Elpidium F. Müller, 1880

Genus Elpidium F. Müller, 1880 View in CoL

Type species (by original designation): Elpidium bromeliarum F. Müller, 1880 .

Diagnosis: see Pereira et al. (2022 2023).

Other species: See table 1.

Elpidium chacoense sp. nov. Díaz, Campos and Martens ( Figs. 2–4 View Fig View Fig View Fig ) urn:lsid:zoobank.org:act:5D5A4BD9-8FA2-49E9-BD31-FEE429035C19

Diagnosis: medium-sized Elpidium (L> 0.7 mm) with slightly dimorphic carapace. Males shorter than females. Females with posterior part expanded into a brood pouch. Surface of carapace smooth, set with few setae, colour light brown covered with dark brown patches. Carapace broad, larger in width than in height, ventral side flat with a subtle ventro-lateral ridge on each valve, at the edge of the flat area (outer lists, arrowed in Fig. 3F View Fig ). In dorsal and ventral views, carapace relatively elongated with maximum width at mid-length.

In lateral view, carapace elongated with LV overlapping RV along both anterior and posterior margins. Selvage on RV well-developed, especially anterior and posterior margins. Hinge of protodont type; ventral rim well-developed, dorsal rim short.

A1 with 5 segments, the first bearing a small sub-apical tiny seta on the ventral margin. A2 dimorphic, terminal segment of male with one strongly serrated claw and two more weakly serrated claws; female with three smooth claws. Mandibular coxa with 8 strong cuspate teeth. Second and third maxillular endites with two setae and two spatula-like claws each; palp reduced, not segmented, tapering, with two apical setae. Hp with DL triangular, with a pointed tip and with a large, broad and curved finger-like projection at the base of the internal margin, long ds with swollen base present near the basis of the DL. LR present and shaped in a clasping process. Copulatory Process broad, hook-like, distal glans and ejaculatory duct undifferentiated.

Type material: Holotype: a male (MLP-Cr 27336) with valves stored dry in micropaleontological slides for scanning electron microscopy and with soft parts dissected in glycerine in a sealed slide. Allotype: a female (MLP-Cr 26337) dissected and stored like the holotype. Paratypes: 10 females and 4 males (MLP-Cr 26338) dissected and stored like the holotype.

Type locality: Tank-bromeliads from ( Aechmea distictchanta Lem. ) from Chaco National Park, Chaco province, Argentina. Geographical coordinates: (26°43'S, 59°30'W). Material collected on 09.10.2009 by Raúl E. Campos.

Derivation of name: The species is named after Chaco province in Northeastern Argentina from where the specimens here described were collected.

Description of the male: CpL (not figured): medium-sized; oval-shaped, dorsal margin rounded with greatest height at mid-length; posterior margin rounded, ventral margin straight, anterior margin bluntly pointed; surface smooth with sparse setae, natural colour light brown with dark spots. pigmented naupliar eye present; carapace less pigmented at the eye region. CpD and CpV ( Fig. 2C View Fig ) with anterior margin slightly more pointed than posterior margin, with maximum width at mid-length, LV overlapping RV along all margins, ventral area flat.

RVe ( Fig. 2A View Fig ) with posterior margin rounded; antero-ventrally with a slightly developed flange, ventral margin slightly concave, not straight.

LVe ( Fig. 2B View Fig ) slightly longer than RVe and of similar shape. Inner morphology of both valves as in the female (see below).

Antennula: as in the female (see below; not illustrated).

Antenna ( Fig. 4B View Fig ): Protopodite two-segmented, endopodite three-segmented. First segment of protopodite (coxa) relatively short and ring-shaped, second one (basis) long, wide and curved. Exoopodite consisting of a long, two-segmented spinneret seta. First segment of endopodite relatively short, bearing a long ventro-apical seta. Second segment long and narrow, dorsally with two sub-apical setae, ventro-medially with a short seta and an aesthetasc (Y), and apically with two sub-equal setae. Third (terminal) segment small, with three claws, ventral one strongly serrated and the other two slender. Hyaline organ absent.

Mandibula and Maxillula: As in the female (see below, not illustrated).

First thoracopod ( T 1- Fig. 4F View Fig ): Four-segmented; basis with a pappose dorsal seta, a medium-sized medio-ventral seta and two apical stout setae; second segment quite long, with a strong ventro-apical seta; third segment without setae; terminal segment with an apical claw and a small vestigial seta.

Second thoracopod ( T 2 - Fig. 4H View Fig ): Similar to the preceding thoracic limb; basis with a long medio-proximal dorsal seta, a medium-sized medio-ventral seta and a long ventro-apical seta; second segment long, with a strong ventro-apical seta longer than the tip of following segment; third segment devoid of setae; terminal segment with an apical claw and a small vestigial seta.

Third thoracopod ( T 3 - Fig. 4G View Fig ): Basis with a medio-proximal dorsal seta, a medio-ventral seta and a ventro-apical seta; second segment quite long, with a plumose ventro-apical seta; third segment devoid of setae; terminal segment with a very long and slim apical claw.

Hemipenis (Hp - Figs. 3D View Fig , 4I View Fig ): Consisting of a major muscular body, a triangular distal lobe ( DL), with a bluntly pointed tip and with a finger-like projection at the base of the ventral margin, dorsal seta present near the basis of the distal lobe; lower ramus ( LR) present and shaped as a clasping process. Copulatory process (cop) broad, hook-like, distal glans and ejaculatory duct undifferentiated.

Description of female: Carapace (CpD Fig. 3E View Fig , CpV Fig. 3F View Fig ): Similar in shape as in the male, but slightly larger and displaying a more pointed ventral margin, expanded in the posterior half forming the brood pouch in which eggs are stored and partially developed.

Right valve (RVe - Fig. 3A View Fig , RVi Figs 3D, G, H, I, J View Fig ): moderately elongated; anteriorand posterior margins rounded, anterior margin produced towards the ventral side; ventral margin nearly straight; dorsal margin arched, with greatest height slightly posterior to the middle; calcified inner lamella narrow; well-developed selvage strongly inwardly displaced along the anteroventral and to lesser extend also along postero-ventral margin, marginally inwardly displaced along ventral margin, postero-ventral selvage set with a row of (calcified?) setae. A row of four elongated central muscle scars, situated slightly in front of the middle of the valve (creating brood space in the posterior half of the Cp) ( Fig. 3G View Fig ).

Left Valve (LVe - Fig. 3B View Fig , LVi - Fig. 3C View Fig ) with shape similar but slightly longer than RV; marginal valve structures comparable as in the RV but with also postero-ventral selvage marginal.

Antennula (A1 - Fig. 4A View Fig ) six-segmented. First segment large with an apical expansion, carrying a tuft of tiny pseudochaetae. Second segment with a large and long ventral seta. Third segment smaller, with a short dorso-apical seta. Fourth and fifth segments partly fused. Fourth segment with two dorso-apical setae (one reaching tip of fifth segment, one half that length) and one short ventro-apical seta. Fifth segment with a group of three unequal dorso-apical setae and one ventro-apical seta. Sixth (terminal) segment with two long setae, one short setae and a short aesthetasc (Ya).

Antenna (A2 - Fig. 4C View Fig ) with general structure as in the male. Second endopodal segment with one sub-apical seta and ventrally with one large claw. Terminal segment with 3 claws, all of them smooth.

Mandibula (Md - Fig. 4D View Fig ): Coxal endite with eight strong cuspate teeth and two setae on inner edge and one long serrate seta on outer edge (near the articulation with the palp), with three interdental setae. Externally with an exopodite (respiratory plate) with three long rays and two reflexed setae. Palp four-segmented, with basis and three endopodal segments. Basis set with two large, plumose and subequal setae, and a respiratory plate (the exopodite) with three long and broad setae and one much shorter seta, all set with setules. First endopodal segment with two setae, one long, plumose and one short. Second endopodal segment with five setae, three long ones and two thin ones, the latter subequal in length. Third endopodal (terminal) segment very small, sub-quadrate, with three apical setae, two long of similar length and one shorter than the other two.

Maxillula (Mx1 - Fig. 4E View Fig ): large respiratory plate, an unsegmented palp and three endites. First endite with three setae, the next two endites with three setae each and two claws. Palp tapering, with two apical setae. Respiratory plate well-developed, with 16 long rays.

T 1- T 3: as in the male (not illustrated, see above).

Abdomen ( Fig. 4J View Fig ): Genital operculum rounded, internally connected by tubes and trabecula; caudal ramus consisting of one smoothly rounded lobe, with 2 serrate seta closely placed together and one larger serrate seta displaced towards the external side.

Measurements: Holotype: Male: LV: L = 764 μm, H = 447 μm; RV: L = 760 μm, H = 446 μm.

Allotype Female: LV: L = 780 μm, H = 432 μm; RV: L = 767 μm, H = 400 μm; CpW = 590 μm, CpL = 800 μm, CpH = 423 μm.

Paratypes: Females: RV: L = 721–760 μm (n = 8, 742 ± 21.10 μm); H = 447–554 μm (n = 8, 493 ± 32.86 μm). LV: L = 730–764 μm (n = 8, 747 ± 22.52 μm); H = 500–580 μm (n = 8, 521 ± 48.2 μm). Males: RV: L = 680 μm (n = 1); H = 388 μm (n = 1). LV: L = 710 μm (n = 1); H = 400 μm (n = 1).

Differential Diagnosis

Elpidium chacoense sp. nov. differs from E. wolfi , E. littlei , E. herberti and E. laeaesslei in the absence of a copulatory process with a differentiated ejaculatory duct and distal glans, which is a synapomorphy uniting these four species in one of two main clades in the morphological phylogeny of Elpidium proposed by Pereira et al. (2022). All four species in this clade were described from Jamaica. These species also lack the large thump-like expansion on the dorsal lobe of the Hp, which is prominent in E. chacoense sp. nov.

Elpidium laesslei , in addition, has a very prominent external ornamentation on the valves which is lacking in any of the other Elpidium species, including E. chacoense sp. nov.

The second clade of Pereira et al. (2022), to which E. chacoense sp. nov. belongs, further consists of E. merendonense from Honduras, E. inaequivalve , E. purperae and E. pintoi from Cuba, E. maricaoense from Puerto Rico and Florida, E. martensi from Jamaica and E. bromeliarum and E. litoreum from Brazil. Elpidium litoreum and E. bromeliarum , both from Brazil, as well as E. pintoi , E. purperae , E. inequivalve and E. martensi from Cuba, also lack the large thumb-like expansion on the DL. The Hp structure of E. maricaoense was previously describe by Colin and Danielopol (1981, fig. 12D) from Puerto Rico and Florida but differs mainly in the shape of DL and of the cop.

The six new species described by Pereira et al. (2023) were not included in the phylogenetic analyses of Pereira et al. (2022), but also differ from the new species described here. Of these six species, only E. higutiae shows the large thumb-like expansion of the DL of the Hp, but in this species the LR of the Hp is pointed (rounded in E. chacoense sp. nov.), while the Cp and valves are much more rounded in lateral view. Elpidium eriocaularum has a thumb-like expansion about half the size of that in E. chacoense sp. nov., while Elpidium cordiforme , E. picinguabaense , E. purium and E. oxumae lack the thumb-like expansion and have very different carapace shapes.

Host plants

The present ostracods were collected from the phytotelmata in the Apiacean species Eryngium pandanifolium Cham. and Schltdl., 1826 and in the Bromeliaceaen taxa Aechmea distichantha Lemaire, 1853 and Vriesea friburgensis Mez., 1894 at Iguazú National, Park (Misiones province); and from A. distichantha at Chaco National Park (Chaco province) and at El Toba stream (Santa Fe province). All are native plants in these areas.

Eryngium pandanifolium occurred in moist soil and grew near the edges of streams and in fields prone to flooding. The imbricate arrangement of Eryngium leaves delimits axils, which increase in age from the centre to the periphery. The internal axils hold freshwater and debris, the intermediate ones a semi-liquid interface, and the oldest, wet or dry slime and debris. Ostracods collected from these plants were sampled around the Iguazú falls in 2006. Aechmea distichantha is a typical bromeliad with imbricate leaves, which delimit axils on the base ( Benzing 2000). Adult plants contain a central cavity called tank or cistern which can hold several litres of rain water in a single plant. This bromeliad grows on dry or wet soil or on trees, indistinctly, in or out of the forest. The ostracods collected from this bromeliad come from Iguazú (2006), Chaco (2009) and Santa Fe (2009).

Vriesea friburgensis is a perennial bromeliad, with both epiphyte and terrestrial growth forms, and grows in high humid environments. In Iguazú, it is often found in the forest near the falls. Samples of ostracods were taken from epiphyte plants in 2006.