Caenis americani Srinivasan, Sivaruban, Barathy, Malzacher & Isack, 2021

Srinivasan, Pandiarajan, Sivaruban, T., Barathy, S., Malzacher, Peter & Isack, Rajasekaran, 2021, A new charismatic Caenis Stephens, 1835 (Ephemeroptera: Caenidae) from Southern India, Zootaxa 4926 (1), pp. 105-116 : 106-112

Caenis americani Srinivasan, Sivaruban, Barathy, Malzacher & Isack sp. nov.

( Figs 1–28 )

Materials Examined. Holotype (in alcohol): Female imago (Reg. No. ZSI–SRC/I/E/507), South India, Tamil Nadu, Dindigul district , Kodaikanal , Mangalamkombu , 10°30542’N & 77°67530’E; 1219 m; 16/II/2020, Pandiarajan Srinivasan & Rajasekaran Isack . Paratypes: 4 female imagoes (Reg. No. ZSI-SRC/I/E/508) and 7 female mature larvae (Reg. No. ZSI-SRC/I/E/509); 3 female imagoes ( AMC /ZN/181) and 4 female mature larvae ( AMC / ZN/182), same data as holotype .

Mature larva (in alcohol) ( Fig. 1 ): Body length: 4.7–4.9 mm; caudal filaments length: 2.6–3.2 mm; antennae length: 1.9 mm. Head: length 0.45 mm, width 1.20 mm. Middle ocellus larger than lateral ocelli. Hind margin without setae, setaceous type of antennae and presence of thin minute hairs on lateral sides just above each articulation of antennal segment. Pedicel with 4–5 simple setae on lateral margins ( Fig. 3 ), genae not distinctly bulged. Mouthparts: Labrum ( Fig. 4 ) length 0.17 mm, width 0.36 mm, 2.1 times broad as long, emargination with thick setae, anterior margin with row of long setae and scattered small stout setae on entire dorsal surface. Lingua of hypopharynx with very short spines on apical margin; apical margin emarginate ( Fig. 5 ). Mandibles ( Fig. 6, 7 ) each with single dorso-lateral row of long setae below incisors. Maxillary palp ( Fig. 8 ) three-segmented and with ratio of 1: 0.8: 1.1; segment III covered with row of long stout setae near proximal end of inner margin. Labial palp ( Fig. 9 ) three-segmented, segment I and II sub equal in length, segment III shorter and conical and 1/3 rdof length of segment II, segment I with row of setae on outer margin, segment II with row of setae on surface near outer margin and row of long simple setae on outer margin, segment III with 2 transverse row of stout setae near tip of distal margin.

Thorax: Pronotum lateral margins denticulate; slightly concave with thin minute setae present on anterolateral and posterolateral margins; setae on anterolateral margins usually longer than those on posterolateral margins. Mesonotum distinctly bulged near anterolateral margins and numerous simple long setae present on lateral margins ( Fig. 10 ), with denticulations. Legs: No coxal processes, with smooth margins. Foreleg ( Fig. 11 ): lengths of femur: tibia: tarsus: claw viz., 0.86: 0.64: 0.51: 0.16 mm. Trochanter with row of thin simple setae on outer margin; femur with transverse row of 9–10 bifid bristles ( Fig. 12 ) on ¾ of distal region; rows of simple setae on inner and outer margins; tibia with 3–4 bipinnate setae on dorsal margin, cluster of 7–8 long, robust setae on apex of distal region of ventral margin; tarsi with 2 stout spines at subapical proximal part of ventral margin, bipinnate setae on both margins and surface bifid bristles present; cluster of bipinnate setae at apex of distal region of tarsi; foreclaw, long and slender without denticles ( Fig. 13 ). Midleg ( Fig. 14 ): lengths of femur: tibia: tarsus: claw viz., 0.85: 0.58: 0.48: 0.20 mm. Trochanter similar to that of foreleg; femur with bifid bristles of various sizes scattered on surface ( Fig. 15 ), row of thin, simple setae on half of ventral margin and entire dorsal margin; tibia with row of stout spines on ventral margin and on surface longitudinal row of bipinnate setae, numerous bipinnate and unipinnate setae scattered on surface and lateral margins; tarsi with row of spines directed medially and also 1–2 comb shaped bristles along with spines on ventral side; on dorsal side, numerous bipinnate and unipinnate setae scatteredly on lateral margins and surface; claw same as that of foreleg ( Fig. 16 ). Hindleg ( Fig. 17 ): lengths of femur: tibia: tarsus: claw viz., 0.91: 0.60: 0.51: 0.22 mm. Trochanter with 4–5 long, simple setae on surface; femur with unevenly distributed and more scattered simple setae on dorsal and ventral margins and bifid bristles scattered all over surface; tibia with cluster of spines on apical area of distal region, longitudinal row of bifid bristles on surface; tarsi with longitudinal row of 4–5 comb-shaped pinnate bristles along with longitudinal rows of spines on its ventral face and on dorsal side, 2 long and simple setae on distal region and 4 simple setae on proximal apex region; hindclaw long and slender, with row of very fine microdenticles ( Fig. 18 ).

Abdomen: Tergum I without posteromedian process; tergum II with posteromedian process with rounded tip at apex; posterolateral processes on terga I, II and III absent; terga IV–VI with size of posterolateral spines increasing posteriorly and reaching maximum on tergum VII; terga VIII–IX with small posterolateral spines compared to VII. Hind margins of terga VII and VIII with bristles ( Fig. 19 ); terga IX and X with small denticles ( Fig. 20 ). Gill 1 about half as long as gill 2. Gill 2 Y-ridge complete and well-developed ( Fig. 21 ); dorsal surface with combination of bifid bristles and small spine-like processes ( Fig. 22 ). Ventral row of microtrichia originates at some distance from base; laminar and slightly pinnate microtrichia ( Fig. 23 ). Gills 3–6 with more numerous filaments ( Fig. 24 ). Near hind margin of sternum IX, broad shagreen field of 4–5 rows of small denticles present. Hind margin of sternum IX slightly concave and emarginated with numerous cuts, and dense robust setae present medially, lateral margins with simple setae. Sternite IX strongly elongated posteriorly ( Fig. 25 ).

Color pattern: Head: ocelli black; vertex dark brownish with branch-like markings, ecdysial cleavage dark brownish, frons milk whitish color, genae light brown, mouth parts pale, venter of head whitish. Thorax: pronotum dark brown with one large and one small paramedian spot in each half of pronotum, lateral margins pale yellowish, mesonotum with median areas golden yellowish and lateral areas including lateral lobes thick brownish. Pro- and mesosternum light brown. Legs: Femora whitish with dark tinge in distal region, tibia and tarsus whitish with irregular blackish tinge, claw light reddish proximally. Abdomen: tergum I blackish brown; tergum II yellowish brown; tergum VII–X yellowish with dark brown tinges and tergum VII slightly paler than others; terga VII–IX each with 3 longitudinal pale stripes; middle one broader compared to others. Operculate gill thick brownish almost completely shaded with faint black tinge on outer half of Y-shaped ridge; minute black spots scattered on inner side; brownish tinges present on proximal and distal areas on inner side of Y-ridge. Caudal filaments pale yellowish and blackish at articulations; first articulation dark black. Sternum yellowish with light blackish tinge on lateral areas.

Female imago (in alcohol) ( Fig. 2 ): Body length: 3.2–3.5 mm; wing length: 3.0– 3.2 mm; caudal filaments length: 2.1–2.3 mm. Wing venation: CuA1 forked with CuA2 immediately in distal part of CuA1-CuP vein ( Fig. 26 ). Head: Ocelli large. Base of antenna distinctly dilated; pedicel three times length of scape. Thorax: Lateral margins of pronotum slightly concave and rounded, protruding anteriorly; broad mesonotum; prosternal ridges forming triangular structure, open anteriorly with straight lateral areas ( Fig. 27 ). Abdomen: Absence of posteromedian finger-like processes on tergum II; lateral filaments short and apex of lateral filaments blunt and increasing in size from tergum III–VIII, reaching maximum size on tergum IX. Hind margin of sternum IX slightly emarginated medially; subanal plate clearly visible with fuzzy appearance and nearly straight ( Fig. 28 ). Subgenital plate not clearly visible.

Coloration: Head: yellow with thick blackish tinges, ocelli whitish, transverse medial line yellowish; venter of the head whitish, scape and pedicel whitish and antennae whitish with blackish tinge at articulations. Thorax: Pronotum dark brownish medially and yellow washed with brownish tinges laterally. Mesonotum yellowish and in distal part filled with black tinges and subapical area of median longitudinal ridge whitish; scutellum yellowish shaded with blackish. Prosternal triangular ridge reddish. Wing base and veins grayish; subcosta region blackish. Abdomen: tergum X whitish, terga III–IX whitish washed with black sparse pigmentation; terga I and II with irregular dark blackish tinges. Sternum VII–IX yellowish with black longitudinal stripes on lateral side; caudal filaments pale red.

Etymology. The species name “ americani ” is made in honour of the institute The American College, Madurai, South India for their support of finding this new species.

Diagnosis. The larva of C. americani sp. nov. can be distinguished from all other species of Caenis by the following combination of characters: i) distinct bulging margin of mesonotum ( Fig. 10 ), ii) hind claw long and slender with a row of very fine microdenticles ( Fig. 18 ) and iii) shape of sternite IX and its shagreen field ( Fig. 25 ).

Ecology. The larvae and imagoes of C. americani sp. nov. were collected in the stream of Mangalamkombu, Kodaikanal, Dindigul district (5–7 m wide, 1.1–1.3 cm depth). The water current was about 1.43 m /sec. The water temperature ranged between 20°C–22°C and pH 7.2–7.7. Substratum was high cobble and decayed leaf litter, with slow current.

Molecular divergence. Due to limited molecular studies carried out on the Caenidae , especially in the Oriental region, our molecular comparison becomes restricted to a certain extent. From the COI based comparisons using K2P distance values, C. americani sp. nov. shows interspecific divergence within the range 0.189 –0.246 ( Table 2) with related species (based on NCBI database comparisons), the most similar species being C. horaria Linnaeus, 1758 and C. lactea Burmeister, 1839 , with 0.189 and 0.199 divergence respectively. The K2P neighbor joining tree of COI reveals that C. americani sp. nov. forms a separate monophyletic clade ( Fig. 29 ).

Figures

FIGURE 29. Kimura-2-parameter Neighbor joining COI tree of Caenis americani sp. nov.

FIGURES 1–3. Caenis americani sp. nov. 1—Female nymph, 2—Female adult, 3—Base of the antennae of nymph

FIGURES 4–9. Mouthparts of Caenis americani sp. nov. 4—Labrum, 5—Hypopharynx, 6—Left mandible, 7—Right mandible, 8—Maxilla, 9—Labium

FIGURES 10–18. Mature nymph of Caenis americani sp. nov. 10—SEM view of Lateral lobes in mesonotum, 11—Foreleg, 12—Shape of bifid bristles in foreleg, 13—SEM view of Foreclaw, 14—Midleg, 15—SEM view of bristles in surface of Midfemora, 16—SEM view of Midclaw, 17—Hindleg, 18—SEM view of Hindclaw having microdenticles

FIGURES 19–25. Mature nymph of Caenis americani sp. nov. 19—SEM view of tergum VII, 20—SEM view of tergum X, 21—SEM view of Gill II, 22—SEM view of surface of operculate gill showing small spine like processes, 23—SEM view of microtrichia in operculate gill, 24—SEM view of Gill III, 25—Shape of sternum IX

FIGURES 26–28. Female adult of Caenis americani sp. nov. 26—Wing venation, 27—Prosternal ridge, 28—Subanal plate