Alitropus typus H. Milne Edwards, 1840

Vigneshwaran, P., Ravichandrana, S. & Kumar Ajith ,, T. T., 2022, Redescription of the monotypic micro-predatory isopod genera Alitropus H. Milne Edwards, 1840 and Barybrotes Schioedte & Meinert, 1879 (Isopoda, Cymothoida), with a taxonomic key to the Cymothooidea Leach, 1814 from India, Journal of Natural History 55 (45 - 46), pp. 2909-2934 : 2914-2923

publication ID	https://doi.org/ 10.1080/00222933.2021.2008542
DOI	https://doi.org/10.5281/zenodo.6312591
persistent identifier	https://treatment.plazi.org/id/03DF5711-FFEC-FFFF-11B5-FF2574C1B42F
treatment provided by	Plazi
scientific name	Alitropus typus H. Milne Edwards, 1840
status

Treatment

Alitropus typus H. Milne Edwards, 1840 View in CoL

( Figures 1–5 View Figure 1 View Figure 2 View Figure 3 View Figure 4 View Figure 5 )

Alitropus typus H. Milne Edwards, 1840: 247 View in CoL , pl. 33, figs 1–7. — Schioedte and Meinert, 1879: 404, pl. 13, figs –12. — Stebbing, 1911: 180. — Ingle and Fernando, 1963: 106, fig. 4. — Pillai, 1967: 280, fig. 7A–D, pl II, figs 3–4. — Moreira and Sadowsky, 1978: 109. — Bruce, 1983: 782, fig. 17. — Bruce 2009: 214. — Ho and Tonguthai, 1992: 203, figs 1–20.

Aega (Conilera) interrupta Von Martens, 1868: 58 , pl. 1, fig. 3a–c.

Alitropus foveolatus Schioedte and Meinert, 1879: 406 View in CoL , pl. 13, figs 13–16.

Rocinela typus Weber, 1892: 553 . — Nierstrasz and De Marees van Swinderen, 1931: 399, figs 4, 5.

Rocinela mundana Lanchester, 1902: 378 , pl. 35, figs, 9a.

Rocinela simplex Chilton, 1926: 182 View in CoL , fig. 4a–m.

Alitropus dimorphus Pillai, 1954: 8 View in CoL .

Type and locality. Milne Edwards (1840) reported a specimen from the unknown specific locality in the Bay of Bengal. The marine isopods identified by Milne Edwards (1840) were part of the collection from the Bay of Bengal, Indian Ocean, of which most (if not all) are deposited at the Museum National d’Histoire Naturelle, Paris, France (MNHN). There is no indication from the online catalogue of MNHN that the type for Alitropus typus is there; requests were made for the material, but it is not held in the collection of the Museum National d’Histoire Naturelle, Paris (Laure Corbari, personal communication) or the Zoological Museum of Copenhagen, Denmark (JØrgen Olesen, personal communication). Attempts to contact the various international museums and to obtain this material or determine the whereabouts of the holotype have been unsuccessful. On the other hand, the Zoological Museum, Museum für Naturkunde, Berlin (ZMHB) allowed us to examine the holotype specimen of Alitropus typus (as Aega (Conilera) interrupta Von Martens, 1868 ) ( Figures 4 View Figure 4 and 5 View Figure 5 ). Aega (Conilera) interrupta Von Martens, 1868 was placed into junior synonymy with A. typus by Schioedte and Meinert (1879); therefore, a conservative approach is used here: the material examined is tentatively considered conspecific with A. typus until the type or additional material can be checked against it. A neotype was not designated as the species is easily recognisable. Should a neotype be designated, it would ideally be from the type locality.

Material examined. Non-type: All from Vellar estuary, Tamil Nadu, southeast coast of India (11°29 ʹ N, 79°46 ʹ E) from the host Etroplus suratensis, Coll P. Vigneshwaran. 1 male (19.0 mm), 18 June 2016 (ZSI/ MBRC D 1 -568); 2 males (14–19 mm), 18 June 2016 ( CAS / MBRM C- 322 to C- 323); 1 female (15.0 mm), 20 June 2016 ( CAS / MBRM C- 324).

Additional material. Holotype female (15.0 mm; ZMHB No. 3366) (as Aega (Conilera) interrupta Von Martens, 1868 ).

Redescription of male ( Figures 1–3 View Figure 1 View Figure 2 View Figure 3 ). Body ( Figure 1 View Figure 1 (a)) comparatively long, dorsoventrally flattened, symmetrical, parallel-sided, almost ovate, 2.4 times long as wide, widest between pereonites 4–6. Cephalon is triangular, narrower than pereonite 1, anteromedian margin slightly produced. Eyes large ovate and distinct, 0.6 times wider than cephalon. Bases of antennae concealed beneath the median point of the cephalon. Pereonite 1 longer than others and cephalon, anterolateral margins slightly produced; pereonites 2–4 subequal in length; posterolateral margins of pereonites 5–7 slightly produced. Coxae comparatively broad, with rounded apex, partially visible in dorsal view. All pleonites visible dorsally, abruptly narrower than pereion, lateral parts were drawn out into acute processes; pleonites 1–4 subequal in length and width. Pleotelson broader than pleonite 5, broadly rounded apically bearing stout spines among long setae.

Antennula ( Figure 1 View Figure 1 (b)) short, reaching slightly beyond posterior margin of cephalon, with distinct peduncle and flagellum; peduncle article 1 enlarged, article 3 long, longer than the combined length of article 1 and 2, more or less rectangular with trisinuate distal margin bearing 3 setae; flagellum with 5 articles, article 1–4 proximal subequal in length and width, nearly rectangular, laterally setose; terminal article small, tapering, bearing 5 setae; median seta longer than others.

Antenna ( Figure 1 View Figure 1 (c)) longer then antennula, reaching pereonite 3; peduncle article 4 longest, article 5 slightly shorter and narrower than article 4, articles 4 and 5 distally feebly setose; flagellum with 13 articles; all articles with groups of minute setae on the superior distal corner of each segment, apex setose.

Frontal lamina minute. Mandibular ( Figure 1 View Figure 1 (d)) apex narrow; palp with 3 articles, longer than mandible; article 2 bears 8 distinct setae which decrease in length distally; article 1 slightly recurved, inner margin bearing a row of pectinate setae. Maxilla ( Figure 1 View Figure 1 (e)) apex slightly serrated, mesial lobe immersed into its lateral side with 2 hooked spines, lateral lobe with 1 spine. Maxillula with a slightly curved apex, with 2 stout and 3 elongated spines. Maxillula with 1 broad and 2 slender terminal spines. Maxilliped ( Figure 1 View Figure 1 (f)) palp with 3 articles, terminal article with 5 hooked spines.

Pereopods 1–3 ( Figure 2 View Figure 2 (a–c)) proximal superior lateral margin of basis bears a few setae, ischium and merus with 2 spiny setae on their superior anterolateral margins, carpus slightly immersed in merus, propodus long and slender with 2 minute setae at its superior lateral margin; dactylus falcate with a single unguis. Basis and ischium of pereopod 1 long. The number of spines progressively increases in pereopods 1–7. Basis of pereopod 4 ( Figure 2 View Figure 2 (d)) bears a single long spine on its inferior anterior margin; inferior lateral margin of ischium bears 4 groups of stout spines marginally and 2 spines submarginally and the superior distal margin has a single long spine and 2 stout forked spines; merus and carpus elongated bearing groups of stout spines on its inferior lateral margins and forked spines on its superior anterior margin; carpus bears a slender spiny seta with forked tip at its superior distal corner; propodus with 3 equidistantly spaced stout spines along its inferior lateral margin and 3 slender spiny setae on its superior anterior corner; dactylus bears 2 slender spines near its distal margin, single unguis present.

Pereopod 7 ( Figure 2 View Figure 2 (e)) longest; basis bears 4 spines; ischium distally with 2 spines, 1 long and 1 short; merus bears 2 superior spines, 1 long and 1 short, and 2 short inferior spines; carpus bears 3 superior spines, of which 2 are long and 1 short, and 3 inferior spines; propodus with 4 long spines on the superior distal margin and 1 long and 2 short spines on the inferior margin. Pereopods 5 are similar except for the presence of more spines. Dactylus of all pereopods non-spinose, crescentic and recurved on propodus.

Pleopod 1 ( Figure 3 View Figure 3 (a)) with peduncle nearly as long as wide; rami subequal in length, both with marginal setae. Pleopod 2 ( Figure 3 View Figure 3 (b)) with rami subequal in length, endopod possesses the rudiment of appendix masculina, basis bears 6 small setae. Pleopods 3–5 ( Figure 3 View Figure 3 (c–e)) truncate, with partial suture on exopod. Pleopod 5 smallest; endopod bears a single spiny structure on the proximal inner lateral side and several short spiny projections on the distal lateral side.

Uropods ( Figure 3 View Figure 3 (f)) well developed with a long peduncle, exopod long and narrow, slightly shorter than endopod margin, armed with 6 spines among long spiny setae; endopod apex broad and slightly rounded, apex bears 6 spines and long spiny setae, margins of rami profusely setose.

Female ( Figures 4 View Figure 4 and 5 View Figure 5 ). Body ( Figures 4 View Figure 4 (a) and 5(a)) 3.3 times as long as greatest width, widest at pereonites 5 and 6. Rostral point slightly projecting anteriorly, not ventrally folded. Eyes large, not medially united, separated by about 9% width of the cephalon. Coxae 2–3 ( Figures 4 View Figure 4 (c) and 5(b)) each with the posteroventral angle with small distinct produced point (ventral); coxae 5–7 with incomplete oblique carina. Pleon with pleonite 1 visible in dorsal view; pleonite 4 with posterolateral margins extending to but not beyond posterior margin of pleonite 5; pleonite 5 with posterolateral angles free, not overlapped by lateral margins of pleonite 4. Pleotelson ( Figure 4 View Figure 4 (e)) 0.6 times as long as anterior width; lateral margins convex.

Antennula peduncle article 2 anterodistal lobe extending to end of article 3; article 3 as long as combined lengths of articles 1 and 2; flagellum with 5 articles, extending to midpoint of the eye. Antenna peduncle article 2 inferior surface without distinct longitudinal suture; article 4 as long as combined lengths of articles 1–3, with long distal seta, article 5 flattened and expanded, 2.5 times as long as article 4; flagellum with 12 articles, extending to posterior of pereonite 3. Frontal lamina flat, as wide as long, lateral margins converging posteriorly, anterior margin produced, forming a median angle, posterior margin with abutting clypeus ( Figure 5 View Figure 5 (c)).

Pereopods 1–3 prehensile; outer distal corner of ischium with 2 simple setae, that of merus with 3 simple setae and 2 forked spines; medial surface of merus, carpus and propodus bearing 2, 1 and 2 small tubercle-shaped spines, respectively. Pereopods 4–7 ambulatory; distal ends of merus and carpus armed with simple setae, forked spines, and blunt spines with subterminal flagellum; medial surface of ischium, merus, carpus and propodus bearing various numbers of blunt spines each with the subterminal flagellum.

Pleopod 1 carrying 5 obtuse, brush spines and 7 plumose setae on the medial surface of peduncle; exopod fringed with plumose marginal setae; endopod 1.9 times as long as wide, distally rounded, lateral margin straight, with plumose marginal setae on distal onethird, mesial margin with plumose marginal setae on distal margin only; peduncle 1.9 times as wide as long. Pleopod 2 exopod and endopod with plumose marginal setae, appendix masculina basally swollen, 1.3 times as small as endopod, distally acute. Medial surface of peduncle in pleopod 5 naked, that of pleopod 4 with 4 spines and 2 setae and that of pleopod 3 with 4 spines and 4 setae.

Exopod of uropod fringed with plumose marginal setae and bearing 6 short spines on outer margin; endopod larger than exopod, with 3 spines on outer-distal surface.

Distribution. Alitropus typus is widely distributed in the Indian Ocean, from India ( Pillai 1967; present study) to Pakistan ( Ahmad et al. 2016; Shah et al. 2017), Sri Lanka ( Ingle and Fernando 1963; Fernando and Hanek 1973), Indonesia ( Milne Edwards 1840; Schioedte and Meinert 1879), Thailand ( Chilton 1926; Ho and Tonguthai 1992), and Sumatra ( Nierstrasz and De Marees van Swinderen 1931); and in the Pacific Ocean, from eastern Australia ( Bruce 1983).

Prey. Alitropus typus is a common micro-predatory species found in fresh- and brackishwater fishes. The present material was collected from green chromide fish Etroplus suratensis (Bloch, 1790) , and Mozambique tilapia fish Oreochromis mossambicus (Peters, 1852) . Previously this species was recorded from bronze featherback fish Notopterus notopterus (Pallas, 1769) ( Von Martens 1868) , slender rasbora fish Rasbora daniconius (Hamilton, 1822), freshwater catfish Wallago attu Bloch and Schneider, 1801 ( Fernando and Hanek 1973; Ahmad et al. 2016), striped snakehead fish Channa striata (Bloch, 1793) (as Channa striatus ) (Nair and Nair 1983), Mugil sp. ( Bruce 1983) , mekong giant catfish Pangasianodon gigas Chevey, 1931 ( Ho and Tonguthai 1992), mrigal carp Cirrhinus mrigala Hamilton, 1822 , rohu Labeo rohita Hamilton, 1822 , freshwater garfish Xenentodon cancila (Hamilton, 1822) , common carp Cyprinus carpio Linnaeus, 1758 , bullseye snakehead Channa marulius (Hamilton, 1822) , pabdah catfish Ompok pabda (Hamilton, 1822) , Indian river shad Gudusia chapra (Hamilton, 1822) ( Ahmad et al. 2016) and kuria labeo Labeo gonius (Hamilton, 1822) ( Ahmad et al. 2016; Shah et al. 2017).

The juveniles are ectoparasitic on fishes until they attain sexual maturity. The adults are, however, free-living and cling to a fish only for the purpose of feeding.

Remarks. Alitropus typus is characterised by the comparatively slender and elongated body; cephalon is broadly triangular with antero-median margin slightly produced; pereonite 1 as long as the cephalon, female pereonites 5 and 6 are much shorter than in the male; frontal lamina minute; mandible palp longer than mandible; maxillula with 1 broad and 2 slender terminal spines; pleotelson broadly rounded as broad as pleonite 5, margin finely setose; peduncle of uropod long; exopod narrow. The combination of the above-mentioned characters clearly separates this genus from other genera of the family Aegidae .

A number of species have been placed in synonymy with A. typus . Of these, figures of whole specimens have been given by Nierstrasz and De Marees van Swinderen (1931) and of most appendages by Chilton (1926) (as Rocinela simplex ). The mouthparts and pleopods have not been described in detail, and are here figured.

The species Rocinela simplex Chilton, 1926 was transferred to A. typus by Ingle and Fernando (1963). In the illustrations given by Chilton (1926), fig. 4(a) is described as that of the female and fig. 4(d) as that of an immature male. However, there appears to be a slight confusion. Chilton’s fig. 4(a) depicts an adult male and fig. 4(d) shows an immature male or female. Chilton appears to have been unaware of the sexual dimorphism which this species shows. Chilton’s fig. 4(k) is likely to be the second pleopod of the specimen marked 4a and not that of 4d.

The morphology of the mouthparts of A. typus seems variable, as noted in the available descriptions. As seen from Pillai’s (1967) description, the last segment of the mandibular palp bears a single apical seta. In the present material, the last segment of the mandibular palp bears a comb of setae that very closely resembles the figure by Bruce (1983) except that the setae are pectinate. The first maxilla also differs from that described by Pillai (1967) and Bruce (1983) in having 2 stout apical teeth and 3 elongated spines. The second maxilla has been described as having 3 spines on its inner lobe ( Pillai 1967), 2 hooked spines on the endopod and 1 spine on the exopod ( Bruce 1983). The present specimen reveals 2 hooked spines and 3 small spiny setae on the inner lobe of the maxilla. The maxillipeds’ terminal article has 5 hooked spines, as was observed by Bruce (1983), but Pillai (1967) mentioned only 3 hooked spines. Despite these variations in the number of spines of the mouthparts, it is evident from the rest of the details that the present material is A. typus .

Alitropus typus occurs predominantly in marine water habitats. Milne Edwards (1840) stated ‘Trouvé dans le golfe du Bengale’ (‘Found in the Bay of Bengal’). Later, Schioedte and Meinert (1879) examined the specimen collected by Von Martens from the Museum für Naturkunde in Berlin, Germany, and his label stated ‘Specimen typicum Martensii, in flumine Kapuas ad Sintang insulae Borneo captum’ (‘ Type specimen collected by Von Martens, from the river Kapuas Kalimantan (as Borneo Islands) Barat, Indonesia were captured’). Also, the museum type specimen container mentions the type locality as Kalimantan (as Borneo Islands) Barat, Indonesia (0°4 ʹ N, 111°30 ʹ E) (170 km from the nearest coast). Von Martens (1868) also reports this species from the gills of bronze featherback fish ( N. hypselonotus ), collected from the Kapuas River near Sintang, Borneo. More recently, Bruce (1983) reported its occurrence in freshwater habitats of Wivenhoe Dam, Australia (175 km from the nearest coast), and Ho and Tonguthai (1992) found this species on the giant catfish ( P. gigas ) from freshwater habitats of Chiang Khong (500 km from the nearest coast).

The present materials of A. typus were collected from brackish-water habitats, occurring on green chromide fish E. suratensis and Mozambique tilapia fish O. mossambicus in Vellar estuary, Parangipettai, India (11°29 ʹ N, 79°46 ʹ E) (2 km from the sea mouth). Similarly, Pillai (1967) collected specimens from a brackish-water habitat. This species has been collected in coastal low-salinity habitats both in India at Vellar estuary (salinity 11 ppm; present study) and in Thailand ( Chilton 1926). Rameshkumar and Ravichandran (2010) reported that the A. typus collected on Cuddalore coast (the Vellar estuarine environment, Parangipettai) from the host species tilapia Oreochromis mossambicus (Peters, 1852) is a misidentification; without a description and access to the specimens, we exclude this record from synonymy.