Cerapus rivulus sp. nov.

Figs 2, 3, 4, 5, 6, 7, 8, 9, 10

Material examined.

Holotype. THAILAND • ♂, 10 mm; Samut Songkhram Province, Mae Klong River, Darunanukroh School; 13°29'41.0"N, 99°55'25.3"E; 5 m depth; 10 Apr 2021; C. Katnoum leg.; THNHM-lv-19376.

Paratype. THAILAND • 5 ♂, 5 ♀; same data as for holotype • THNHM-lv-19379 .

Type locality.

Darunanukroh School (13°29'41.0"N, 99°55'25.3"E) Mae Klong River, Amphawa District, Samut Songkhram Province, Thailand.

Habitat.

Freshwater and brackish water (0.05-2.03 ppt) in Mae Klong River of Amphawa District, Samut Songkhram Province, Thailand.

Etymology.

The species is named after their habitat which is located in the river ( Rivulus: Latin for river/stream).

Description.

Based on holotype, male, size 10 mm, body elongated, cylindrical THNHM-lv-19376.

Head. (Fig. 3A) Eye medium, 0.1 mm (0.14 × head length); rostrum short (0.16 × length of head), acute; Head lateral cephalic lobe well-developed and distally round. Antenna 1 (Fig. 3B) anterior margin with short setae, posterior margin with long setae, about 0.4 × the body length, peduncle 3.1 × the length of flagellum peduncle article 1 with proximoventral swelling, peduncle article 3 subequal to peduncle article 2, both are slender, several aesthetascs, flagellum with 5 articles, article 1 being the longest, flagellum article 5 the shortest with 1 aesthetasc. Antenna 2 (Fig. 3C) anterior margin with short setae, posterior margin with long setae, slightly shorter than antenna 1, setation similar to antenna 1; peduncle article 5 subequal to article 4.

Upper lip (Fig. 6F) symmetrical, notched, with small setulae apically. Lower lip (Fig. 6G) with inner lobe, smooth, setulose on inner and outer lobes. Mandibles (Fig. 6A, B) left incisor with 4 teeth; right lacinia mobilis 4-cuspidate (4 teeth); right mandible with 3 broad accessory spines; molar triturative; molar flake absent; palp elongated, robust; article 2 1.3 × as long as article 3, with 17 marginal setae; article 3 with 10 long setae. Maxilla 1 (Fig. 6E) inner plate small without seta; outer plate with 7 apical robust setae; palp biarticulate, with 6 serrated apical robust setae and 4 subapical setae. Maxilla 2 (Fig. 6D) outer plate broader than inner plate, with 12 simple setae; inner plate with 12 setae.

Pereon. Gnathopod 1 (Fig. 3D, E) length ratio from basis to dactylus 2.6:1:1:1.7:1.8; coxa 2 × as broad as deep, carpus subequal length to propodus, lobate posterior margin with setae and plumose setae, anterior margin less setae; propodus oval, length 1.7 × the width, posterior margin with setae and plumose setae, palm oblique, serrated and with setae; dactylus well developed, palmar margin serrated, crenulated posterior margin withone robust seta. Gnathopod 2 (Fig. 3F, G) carpochelate, length ratio from basis to dactylus 3:1:2:3.5:3:2.5; coxa broader than deep, subequal to coxa 1; basis longer than wide, with 2 marginal setae on anterior margin; carpus massive, length 1.6 × the width, palm margin transverse with long posterior defining tooth and well-developed anterodistal recurved spine adjacent to propodus articulation; propodus length 2 × the width, with two acute spines and 1 rounded projection on posterior margin, with long setae on anterior and posterior margins; dactylus length 3 × the width, with long setae on posterior margin and one seta on anterior margin.

Pereopod 3 (Fig. 4A) length ratio from basis to dactylus 14:6:4.5:4:5.5:3.5; coxa 2.2 × as broad as deep, with 2 anterodistal setae; basis length 2.2 × the width, anterior margin with 10 setae; merus with 3 anterodistal setae and 3 posterodistal plumose setae; carpus with 2 anterodistal setae and 2 posterodistal setae; propodus with 3 anterodistal setae.

Pereopod 4 (Fig. 4B) length ratio from basis to dactylus 12.5:6.5:6:4:5:3; coxa 1.8 × as broad as deep, with a anterodistal seta; basis, length 1.9 × the width, anterior margin with 8 setae; ischium subequal to merus, with 4 posterodistal setae; merus without ridges, with a posterodistal plumose seta; carpus with an anterodistal seta and 2 posterodistal setae; propodus with 4 posteromarginal setae; dactylus with unguis.

Pereopod 5 (Fig. 4C) length ratio from basis to dactylus 8:3:4.5:3:5.5: 3; coxa 1.2 × as broad as deep; basis globular, length equals to width; ischium subrectangular; merus posterior lobe with 1 plumose seta, smaller anterior lobe with 1 seta; carpus smaller than merus, posterior lobe with fine hair, bearing a seta; propodus subovate; dactylus unguis with 2 accessory hooks.

Pereopod 6 (Fig. 4D) length ratio from basis to dactylus 8.5:2:4:3:5:3; coxa lobate, about 1.3 × as broad as deep; basis length 1.6 × the width, with setae on posterior margin; ischium subrectangular, length 1.25 × width; merus 2.1 × as long as broad with distal setae on both sides; carpus as long as broad, with 3 long setae on anterodistal corner and 2 long setae on posterodistal corner; propodus with 2 anterodistal long setae and 1 short marginal seta; dactylus unguis with 2 accessory hooks, with 1 seta on anterior side.

Pereopod 7 (Fig. 4E) length ratio from basis to dactylus 9:3:5:3:5:3; coxa lobate, about 1.6 × as broad as deep; basis length 2.25 × the width, with setae on anterior margin; ischium subrectangular, length 1.5 × the width; merus 2 × as long as broad with distal long setae on both sides; carpus as long as broad broad, with 1 marginal seta and 3 long setae on anterodistal corner and 3 long setae on posterodistal corner; propodus with 3 marginal setae on both sides and 8 long posterodistal setae; dactylus unguis with 2 accessory hooks.

Pleon. Pleopods 1 to 3 decreasing in size. Pleopod 1 (Fig. 5A) peduncle bearing 2 retinacula, without setae; outer ramus slightly shorter than inner ramus, with 6 articles; outer ramus broader than inner ramus, with 2 articles, first article is the largest. Pleopod 2 (Fig. 5B) peduncle with 2 retinacula, without setae; outer ramus 3.1 × as long as inner ramus, with a single article; inner ramus reduced, 1.6 × as long as broad, 1 plumose seta and single article. Pleopod 3 (Fig. 5C) peduncle with 2 retinacula, without plumose setae distomarginally; outer ramus 3 × as long as inner ramus; single article; inner ramus reduced, about 1.75 × as long as broad, single article with 1 plumose seta.

Urosome. (Fig. 5D) Uropod 1 (Fig. 5E) biramous; peduncle 2.1 × as long as wide, with 3 dorsodistal setae; outer ramus shorter than peduncle, 1.5 × longer than inner ramus, with outer row of spinules and 1 large apical robust seta, inner ramus with a large apical robust seta, corona of short robust setae surrounding large terminal robust seta.

Uropod 2 (Fig. 5D) uniramous; peduncle 3.75 × as long as broad, 6.3 × as long as vestigial ramus; ramus with 1 row of short setae and a terminal seta. Uropod 3 (Fig. 5D) uniramous; peduncle 2.7 × as long as wide, with one seta on inner margin; ramus vestigial, inconspicuously small with 2 hooks. Telson (Fig. 5D) deeply cleft, 0.5 × the length of uropod 3, each lobe with 19 or 20 recurved hooks in two rows and an apical seta.

Female (Figs 7 - 10) (Sexually dimorphic characters). Based on allotype (THNHM-iv-19379), length 3.5 mm (Fig. 7A). Similar to male, except for the following: Antenna 1 (Fig. 7B) with fewer setae and fewer aesthetascs. Antenna 2 (Fig. 7C) more slender than in male. Gnathopod 1 (Fig. 7D, E) dactylus posterior margin serrated, with one spine. Gnathopod 2 (Fig. 7F, G) slightly larger than gnathopod 1; carpus triangular, posterodistal corner with plumose setae and normal setae; propodus palm not serrated, with plumose setae and simple setae; dactylus posterior margin crenulated. Pereopod 5 (Fig. 8C) coxa larger, about 2.7 × as broad as deep, about 6.3 × longer than the basis width; basis slightly globular; dactylus with 2 accessory hooks; with a pair of oostegites on each segment from 4 to 6.

Remarks.

Cerapus rivulus sp. nov. is similar to C. nudus and C. longirostris which has synapomorphic character states, such as: (1) pereonites 1-2 without constriction; (2) pereopod 5 merus without plumose seta on posterior lobe; (3) pereopod 6 coxa without setae on ventral margin; (4) uropod 1 without hook on ventrodistal margin. They are distinguished by the following: (1) head subequal in length to pereonites 1 + 2 [ C. nudus longer than pereonites 1 + 2, C. longirostris subequal to pereonites 1 + 2]; (2) gnathopod 2 propodus twice as long as wide [ C. nudus and C. longirostris less than twice as long as wide]; (3) telson deeply cleft (> 50%) [ C. nudus and C. longirostris semi-cleft]; (4) telsonic lobe with 19-20 hooks in two transverse rows [ C. nudus with ten recurved hooks on three transverse rows and C. longirostris with eight recurved hooks on two transverse rows.

This study brings the number of identified Cerapus species from Southeast Asia to seven. Cerapus rivulus sp. nov. can be separated from Southeast Asian congeners by a combination of characteristics as follows (other species in paratheses): absence of constriction between pereonite 1 and 2 (present in C. yuyatalay, C. bumbumiensis and C. longicervicum); antenna 1 peduncular article 1 shorter than article 3 (longer in C. chaomai, C. bumbumiensis and C. longicervicum); gnathopod 2 propodus with tooth in male (without tooth in C. chaomai, C. yuyatalay, C. bumbumiensis and C. longicervicum); telson fused with pleonite 3 (not fused in C. yuyatalay, C. bumbumiensis, and C. longicervicum); telson with more than ten hooks (fewer than ten in C. bumbumiensis).

C. tubularis was described from Long Island Sound, New York, U.S.A. and after that reported in Japan by Morino (1976). However, Lowry and Berents (1989) re-described C. tubularis and excluded the record of Morino (1976). Lastly, White (2015) reported Cerapus sp. in Singapore and noted that the amphipod resembles C. tubularis and possibly was introduced to Singapore. The presence of C. tubularis in the eastern Pacific and South China Sea is still unclear and needs clarification.

Geographic distribution.

The Cerapus are distributed worldwide and mainly inhabit marine and brackish water (Fig. 11) (Lowry and Thomas 1991; Lowry and Berents 2002; Drumm 2018; Nurshazwan et al. 2020). Interestingly, C. rivulus sp. nov. was found in freshwater and brackish water (salinity less than 3 ppt), so this is the first report for freshwater habitats. They were dominant in the coarse filter substrate used for benthos sampling because the tubes are retained on the coarse filter.

Biological notes.

Generally, tube-building amphipods build tubes using amphipod silk which is observed as silk strands adhering to the tip of the dactylar surface close to the pore on P3 and P4 (Kronenberger et al. 2012). Cerapus rivulus sp. nov. also presents this characteristic (Fig. 12). The outer surface of the tube consists of uniformly coarse sediment and detritus with a mix of fragmented fibres and algae (Fig. 2) (small tubes of juvenile attached to the larger tube) (Fig. 13A-D); the inner surface, on the other hand, exhibits a fine network of amphipod silk. Laboratory observations on C. rivulus sp. nov.: feeding behaviour, both antennae 1 and 2 were used to gather food and move chunks of organic material back inside their tube. From the gut content analysis, the major food items consist of fungus, algae, diatoms, organic substances and protozoa. Moreover, mating behaviour showed that males wander about the habitat investigating the tube of female individuals. During these encounters, males try to contact females with their antennae. After that, females withdraw the posterior part of the body into their tubes, while males use their urosome to stimulate the ventral surface of the female pleosome. These situations are assumed to be a possible prelude to mating (Fig. 14).