Northern mayfly (Branchiobaetis borealis) and Greater Bay mayfly (Branchiobaetis megasinus) based on larval stage

NEWS - Northern mayfly (Branchiobaetis borealis sp. nov.) and Greater Bay mayfly (Branchiobaetis megasinus sp. nov.) were described as new species based on the larval stage, Megabranchiella longusa Phlai-ngam & Tungpairojwong 2022 which was previously distributed only from Thailand, was recorded from China for the first time.

Baetidae are distributed almost worldwide but are mostly diversified in the tropics. Southeast Asia, known for its expansive tropical rainforests and river systems that represent one of the most biodiverse ecosystems on Earth, is one of the richest hotspots for mayflies on a global scale.

However, Baetidae have received less attention from taxonomists compared to those from Europe and North America. Although many Ephemeroptera taxonomists have made major contributions to the study of baetid fauna in the region, knowledge of baetid systematics is still limited and the diversity of baetid species in Southeast Asia is under severe threat of extinction due to rapid economic development and urbanization.

The baetid fauna of South and Southwest China shows significant affinities with the baetid fauna of mainland Southeast Asia and even shares many baetid taxa with island Southeast Asia. Branchiobaetis is a small genus with 7 species and was previously known only from Indonesia, Malaysia and the Philippines.

The morphology of the two new species differs from the other species in the larval stage by smooth antennal scapes without spatulate setae, inner margin of pedicel with small triangular denticles distolaterally; mandibles without blade-like incisors; maxillary accessory gills reduced to small tongue-like structures.

However, confirmation of both belonging to the genus Branchiobaetis is a single finger-like accessory gill ventrally on the coxal articulation of the foreleg; accessory gills externally laterally between the stipes and cardo at the base of the maxillae; bubble-like membranous swellings between the coxa and trochanter of the leg; a single row of small hook-like setae curved apically along the dorsal margin of the femur, tibia and tarsus of the leg.

B. borealis in adult larvae has a body length (mm): female 7.2-8.5, male larvae slightly shorter than females, 6.0-7.5; antenna 2.0-3.0; cerci 3.0-4.0, paracercus ~3/4 length of cerci.

Abdominal tergites with contrasting colour pattern; paraglossae of labium with three rows of long, robust, distally pectinate setae distoventrally; femur with row of long, stout, clavate setae along dorsal margin, villopore well developed on all legs; claw without subapical seta; untypical folding way of gonostyli bud.

The specific epithet borealis is a Latin masculine adjective meaning "northern", referring to the fact that the new species probably represents the northernmost distribution of the genus in the Oriental Region. Distribution in Yunnan (Lushi, Weixi, and Yunlong), China.

The species is found in fast-flowing unshaded rivers with gravel substrates at elevations of 1036-2523 m. Three specimen collection sites in the watersheds of the Jinsha River (upstream of the Yangtze River), Nujinag River (upstream of the Salween River) and Lancang River (upstream of the Mekong River) suggest that B. borealis may be common in the Three Parallel Rivers of the Yunnan Protected Area.

B. megasinus in larvae has body length (mm): female 7.0-8.5, male larvae slightly shorter than females, 6.0-7.5; antenna 3.0-4.0; cerci 3.0-4.0, paracercus ~4/5 length of cerci.

Abdominal tergites almost uniformly brown except for with a cream oval macula anterior medially on tergite V, legs with contrasting cream and brown alternating bands; paraglossae of labium with two rows of long, robust, distally pectinate setae distoventrally; femur with row of long, stout, blunt pointed setae along dorsal margin, villopore well developed on all legs; claw without subapical seta.

The specific epithet is a combination of the words mega- (derived from Greek, meaning large) and sinus (from the Latin masculine adjective meaning bay or ravine). The name refers to the type series found from the Guangdong-Hong Kong-Macao Greater Bay Area.

Distribution in Guangdong (Shenzhen, Guangzhou, Boluo, Longmen) and Hong Kong (Tai Po Kau, Shing Mun, Chuen Lung, Ma Po Mei, Ng Tung Chai, Mui Tsz Lam, Shek Mun Kap Forest Rivers).

This species usually lives in fairly fast-flowing and well-aerated lowland forest rivers (elevation under 300 meters) with gravel and cobblestone substrates. The physicochemical parameters of the type location (December) are river width 5-7 m, water depth 10-20 cm, water temperature 18.3C, current speed 0.25 m/s, DO 9.4 mg/l, pH 8.0 and TDS 43.9 mg/l.

The male imago of B. megasinus differs from B. javanicus based on normal turbinate eye precursors based on the exuvia of the last instar male larvae (vs. with a raised area with well-expressed facets in B. javanicus).

Forewing costa serrated with a pointed spine at the base, pterostigma area almost transparent with four or five oblique transverse veins (vs. costa smooth and pterostigma area brown with at least ten oblique transverse veins or small veins). Femur of foreleg curved medially (vs. femur straight, but tibia slightly curved medially).

Genitalia with inner margin of gonostylus segment I with distomedial expansion and outer margin with basal prominence, gonostylus segment III oblong (vs. gonostylus segment I with prominent obtuse angle near its middle; segment III short and triangular).

Original research

Tong X, Zhou Z, Wu B (2024). First contribution to the genera Branchiobaetis and Megabranchiella (Ephemeroptera, Baetidae) in China, with descriptions of two new species. ZooKeys 1216: 115-148, DOI:10.3897/zookeys.1216.129803

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