. muliebris and H. hessleri ). The number of spines on the sympod of the uropod (six or more) is also characteristic of H. nicoleiana , H. szidati , H. paranaensis and H. aotearoae . The presence of a single seta on the dorsal margin of the pleotelson is shared with all European species, one south American species ( H. paranaensis ), the three north American species ( H. otayana , H. muliebris and H. hessleri ), and H. pauliani from Madagascar. Hexabathynella valdecasasi n. sp. is a medium-sized species, as are the majority of the species in the genus except for H. hebrica which is the largest and H. minuta , H. pauliani and H. tenera which are small.