Morphological study describes three new Anacaena species in Japan’s Central Ryukyus

A taxonomic study of small water beetles has identified and formally described three previously unknown species of Anacaena in Japan’s Central Ryukyu Archipelago. The species—Anacaena kumejimana, Anacaena okinawana and Anacaena torikaii—are the only members of the genus currently known from the Ryukyus, where it had not previously been recorded. The researchers based their descriptions on detailed examination of external anatomy and male genitalia, using the shape of the aedeagus as the principal diagnostic character, supplemented by features of coloration, mouthparts and leg pubescence.

The study documents a new taxonomic picture of the genus, but its conclusions are based on the specimens available and the evolutionary origins of the three species remain unresolved. The Ryukyu Archipelago, also called Nansei-shotô, is an island arc containing more than 900 islands across about 1,200 kilometres between Kyushu and Taiwan. Most of its islands are of continental origin, although a small number in the Daitô group are oceanic.

The islands have a complex biogeographic history, including repeated connections and isolations during the past two million years. Their position between the Palaearctic and Oriental regions, subtropical climate and high precipitation help explain the distinctive communities of plants and animals found there. Aquatic beetles have received sustained attention from professional and amateur entomologists, yet unexpected records and taxonomic discoveries continue to emerge.

The new Anacaena species therefore reinforce the view that even relatively well-studied aquatic insect inventories in the archipelago remain incomplete. Anacaena is a widespread genus in the water-beetle family Hydrophilidae and subfamily Chaetarthriinae, with about 130 species described worldwide. Its members are small, generally 2–3 millimetres long, and live in aquatic or semi-aquatic habitats, although some have also been found in forest leaf litter.

Neighboring regions already contained numerous species: one recorded from Honshû and Hokkaidô in Japan, two endemic species in Taiwan, about 15 mostly endemic species in China and at least 17 endemic species in the Philippines. Within the Ryukyus, unidentified Anacaena individuals had emerged during recent field investigations, but their identities remained ambiguous. The study resolved that ambiguity through detailed morphological analysis and formal descriptions of three distinct species.

Dissected structures were cleaned with warm potassium hydroxide solution or proteinase K, rinsed in ethanol, dehydrated and mounted for inspection. Specimens and dissected body parts were photographed with digital cameras and a macro lens, either directly or attached to a microscope, and composite images were assembled with focus-stacking software. The male aedeagus provided the most reliable characters for delimiting species, although its apparent shape could be affected by preparation when the manubrium curved dorsally.

Maxillary palpomeres, the coloration of the pronotum and elytra, and the pattern of metafemoral pubescence supplied additional information. Because some colors vary within a species, the authors emphasized more consistent structural characters, especially those of the aedeagus. Anacaena kumejimana was named from Kumejima Island, where the type series and additional specimens were collected from a shallow pool beside a stream, a pond and the upper course of the Shirase River.

Adults measure 2.1–2.3 millimetres. The dark brown to black pronotum has broad yellowish lateral margins, and a conspicuous yellowish patch occurs near the anteromedial base of each elytron around the scutellar shield. A. kumejimana can be separated from A. okinawana by that elytral patch, the broader yellowish margin of the pronotum and differences in the convexity and dorsal outline of the parameres.

It also resembles A. yunnanensis from China and A. zamboangana from Mindanao in the Philippines, but the Ryukyu species has a horizontal hairline marking the boundary of the pubescent area on the metafemur; the corresponding boundary is rounded in the two Asian species. Anacaena okinawana has the broadest documented distribution among the three new taxa. Its type locality is a tributary of Benoki Dam on Okinawa-jima, while examined material also came from Aka-jima, Geruma-jima and Zamami-jima in the Kerama Islands.

The specimens were collected from small streams, a marsh with slow water flow and a small pool in a concrete drainage. Adults range from 1.9 to 2.6 millimetres in length, and their dorsal coloration varies from yellowish brown to black. A. okinawana differs from A. torikaii in having a wider, triangular median lobe on the aedeagus.

Its separation from A. kumejimana involves both dorsal coloration and details of the parameres, including the extent of basal convexity and the form of the lateral margin in dorsal view. Variation in A. okinawana showed a geographic component. Specimens from Okinawa-jima had slightly smaller aedeagi, with a somewhat narrower median lobe and weakly sinuate to nearly straight lateral faces of the parameres.

Specimens examined from the Kerama Islands had slightly larger aedeagi, a wider median lobe and more evident sinuation along the parameres. Color also differed between the samples: the Okinawa-jima material included both yellowish-brown and black individuals, whereas most examined Kerama specimens were black, and the observed color range appeared smaller there. The researchers regarded the Okinawa-jima and Kerama populations as belonging to one species because the differences were small and partly overlapping.

They also stated that the geographically correlated variation weighs against a recent human-mediated origin for the populations, although the study did not formally test that explanation with genetic data. Anacaena torikaii is currently known only from its type locality near Kinsakubaru in Naze, Amami-ôshima. Examined adults were collected from a small stream in a forest and measure 2.1–2.5 millimetres.

The species has a dark brown pronotum with broad yellowish lateral margins, brown and weakly speckled elytra that are slightly paler at the base, and a uniformly yellowish maxillary palpus. Its clearest diagnostic feature is the slender median lobe of the aedeagus, approximately three times as long as its greatest width. This distinguishes it from the broader, triangular median lobes of A. kumejimana and A. okinawana.

A. torikaii shares genital features with the Chinese A. jiafenglongi and the Laotian A. hajeki, but the shape of the maxillary palpomere and the relative length of the basal piece of the aedeagus separate it from those species. Additional specimens are needed to determine the full extent of variation in its dorsal coloration. All three new taxa can be distinguished from Anacaena asahinai, the species previously known from Honshû and Hokkaidô and also recorded from the Russian Far East, by the pattern of pubescence on the metafemur.

The Ryukyu species have a horizontal hairline across the pubescent area, whereas A. asahinai has an oblique hairline extending over a larger part of the basal femur. Within the examined material, A. torikaii occurs on Amami-ôshima, A. kumejimana on Kumejima and A. okinawana on Okinawa-jima and the Kerama Islands. A. kumejimana and A. okinawana share several structural features that may indicate a close relationship, but similarities between A. torikaii and species in Taiwan, China and Laos leave the evolutionary relationships among the three uncertain.

DNA analysis of all studied populations, alongside further field surveys, is needed to clarify their age, origin and population structure and to determine whether Anacaena also occurs in the unexamined Southern Ryukyus. The descriptions add three species to the recorded aquatic fauna of the archipelago. A total of 49 aquatic hydrophilid species in 19 genera is now reported from the Ryukyus, representing about 60% of the species and 95% of the genera recorded in Japan.

Twelve of the 49 species, or 24%, are endemic to the archipelago. That proportion is higher than the 8% endemism reported for aquatic Dytiscidae in the region, although it is lower than the 71% for Hydraenidae and 74% for Elmidae, groups more strongly associated with running waters. The three Anacaena species were all collected from streams or nearby pools, marshes and drainage habitats, a pattern consistent with the proposed relationship between habitat preference and geographic range.

Shipping may assist dispersal for some aquatic beetles, particularly those attracted to harbour lights or living in lowland standing waters, but this mechanism has not been formally demonstrated for aquatic beetles. The new findings therefore add three recognizable species to an incomplete regional inventory while leaving the evolutionary and biogeographic history of the Ryukyu Anacaena fauna open for further study.

Subject of Research: Taxonomic description of three new Anacaena water beetle species in Japan’s Central Ryukyu Archipelago

Article Title: The genus Anacaena Thomson from the Ryukyu Archipelago of Japan ( Coleoptera , Hydrophilidae )

Article References: Minoshima, Y. N., Kamite, Y., & Fikáček, M. (2023). The genus Anacaena Thomson from the Ryukyu Archipelago of Japan (Coleoptera, Hydrophilidae). Deutsche Entomologische Zeitschrift, 70(1), 143-157. https://doi.org/10.3897/dez.70.96994

Image Credits: AI Generated

DOI: 10.3897/dez.70.96994

Keywords: Anacaena, water beetles, Hydrophilidae, Ryukyu Islands, Okinawa, Amami-ôshima, Kumejima, taxonomy, species description, morphology, genus, Thomson