Symbiocloeon laoense Palatov 2023

Kluge, Nikita J., Vishnevskaya, Maria S. & Palatov, Dmitry M., 2025, Winged stages of Symbiocloeon laoense Palatov 2023 (Ephemeroptera: Baetidae) and systematic position of larvae developing inside mantle cavities of bivalve mollusks, Zootaxa 5728 (2), pp. 287-320 : 289-305

publication ID	https://doi.org/10.11646/zootaxa.5728.2.3
publication LSID	lsid:zoobank.org:pub:DEDCEEF6-772A-44F1-B314-A7C466EDED33
persistent identifier	https://treatment.plazi.org/id/03CD87BD-3D63-FF90-A4FF-F9C7BD15FE53
treatment provided by	Plazi
scientific name	Symbiocloeon laoense Palatov 2023
status

Treatment

( Figs 3–60 View FIGURES 1–6 View FIGURES 7–9 View FIGURES 10–13 View FIGURES 14–18 View FIGURES 19–22 View FIGURES 23–25 View FIGURES 26–29 View FIGURES 30–35 View FIGURES 36–42 View FIGURES 43–47 View FIGURES 48–50 View FIGURES 51–54 View FIGURES 55–60 , 63–65 View FIGURES 63–66 )

Symbiocloeon sp. 2 : Bespalaya, Sousa, Gofarov, Kondakov, Kropotin, Palatov, Vikhrev & Bolotov 2022: 2.

Symbiocloeon laoensis Palatov (in Bespalaya, Palatov, Gofarov, Kondakov, Kropotin, Sousa, Taskinen, Inkhavilay, Tanmuangpak, Tumpeesuwan, Vikhrev & Bolotov) 2023: 16 (larva). Ending of the species name should be changed to -e according to neutral gender of the generic name formed from « Cloëon View in CoL ».

Material examined. LAOS, boundary of Champasak and Attapu provinces, river Xekhamphor upstream village Kenang-Ang ( 22 km E from Thang Beng by road 18), 14°48′45″N and 14°46′16″N, 106°10′57″E, 30.II–4.II.2025, coll. N. Kluge & L. Sheyko, extracted from Corbicula fluminea : 1 L-S-I ♂, 2 L-S ♀, 46 larvae; collected at light: 1 S-I ♂, 1 S-I ♀. Holotype and paratypes, the same locality, 3.III.2018, coll. I.N. Bolotov, I. V. Vikhrev, E.S. Konopleva & Y.E. Chapurina GoogleMaps .

Larva. CUTICULAR COLORATION. In early larval instars cuticle colorless ( Figs 10, 12 View FIGURES 10–13 ), in later instars pigmented dorsally, with maximum pigmentation in last instar ( Figs 11, 13 View FIGURES 10–13 ).

CUTICULAR COLORATION IN LAST LARVAL INSTAR ( Figs 14–18 View FIGURES 14–18 ). Frons between frontal sutures and median ocellus dark brown ( Figs 13 View FIGURES 10–13 , 17 View FIGURES 14–18 ). Pronotum and mesonotum with brown, ochre and colorless areas; fore protopteron with two lighter stripes and fine darker lines along all longitudinal and main intercalary veins ( Figs 13 View FIGURES 10–13 , 18 View FIGURES 14–18 ). Metanotum brown medially, colorless laterally ( Fig. 16 View FIGURES 14–18 ). Thoracic pleura with colorless and brown areas, sterna colorless ( Fig. 16 View FIGURES 14–18 ). Legs entirely colorless ( Figs 16 View FIGURES 14–18 , 19–21 View FIGURES 19–22 ). Abdominal terga II–IX mostly brown, with lighter paired medio-anterior and medio-posterior sigilla; tergum X colorless anteriorly and brown posteriorly ( Fig. 14 View FIGURES 14–18 ). Abdominal sterna mostly colorless, sterna of some posterior segments diffusively darkened by brown in median part ( Fig. 15 View FIGURES 14–18 ). Cerci and paracercus entirely colorless ( Figs 10–14 View FIGURES 10–13 View FIGURES 14–18 ).

HYPODERMAL COLORATION. Not expressed.

SHAPE AND SETATION. Described by Bespalaya et al. (2023). Following details should be added:

Maxilla of « Cloeon - type », i.e. with distal (1st) dentiseta bent in same direction as canines and other dentisetae (Kluge 2017: 94). Among 3 maxillary canines, two canines small and slender; all 3 dentisetae slender, either pointed, or bifurcate, or trifurcate ( Figs 5–6 View FIGURES 1–6 ). Maxillary palp enlarged and banana-shaped, without any setation, with week muscle going to its base, without muscle inside (i.e. one-segmented).

Labium and labial palps without any setae other than setae on dorsal side of paraglossae. Paraglossa greatly widened, with field of densely and irregularly situated small setae on dorsal side of distal half. Labial palp with week muscles going to bases of 1st and 2nd segments, but probably immovable; connection between 1st and 2nd segments wide and immovable. 2nd segment directed medially and overlapping paraglossa from ventral side, with immovable dorso-distal projection (possibly, vestige of 3rd segment lacking muscle).

Legs with few small hair-like setae and few small two-channel setae [term introduced by Kluge et al. (2023)]; each femur with sparse longitudinal row of small, pointed, two-channel setae on anterior side near outer margin ( Fig. 22 View FIGURES 19–22 ); other two-channel setae smaller.

Claws with two symmetric rows of blunt denticles near apex ( Figs 23–24 View FIGURES 23–25 ).

Tergalii unable for rhythmic respiratory movements, able for slow movements only; in living larva usually directed laterally, in one flatness ( Figs 10–13 View FIGURES 10–13 ).

POSE OF SUBIMAGINAL GONOSTYLI DEVELOPING UNDER LARVAL CUTICLE. In male larva of last instar, protogonostyli prominent and cone-shaped. Developing subimaginal gonostyli folded by « Nigrobaetis - type »: all segments directed caudally, 2nd segment sharply bent with convexity directed medially, 3rd (apical) segment deeply inserted into 2nd segment ( Figs 47 View FIGURES 43–47 , 50 View FIGURES 48–50 ) .

Subimago. CUTICULAR COLORATION. Head very light or colorless. Pronotum brown, with darker and lighter areas. Mesonotum mostly brown, with anteronotal protuberance colorless ( Fig. 26 View FIGURES 26–29 ). Pleura mostly colorless, with postsubalar sclerite and lateropostnotal crest brown ( Fig. 27 View FIGURES 26–29 ). Sterna colorless. Legs colorless ( Figs 30–35 View FIGURES 30–35 ). Abdominal terga very light brownish; each segment with pair of darker sublateral spots bearing enlarged microtrichia ( Figs 28–29 View FIGURES 26–29 ); sterna, gonostyli and cerci colorless ( Fig. 28 View FIGURES 26–29 ).

TEXTURE. On all legs of both sexes, initial 1st–4th tarsomeres covered with blunt microlepides, initial 5th

(last) tarsomere covered with pointed microlepides ( Fig. 35 View FIGURES 30–35 ).

Imago, male ( Figs 36–46 View FIGURES 36–42 View FIGURES 43–47 , 48 View FIGURES 48–50 ). Head brown with ochre. Turbinate eyes wide, dull red, with facetted surface darker than stem. Thorax with sclerites dark brown, equally dark on dorsal, ventral and lateral sides; membranous areas ochre. Fore wings colorless, veins ochre, costal vein proximad of costal brace darkened by brownish, costal brace colorless. Cross veins of pterostigma oblique, non-branched, complete and incomplete. 2 marginal intercalaries in each space. Hind wings absent. Legs light ochre; femur, base and apex of fore tibia slightly darkened by brownish ( Figs 41–42 View FIGURES 36–42 ). Tarsus of middle and hind legs relatively short, with 2 apical spines (on initial 2nd and 3rd tarsomeres) (as in Fig. 35 View FIGURES 30–35 ). Abdominal terga brown, each with pair of lighter ochre stripes corresponding to medio-anterior and medio-posterior sigilla and median ochre stripe between them; sternum I brown, sterna II–VIII ochre ( Fig. 40 View FIGURES 36–42 ). Cerci uniformly light ochre.

Abdominal sternum IX, unistyligers, gonostyli, penial bridge and gonovectes brown ( Fig. 43–46 View FIGURES 43–47 ). More or less well outlined brown median styligeral sclerite locates between bases of unistyligers; median sterno-styligeral muscle distinctly developed, not wide, posteriorly attached to median sclerite only ( Figs 43 View FIGURES 43–47 , 48 View FIGURES 48–50 ). Unistyligers about as long as wide, cylindrical. 1st segment of gonostylus (thickest and fused with 2nd) slightly narrowing distally; 2nd segment (longest and arched) equally wide over its length; 3rd segment (terminal) long, equally wide over its length. Penial bridge with prominent, rounded, sclerotized median projection. Gonovectes sharply bent, movably attached, deeply inserted into abdominal segment IX, with free ends bearing gonoducts (as characteristic for Baetovectata).

Imago, female ( Figs 51–54 View FIGURES 51–54 ). Coloration of head, thorax, legs, abdomen and cerci as in male. Head short and wide, eyes small and widely separated. Fore leg ( Fig. 53 View FIGURES 51–54 ): tibia with patella-tibial suture (as on middle and hind legs); tarsus with 2 apical spines on 2nd and 3rd tarsomeres (as on middle and hind legs).

Egg ( Figs 55–60 View FIGURES 55–60 ). Asymmetric, generally irregularly-oval, with one side more or less stretched cone-like. Surface with evenly dispersed small round papillae and irregular folds forming concentric wavy circles around papillae. Micropyle wide, oval.

Dimension. Fore wing length (and approximated body length of imago) 6 mm; larval body length up to 8 mm.

Distribution. Known from one locality in Laos.

Collecting site and larval habitat. All larvae used in this study, were found in mantle cavities of Corbicula fluminea collected near the type locality of S. laoense —in the river Xekhamphor (or Sekhamphon), which serves as the boundary between provinces Champasak and Attapu. Name of this river is transliterated also as «Khampho»

(on Russian map printed in 1974) and «Sein Kaphoe» (in the original description of S. laoense ). Picture of the type locality ( Bespalaya et al. 2023: fig. 1C) was made from this river in the village Kenang-Ang ( Fig. 61 View FIGURES 61–62 ).

Collecting in the river Xekhamphor near the village Kenang-Ang ( Fig. 61 View FIGURES 61–62 ) gave little result: among 959 examined individuals of Corbicula fluminea , only 10 individuals contained 10 larvae of S. laoense . Much more density of S. laoense occurred upstream of this place, especially 3 km to the north, where a road from Ban Nakala reaches the river ( Fig. 62 View FIGURES 61–62 ).

Mantle cavities of Corbicula fluminea contained larvae of S. laoense of various instars, from rather young larvae ( Fig. 12 View FIGURES 10–13 ) to mature larvae ready to molt to subimago. Besides the mayfly S. laoense , some clams contained larvae or pupae of Chironomidae . Each clam contained no more than one insect―either mayfly larva, or chironomid larva or pupa. While larvae and pupae of Chironomidae were located between the mantle and the shell, larvae of S. laoense were located on the gills of clam ( Figs 63–65 View FIGURES 63–66 ). Clams are small (mostly about 10–15 mm length) and mayfly S. laoense is relatively large ( 6 mm length in adult stage), so that mature larva occupies significant part of mantle cavity and can be longer than body of the clam ( Fig. 65 View FIGURES 63–66 ). Most mayfly larvae sited on the clam body immovably, being covered with mucus ( Fig. 65 View FIGURES 63–66 ), but some were clean and were able to move by the clam body ( Figs 63–64 View FIGURES 63–66 ).

Being taken out from the clam and moved into a net cage placed in running water, larvae were able to sit on the net surface of the cage and swim when necessary. Before molt to subimago, larva floats on the water surface in the same manner as other mayflies. It remains to be unclear, how larva ready to molt to subimago escapes from the clam.

Among 9 mature larvae placed into the cage, only one male and two females were able to shed the larval cuticle; among them, one male successfully molted from larva to subimago and from subimago to imago ( Fig. 36 View FIGURES 36–42 ), but died just after this molt, so its cuticle of gonostyli did not harden and crumpled ( Fig. 46 View FIGURES 43–47 ). Both emerged female subimagines died not spreading their wings. The description of winged male and female imagines given here, is completed by examination of two specimens reared from subimagines, which were attracted to on ultraviolet lamp placed close to the river ( Figs 39 View FIGURES 36–42 , 50–51 View FIGURES 48–50 View FIGURES 51–54 ).