Tachaea spongillicola Stebbing, 1907

Vigneshwaran, P., Kumar, T. T. Ajith, Ravichandran, S. & Lal, Kuldeep K., 2022, Redescription and molecular characterisation of the predatory isopod Tachaea spongillicola Stebbing, 1907 (Isopoda: Corallanidae) infesting the freshwater fish Pangasius silasi from India, Journal of Natural History 55 (37 - 38), pp. 2419-2437 : 2422-2431

Tachaea spongillicola Stebbing, 1907

( Figures 1–6 )

Tachaea spongillicola Stebbing, 1907: 40 , pl. 6, 1908: 107. – Thielemann 1910: 21. – Nierstrasz 1917: 102, 1931: 172. – Tattersall, 1921: 419. – Nierstrasz and De Marees van Swinderen 1931: 398. – Shen 1936: 17. – Delaney 1989: 56. – Mariappan et al. 2003: 260, figs 2 and 3. – Roy and Mitra 2014: 4, fig. 1.

Type and locality

The syntype is available at the Natural History Museum, London ( BMNH 1907.10 .19.4 (Ecatalogue: NHMUK 1202421 )) coll . Dr Annandale from Kolkata, India.

Material examined

4 non-ovigerous females (6–8 mm) ( NBFGR /CORTSPO I-01 to I-04); 3 males (3–6 mm) ( NBFGR /CORTSPO I-05 to I-08); 3 male (3.5–6 mm) ( CAS / MBRM C-716 to C-718 ); 3 females (5–7 mm) ( CAS / MBRM C-711 to C-713 ) attached to Pangasius silasi , collected by T . T. Ajith Kumar, 9 April 2019, from the ICAR-NBFGR Germplasm Resource Centre in open waters for indigenous fishes at the Nagarjuna Sagar dam, Telengana, India .

Redescription of non-ovigerous female ( Figures 1–5 ). Body 2.5 times as long as wide, pereonites 3–5 widest, with many chromatophores scattered on dorsal surface ( Figure 1a ). Cephalon 2.4 times wider than medial length, slightly immersed in pereonite 1, with short, broadly rounded rostrum anteriorly. Eyes not medially united, separated, about 22% width of cephalon; each eye made up of ~8 transverse rows of ommatidia, each row with ~5 ommatidia; eye colour pale brown. All pereonites with broad posterolateral regions; posterior margin of pereonite 7 weakly concave, pereonite 1 longer than 2–7, pereonites 4–6 subequal in length, wider than 2–3 and 7. Coxae ( Figure 2a,b ) 2–7 acute, visible in dorsal view, gradually increasing in size; coxae 2–3 each with posteroventral angle rounded; coxae 3–7 extending beyond posterior margin of pereonites; 5–7 without oblique carina. Pleon ( Figures 1a,b and 2a,b ) visible, pleonite 4 longest, 1–3 and 5 subequal in length; pleonite 4 with posterolateral margins extending clearly beyond posterior margin of pleonite 5; pleonite 5 slightly posteriorly produced with posterolateral angles rounded. Pleotelson ( Figure 2a ) 0.8 times as long as anterior width, anterior dorsal surface without 2 sub-median depressions, lateral margins convex, posterior margin evenly rounded, with 9 robust setae and ~38 plumose setae.

Antennula ( Figure 3b ) peduncle with 3 articles; article 1 is 1.9 times as wide as 2 and 3, with 3 setae; article 2 is 0.8 times as long as combined lengths of articles 1 and 3, with 10 setae; article 3 minute, 0.3 times as long as article 2; flagellum with 7 articles (articles 1 and 2 longest); all articles with cluster of aesthetasc setae; articles 6 and 7 minute, extending to anterior of pereonite 1. Antenna ( Figure 3c ) peduncle with 5 articles; article 1 is 0.6 times as long as wide; article 2 shortest, with 2 setae; article 3 is 2.0 times as long as article 2, with 6 setae; articles 4 and 5 of similar length, elongate; article 4 with 3 setae; article 5 with 5 setae; flagellum with 19 articles (article 1 longest with 1 seta), articles 4–19 with 6–8 setae.

Frontal lamina, clypeus and labrum visible anterior to mandibles; labrum may cover distal region of mandibular incisor. Clypeus sometimes encompassing lateral margins of labrum ( Figure 3a ). Mandible ( Figure 3d ) molar process, represented by small 3-spined lobe; lacinia mobilis represented by 1–3-spined lobe, incisor of mandible with 2 cusps. Palp inserted near mandible base with 3-articulate, palp article 2 longer than 1 and 3, with simple marginal setae; palp article 3 with plumose marginal setae and often with serrate setae. Maxillula ( Figure 3f ) with lateral lobe forming single large curved spine; medial lobe with single apical seta. Maxilla ( Figure 3g ) mesial lobe with 2 robust setae, lateral lobe with 1 robust seta, apical lobe without setae. Maxilliped ( Figure 3e ) narrow, 3.9 times as long as wide, basis elongate, palp with 3 articles, palp distal articles with simple marginal setae.

Pereopod 1 ( Figure 4a )basis elongated,2.9times as long as wide,with 7 short setae on both superior and inferior margins; ischium 0.5 times as long as basis, 1.6 times as long as wide, inferior margin with 3 setae, inferior distal margin with 5 setae, superior margin with 3 setae; merus 0.5 times as long as ischium, 0.4 times as long as wide, inferior margin with 3 robust setae and 1 seta, superior distal margin with 3 setae; carpus short, 0.5 times as long as merus, 0.2 times as long as wide,inferior margin with 3 robust setae,inferior distal margin with 3 setae and 8 robust setae, superior distal margin with 2 setae; propodus equal in length to ischium, 1.2 times as long as wide, anterior margin with 6 robust setae, inferior margin with 3 robust setae and 1 seta, superior margin with 4 setae; dactylus falcate, 0.4 times as long as propodus.

Pereopod 2 ( Figure 4b ) basis elongated, 3.5 times as long as wide, superior margin with 7 setae, superior distal margin with 3 setae, inferior margin with 6 setae; ischium 1.2 times as long as basis, superior margin with slightly curved inside with 3 setae, superior distal margin with 3 robust setae and 2 setae, inferior margin with 3 robust setae and 5 setae; merus 0.4 times as long as ischium, 0.5 times as long as wide, superior distal margin extended to carpus with 4 robust setae, inferior distal margin with 4 robust setae; carpus short, 0.3 times as long as merus, 0.3 times as long as wide, inferior distal margin extended to propodus with 7 robust setae, superior distal margin with 4 robust setae and 4 setae; propodus and dactylus similar to pereopod 1.

Pereopod 3 ( Figure 4c ) basis 2.6 times as long as greatest width; ischium 2.1 times as long as basis, inferior margin with 1 seta and 3 robust setae, superior margin with 1 seta and 4 robust setae, superior distal margin with 3 setae; merus inferior margin with 2 robust setae, inferior distal and superior distal angle with 4 and 3 robust setae respectively; carpus 0.3 times as long as proximal width, carpus palm with the blade, carpus blade 0.3 times as wide as palm, inferior margin with 4 robust setae, set as two groups, superior margin with 1 seta; propodus 1.7 times as long as proximal width, inferior margin with 6 robust setae, set as a single group, superior margin with 4 setae; dactylus 1.9 times as long as propodus, with 2 short setae on both superior and inferior margins.

Pereopod 4 ( Figure 4d ) similar to pereopod 5. Pereopod 5 ( Figure 4e ) basis 2.8 times as long as greatest width, with 2–3 short setae on both superior and inferior margin; ischium 0.4 as long as basis, inferior margin with 1 robust seta, superior distal angle with 1 robust seta, inferior distal angle with 2 robust setae; merus 0.5 times as long as ischium, 0.9 times as long as wide, inferior margin with 5 setae, inferior distal angle with 4 robust setae, superior distal angle with 4 robust setae (set as 3 and 1); carpus 0.4 as long as ischium, 1.8 times as long as wide, inferior margin with 1 robust seta, inferior distal angle with 9 robust setae (set as 6, 2 and 1), superior distal angle with 6 robust setae, set as a single group; propodus 0.6 as long as ischium, 3.8 as long as wide, inferior margin with 7 robust setae, superior margin with 4 robust setae and 2 slender setae; dactylus similar to pereopod 3.

Pereopod 6 ( Figure 4f ) similar to pereopod 7. Pereopod 7 ( Figure 4g ) basis 3.0 times as long as greatest width; inferior distal margin with 1 robust seta; ischium 1.9 times as long as basis, inferior margin with 2 setae and 4 robust setae (set as 3 and 1), superior distal margin with 2 robust setae; merus shortest 0.3 times as long as ischium, 0.6 times as long as wide, inferior distal margin with 6 robust setae (set as 2 and 4), superior distal angle with 3 robust setae (set singly); carpus short, 0.4 times as long as merus, 0.4 times as long as wide, inferior margin with 4 robust setae, superior distal margin with cluster of robust setae; propodus equal in length to ischium, inferior margin with 2 robust setae and 5 setae, superior margin with 6 setae and 1 robust seta.

Pleopod ( Figure 5 ) peduncles wider than long, with 4–5 clusters of spines and groups of proximal setae on medial margins, lateral margins often with simple spine. Exopods of pleopods broader, longer and fringed with plumose marginal setae; endopod have similar ornamentation to exopod except medial margin. Pleopods 3–5 differ from pleopods 1 and 2 in having relatively broader rami and outer surface of peduncle drawn out into process tipped with seta. Endopod of pleopod 5 naked, proximal medial margin produced, endopods of 1–4 naked or with sparse setation.

Uropod ( Figure 4h ) basis triangular, with 2 setae on outer margin and 2 setae on distal end: endopod rectangular, 2.2 times as long as with, with robust setae and more than 30 plumose setae around the margin; exopod slender and little shorter than endopod, with 6 teeth and more than 37–38 plumose setae around the margin.

Description of male ( Figures 1c,d , 2c , 5f ). Male individuals were similar to nonovigerous female. Body elongate, 2.2 times as long as wide, longest between pereonites 3–5. Cephalon small, with rounded rostrum anteriorly. Pereonites 1–4 slightly larger than pereonites 5–7; posterolateral corners of pereonites 4–7 increasing in size posteriorly; pleonite 4 with lateral portions produced posteriorly and extending to pleotelson. Pleotelson as broad as long, apically truncate, posterior margin serrate with 4 spines and numerous plumose setae. Pleopods differ from those of the non-ovigerous female in having longer peduncles on pleopods 1 and 2, pleopod 2 ( Figure 5f ) with appendix masculine, with simple apex, arising from proximal medial margin of endopod with short plumose setae, endopods of pleopods 3–5 with proximomedial lobe but not folded. Uropod peduncle posterior lobe about three-quarters as long as endopod. Uropod rami extending to pleotelson apex, with robust setae and plumose setae on lateral and posterior margins; endopod of uropod apically bifid deeply, lateral process prominent, lateral margin straight, without prominent excision, mesial margin sinuate; exopod of uropod not extending to end of endopod, 3.0 times as long as greatest width, apically subbifid, mesial process prominent; lateral margin weakly convex, mesial margin sinuate.

Prey. The type specimen was collected from the freshwater sponge Eunapius carteri (Bowerbank, 1863) (as Spongilla carteri ) ( Stebbing 1907). Later, the species was collected as ectoparasites of freshwater prawns Macrobrachium lamarrei (Edwards, 1837) , M. malcomsonii (Edwards, 1844) and M. nobilii (Henderson and Matthai, 1910) , as reported by Mariappan et al. (2003) in southern India, and by Roy and Mitra (2014) in West Bengal. The present materials were collected from P. silasi , a new host for this species ( Table 1 ).

Distribution

The distribution of T. spongillicola is restricted to India; it has been collected from Kolkata, West Bengal ( Stebbing 1907; Roy and Mitra 2014), Cauvery River ( Mariappan et al. 2003) and Krishna River at Nagarjuna Sagar Dam (present study), India ( Table 1 ).

Remarks

Tachaea spongillicola can be distinguished from its congeners by the cephalon slightly immersed in pereonite 1, pleonite 4 with posterolateral margins extending beyond pleonite 5, incisor of the mandible with two cusps; the apical lobe of the maxilla without seta; inferior distal margin of carpus of pereopods 1–3 extended to propodus, pereonite 1 is as long as pereonite 2.

Tachaea spongillicola and T. koreaensis (Song and Min, 2018) have similar body proportions, the body being 2.3 times longer than its greatest width, cephalon with broadly rounded rostrum,posterolateral margins of pleonite 4 extending beyond posterior margin of pleonite 5, pleonite 5 slightly posteriorly produced with posterolateral angles rounded. Tachaea spongillicola differs from T. koreaensis in the following characters: the pereonite 1 is as long as pereonites 2–7, the incisor of the mandible with two cusps, the apical lobe of the maxilla without seta, and the posterior margin of the pleotelson with nine robust setae. In contrast with T.koreaensis , the incisor of the mandible with one cusp,the apical lobe of the maxilla with one seta, and the posterior margin of the pleotelson with 10 robust setae. Among them, the present species is most closely related to T. tonlesapensis Nunomura, 2006 reported from the freshwater area of lake Tonlé Sap Cambodia, but the former is separated from the latter by the following features: (1) narrower pleotelson, (2) longer stylus of male second pleopod, (3) absence of wide vibratory lamella of maxilliped in both sexes,(4) numerous flagellar segments of both antennae, (5) smaller eyes and (6) numerous segments of the mandibular palp.

Stebbing (1907) noted in T. spongillicola that the pleotelson is broadly rounded in females but is apically truncate in males. Based on this information Stebbing (1907) synonymised Tachaea incerta Hansen, 1890 with T. crassipes Schioedte and Meinert. Stebbing (1907) distinguished T. spongillicola from T. crassipes by the apical article of the maxillipeds as well as the decreased setations of the pereopods and the shorter pleon (relative to pereion length) in the former species.

Occurrence of Tachaea spongillicola

A total of 40 P. silasi available in the cages were examined for the isopod infestation; 35 were infested with 186 T. spongillicola , corresponding to an overall prevalence and intensity of 87.5% and 5.31, respectively. Among the total of 126 T. spongillicola collected, 74 were non-ovigerous females and 52 were males. Gross features on the infected fish showed lesions on various parts of the body. The isopod attached at the base of the fins, head and abdomen, and to the gill cavity; we noted also that it was attached to weak and dead fishes only. In severe case, haemorrhages were seen all over the body of the infected fish, leading to its death ( Figure 6 ). The isopod sucked the fish’s blood and it may have detached when its stomach was full.

Figures

Figure 1. Tachaea spongillicola Stebbing, 1907, (a–b) non-ovigerous female (7.2 mm; NBFGR/CORTSPO I-01); (a) dorsal view; (b) ventral view; (c–d) male (5.4 mm) (NBFGR/CORTSPO I-05); (c) dorsal view; (d) ventral view. Photographs taken under a Leica M-205A stereomicroscope.

Figure 2. Tachaea spongillicola Stebbing, 1907, (a–b) non-ovigerous female (7.2 mm; NBFGR/CORTSPO I-01). (a) dorsal view; (b) lateral view; (c) male dorsolateral view (5.4 mm) (NBFGR/CORTSPO I-05).

Figure 3. Tachaea spongillicola Stebbing, 1907, non-ovigerous female (7.2 mm; NBFGR/CORTSPO I-01). (a) mouthparts; (b) antennule; (c) antenna; (d) mandible; (e) maxilliped; (f) maxillula; (g) maxilla.

Figure 4. Tachaea spongillicola Stebbing, 1907, non-ovigerous female (7.2 mm; NBFGR/CORTSPO I-01). (a–g) pereopod 1–7; (h) uropod.

Figure 5. Tachaea spongillicola Stebbing, 1907, non-ovigerous female (7.2 mm; NBFGR/CORTSPO I-01). (a–e) pleopod 1–5; (f) pleopod 2 (male, 5.4 mm).

Figure 6. Gross features of the infested fish Pangasius silasi with arrows showing infestation with Tachaea spongillicola at caudal and anal fins and lesions on various parts of the body.