ECL-20100701-1: 1 nymph, TAIWAN, Baishih River (24.882695°N, 121.656242°E), 1.vii.2010. ECL-20100707-2: 1 nymph, TAIWAN, Baishih River (24.882695°N, 121.656242°E), 7.vii.2010. ECL-20100707-3: same data as ECL-20100707-2.

Immature nymph. Body length 1.5–2 mm, excluding caudal filaments. Head yellowish with a small red median ocellus, width about 3 times longer than length. Carapace coloration orange, with two white eye-spot markings on each side close to the mid line, about 2/3 of the distance from the base of the head. Distal end of carapace with a concave exhalent notch (Fig. 3A–B).

Figure 3. 

Prosopistoma sp. whole nymph: A Dorsal view B Ventral view.

Head. Antenna with 5 segments, segment III longest (Fig. 4A). Labrum narrow, 3 times wider than long, surface with stout setae, anterior margin with sparse setae (Fig. 4B). Left and right mandibles similar, outer canine longer than the inner one, outer canine with three apical teeth, outer tooth the smallest with smooth outer margin, inner tooth the largest, with three short spines along the inner margin. Inner canine with two apical teeth, inner tooth larger with outer margin smooth, inner margin with two small spines. Two smooth setae below the inner tooth (Figs 4C–D). A single simple seta present lateromedially on each mandible (Fig. 4C). Maxillae with1 rigid canine at tip, with 3 subequal dentisetae and 3 stout setae (Fig. 4E–F). A simple seta at 2/3 of the sclerotized section of galea-lacinia (Fig. 4E-F). Maxillary palp 3-segmented, with segment II the longest (Fig. 4E). Labium composed of prementum and postmentum. Prementum trapezoid, cutting edge with fine teeth (Fig. 4G). Postmentum with large notch, to house the prementum (Fig. 4H). Labial palp 3-segmented, with the second the longest (Fig. 4G).

Figure 4. 

Prosopistoma sp.: A Antenna B Labrum C Mandible D Magnified view of inner and outer canine of mandible E Maxillae F Tip of Maxillae G Prementum with labial palps H postmentum. Scale bar in μm.

Legs. Dorsal and ventral margins of fore femur smooth (Figs 5A, 6A). Ventral margin of fore tibia with a row of 4 serrated setae (Figs 5B, 6C). Apical serrated setae on tibiae of legs II and III (Figs 5D, F, 6B, E–H). Claws of all legs sharp and without denticles (Fig. 6D).

Figure 5. 

Prosopistoma sp.: A Leg I B Apex of ventral margin of tibia showing 4 serrated spines C Leg II D Apex of ventral margin of tibia of leg II E Leg III F Apex of ventral margin of tibia of leg III G Gill I H Upper lamellae portion of gill I. Scale bar in μm.

Figure 6. 

SEM of Prosopistoma sp.: A Leg I B Dorsal margin of tibia of leg I C Ventral margin of tibia of leg I, showing 4 serrated spines (indicated by white arrows). Note spines 1 and 4 are broken D Claw of leg I E Leg II F Fore-tibia of leg II G Leg III H Apex of fore-tibia of leg III. Scale bar in μm.

Abdomen. Posterolateral projections of abdominal segments VII-IX sharp and with pointed apex (Fig. 3B). Three caudal filaments short and setose (Fig. 3B). Gill I with long upper lamellate portions, lamellate margin serrated, lower portions divided into several branches (Fig. 5G–H). Gill II leaf-like unbranched. Gill VI tiny, unbranched.

Distribution. At present, this unnamed species is only recorded in Baishih River from Taiwan.

Habitat. The collection site BA1 is an undisturbed forested-stream (356 m a.s.l., Fig. 2) with wetted width (6.3–10.5 m) and depth (0.2–0.7 m) relatively constant throughout the year. This site is generally oligotrophic (nitrate-nitrogen < 0.01 mg/L, ammonium-nitrogen = 1.40±0.28 mg/L, total phosphorus = 0.10±0.06 mg/L). Nymphs were found within the riffles with accumulated leaf packs on the bed substrates dominated by gravels and pebbles, moderate to high current velocity (26.7–65.1 cm/s) and high dissolved oxygen level (7.3–9.6 mg/L). Nymphs were rare and they contribute to the relative composition of the mayfly community by 0.19% during the study period. Dominant families of mayfly nymphs collected in the same habitat included Baetidae, Heptageniidae, Leptophlebiidae and Caenidae.

According to the diagnostic key in Shi and Tong (2013), the immature nymphs of Prosopistoma sp. are morphologically similar to P. ocellatum. The coloration pattern of the collected immature nymphs in Taiwan is similar to the immature stage of P. ocellatum and P. annamense. However, as the important diagnostic characteristics, such as number of setae on fore tibia and number of antennal segments, are likely to change with ontogenetic shift, we cannot properly diagnose our specimens due to the lack of mature nymphs collected through extensive sampling in the present study.

Habitat of the nymphs of Prosopistoma sp. are similar to most Prosopistoma such as P. annamense, P. olympus and P. ocellatum. Their habitats are generally located in the undisturbed upstream site (altitude = 200–800 m a.s.l.) commonly characterized by stony streambed, shallow water depth, and moderate to high current velocity (Soldán and Braasch 1984, Sartori and Gattolliat 2003, Shi and Tong 2013) except that nymphs of P. annamense were recorded in the large urban river Xiangjiang from China (Liu et al. 1984).

In this study, the finding of three immature nymphs of Prosopistoma sp. from the upstream site of Baishih River represents the first record of this rarely collected genus in Taiwan. Thus, further collections should be conducted at more river sites to obtain the mature nymphs to ascertain the taxonomic status of this Prosopistoma sp. in Taiwan.