Potamophilus acuminatus (Fabricius, 1792)

Potamophilus acuminatus (Fabricius, 1792) View in CoL

Material examined. Adults. 2 ♂♂, 1 ♀ (CKB, CBT): MOROCCO, Tétouan Province , Afertane Oued Laou 35°21′11″ N, 5°11′9″ W, 30 m a.s.l., 9. VI. 2022, J. Kodada leg. GoogleMaps ; 4 ♂♂, 5♀♀, 41 specimens (CKB): SLOVAKIA, Latorica River , ca. 48°30′19.213″N, 22°2′50.328″E, 15. 7. 1992, J. Kodada leg. GoogleMaps

Larvae. 5 specimens (CKB): SLOVAKIA, Salka env., Ipeľ River, 47°53′06.5″N 18°45′47.9″E, 6. 05. 2022, J. Kodada & D. Selnekovič leg. GoogleMaps ; 10 specimens (CKB): SLOVAKIA, Vrbovka env., Ipeľ River 48°05′19.7″N 19°24′35.6″E, 26. 08. 2020 S. Krčmárik leg. GoogleMaps ; 15 specimens (CKB): SLOVAKIA, Latorica River , ca. 48°29′58.055″N, 22°3′10.355″E, 25. 8. 2017, J. Kodada leg GoogleMaps ; 15 specimens (CKB): SLOVAKIA, Latorica River , ca. 48°29′58.055″N, 22°3′10.355″E, 15. 7. 2019, J. Kodada leg. GoogleMaps

Potamophilus acuminatus is the largest European Elmid and the single representative of the subfamily Larainae LeConte, 1861 .

Barcoding. Maximum likelihood and neighbour joining trees, inferred from the COI dataset (603 nucleotide positions), show identical topologies. Therefore, posterior probabilities (PP) were mapped along with bootstrap support on to the best-scoring ML tree. The specimens from Morocco grouped with specimens of P. acuminatus from western and central Europe with maximum statistical support ( Fig. 1 View FIGURES 1–2. 1 ). Given the taxon sampling, the Moroccan specimens were depicted as their nearest relatives with strong support (100% ML and NJ bootstrap). Sequences from four widely separated, allopatric populations of the well-supported P. acuminatus clade show pairwise genetic distances ranging from 0.000 –0.035 (uncorrected p -distance, Table 2 View TABLE 2 ). Specimens were grouped into two main lineages with high statistical support, separated by a relatively high uncorrected p -distance (0.033 –0.035). The first lineage includes specimens from France, Germany and Slovakia and the second one represents specimens from Morocco.

Haplotype network analyses supported the monophyletic origin of P. acuminatus specimens by 71 shared nucleotide positions within the barcoding fragment. However, the haplotype from Morocco differs in 20 nucleotide positions from the most frequent European haplotype ( Fig. 2 View FIGURES 1–2. 1 ). This deep intraspecific divergence is probably congruent with the divergent geographic distribution of populations.

All examined adults are morphologically very similar and all barcoded specimens were grouped into a single well-supported, unstructured lineage in the tree inferred from amino acid sequences. Their very uniform genital morphology also confirms this status.

The separation of P. kelabitensis from P. acuminatus is corroborated by their deep genetic divergence in the barcoding COI gene (0.131 –0.139 uncorrected p -distance).

Diagnosis of the adult ( Figs 3, 4 View FIGURES 3–4 ). Body elongated, parallel-sided, length excluding head: 6.1–7.7 mm and maximal elytral width: 2.4–3.0 mm. Colour predominantly black, except reddish-brown antennal segments 1–4, distal portion of femora, trochanters, anterior face of femora, claws and abdominal apex. The surface with dense short yellowish decumbent setae and moderately long and less dense, dark suberect setae.

The pronotum gradually expanded posteriad, broadest in front of the posterior margin and deeply excised in hind angles; sides smooth, not crenate, finely arched or nearly straight; the pronotal disc with broad, distinct depression in front of the scutellum. Pronotal surface with more or less elongated punctures that vary in size; longest diameter of largest punctures ca. twice the facet diameter; smallest punctures slightly smaller than facets. Punctures moderately irregularly spaced, from nearly confluent to separate by distances up to the largest puncture diameter.

Elytra long and narrow (ca. 3.6–4.0× as long as pronotum), moderately convex dorsally, with ten punctate striae and one accessory basal stria between the sutural and second stria. Sutural interval is raised in the posterior 0.8 of its length, other intervals flat to feebly convex. Strial punctures on elytra moderately coarse, sharply impressed, rounded and separated by about their diameter. Apex of each elytron produced and angulated in males while acuminating and protruding with divergent apices in females.

Meso- and metatibia are weakly sinuous; mesotibia in males usually more strongly enlarged distally with an indistinct median tubercle.

The aedeagus is trilobate and symmetrical; parameres in the apical portion are narrowed, flattened, abruptly bent ventrad and less sclerotised; apices do not reach the apex of the penis.

Diagnosis of the mature larva ( Figs 7–10 View FIGURES 5–10. 5–6 ). Length from anterior margin of head to apex of abdomen: 12.5 mm; greatest width across metanotum: 3.0 mm. Body elongated, tapered dorsad, ventrally almost flat, dorsally convex; dorsal sagittal line present from prothorax up to sixth abdominal segment; cross-section subtriangular. Prevailing ground colour black with yellowish-brown mouth parts, antennae and claws. Head with a cluster of 5 stemmata on each side; without tooth on anterior margin between base of antenna and clypeus. Antennae short and three-segmented. Frontoclypeal suture distinct. Mandibles short and wide, very strong, with three apical teeth; penicillus with short, dense setae. Maxilla with palpus, four-segmented; galea and lacinia separate and apex of each densely setose. Labrum with anterior margin nearly straight, with a fringe of dense short setae. Labium with palpus, short, two-segmented; postmentum undivided; gula large.

The pronotum ca. twice as wide as long, with transverse depression in the anterior third, anterolateral and posterolateral angles rounded; sides moderately arcuate, slightly converging anteriad; surface with three pairs of smooth areas lacking granulation (signa) situated on each side of meson. Mesonotum and metanotum about 2.6× as broad as long, 0.7× as long as the pronotum and each with six pairs of signa. Margins of thoracic segments moderately explanate. Terga of all thoracic segments with two longitudinal rows of short setae situated on each meson side. Ventral side with a small subtriangular presternum. Prothorax with one anterolateral and two lateral sclerites on each side of meson and a sizable subtriangular sclerite between coxae; sclerites do not surround coxae posteriorly; lateral sclerites are well-discernible in cleared specimens. Ventral portions of both meso- and metathorax with five sclerites: one large anterior sclerite projecting widely between coxae; each lateral side with two moderately large lateral sclerites, the latter not projecting behind coxae. Legs stout, five-segmented, about 0.5× as long as thorax width, all similar in shape and length.

Abdominal terga 1–8, each with four rows of setae on each side of meson, admedian rows more distinct than sublateral ones. Sternopleural sclerites are present on segments 1–7; sclerites become progressively narrower posteriad; ventrites 1–8 subequal in length. Terminal segment with dorsal longitudinal mesal keel, as long as combined length of the four preceding segments, gradually narrowed posteriad; apex strongly bifurcated; operculum sub-pentagonal ca. 0.3× segment length, opercular hooks long and narrow. All spiracles, small and subequal in form and size, present anterolaterally on the mesothorax and lying more laterally on abdominal segments 1–8; all spiracles open on small tubercles.

Habitat of specimens examined ( Fig. 5 View FIGURES 5–10. 5–6 ). Morocco. Three adult specimens were collected downstream of the Oued Laou River near Afertane Village. The permanent river is ca. 30 m wide, with a rocky substrate. Near the river grow native wild oleander shrubs ( Nerium oleander L.) and bunches of Carex sp. Adult specimens were collected either from exposed submerged roots of Carex sp. or of Nerium oleander , although, as both plants were growing next to each other, it was impossible to identify which plant the individuals were on. Further collecting on surrounding vegetation was unsuccessful and no waterlogged wood was found in this river section. The area is subject to adverse impacts, such as domestic water pollution, grazing, extraction of water, sand and gravel, bathing and the use of detergents for washing clothes. Slovakia. The Latorica River flows from Ukraine to eastern Slovakia on lowlands and gives rise to the Bodrog River, itself a tributary of the Tisza River in Hungary. The river is ca. 10– 15 m wide, regulated (canalised) at collecting places with sandy substrate. As riparian vegetation is dense, mainly with Salix sp. , Populus alba L. and P. nigra L., numerous fallen branches, broken trees or exposed rootlets are in the water. Hundreds of adult specimens were observed swarming during the hot summer days in July 1992, so a sample of about 100 specimens was taken. On submerged branches, numerous large larvae were observed; Macronychus quadrituberculatus P.W.J. Müller, 1806 , co-occurs very frequently on the same microhabitat.Adults and larvae were repeatedly observed in 2017 and 2019 and a persisting high population density of P. acuminatus was confirmed. The Ipeľ River is a left-hand tributary of the Danube flowing through a large alluvial floodplain in southern Slovakia. The river is 20–30 m wide and shallow, with macrophytes and gravel substrate at collecting places near the village of Salka ( Fig. 6 View FIGURES 5–10. 5–6 ). Riparian vegetation is dense, but confined to the narrow area around the river; mainly Salix sp. , Populus alba and P. nigra are present here. From the Ipeľ River estuary to the Danube, P. acuminatus has already been recorded (Brtek & Rotschein 1964). Searching on waterlogged wood in 2022 revealed eleven larvae; the population density here was relatively low and no adults were found. On the contrary, the population density of M. quadrituberculatus was higher. Sampling in the river sections upstream near the villages of Vrbovka, Bušince and Balog nad Ipľom showed higher population densities and even adults were captured by S. Benedikt (https:// www.biomonitoring.sk/OccurenceData). Potamophilus from Slovakia is known from a few published records only: Šamorín (Kelecsényi 1896), Bratislava ( Roubal 1936, Kodada 1991), the River Danube near the estuary of the Ipeľ River (Brtek & Rotschein 1964), Rajčianka River near Čičmany ( Šporka et al. 1998), Hronský Beňadik ( Kodada 1991), Latorica River near the bridge of road Kráľovský Chlmec—Veľké Kapušany ( Majzlan 1997), the Bodrog River and Ondava River near Kladzany ( Jászay & Harman 2019) and Gajary ( Majzlan 2014). The distribution, including unpublished data, was summarised by Kodada et al. (2003): the Danube River Basin, the lower Váh River Basin, Hron and Ipeľ Rivers, the Latorica River Basin and the Tisa River. Larger populations with observable adults and easily findable larvae are still preserved in the Latorica, Ondava, Bodrog and Tisa Rivers in eastern Slovakia and the middle course of the River Ipeľ in southern Slovakia. Most lowland rivers in Slovakia are in a suboptimal condition, usually degraded on water regimes during the 20 th century by numerous regulations and often affected by different kinds of pollution. Fine-grained silt, driven by run-off from arable land, resulting in the silting of interstitial space of the stream bed substrate, also has an adverse impact. Larvae are associated with submerged wood and, as this microhabitat is removed as part of the watercourse and riparian vegetation maintenance, the population is adversely affected to the point of extinction. Currently, P. acuminatus is classified as Critically Endangered in Slovakia ( Holecová & Franc 2001).

Distribution ( Fig. 11 View FIGURE 11 ). Afghanistan, Albania, Austria, Azerbaijan, Belarus, Bulgaria, Croatia, Czech Republic, France, Germany, Greece, Hungary, Israel, Italy, Lebanon, The Netherlands, Poland, Romania, Russia, Serbia, Slovakia, Slovenia, Spain, Syria, Tunisia, Turkey, Turkmenistan and Ukraine ( Jäch et al. 2016). The species is here recorded for the first time in Morocco. This record increases Moroccan Elmidae richness to 22 species and the Rif Mountains to 15 ( Bennas & Sáinz Cantero 2007; Benamar et al. 2022).