A Possible Relation between the Occurrence of a Dendritic Organ and the Distribution of the Plotosidae (Cypriniformes)

W. J. R. LANZING1

ABSTRACT: Three marine of Plotosidae are found along the coastlines of the Indian and Pacific oceans, but the other 25 species occur exclusively in the Australian region. The majority of the Plotosidae are freshwater inhabitan ts, some of which are indigenous to both Australia and N ew Guinea. The marine members of the family and two freshwater members possess a dendritic organ. It is sug­ gested that this organ has an osmoregulatory function.

IN THEIR DESCRIPTION of the renee of a dendritic organ and the distribution anguillaris, Bloch ( 1794) and Lacepede of the Plotosidae in marine and freshwater en­ (1803) mention the presence of a peculiar vironments. external structure situated posterior to the vent and between the pelvic fins. Cuvier and Valen­ DISTRIB UTION OF THE SILUROIDEI ciennes (1840) observed that this structure had no connection with the urogenital system, The siluroids of the Austral-Asian region but was attached to the last abdominal verte­ are best represented in the area bounded by bra by means of a long tendon. According to Thailand, Vietnam, and Indonesia. The follow­ Brock ( 1887) and Hirota (1 895) this so-called ing families occur in this region : Akysidae, dendritic organ consists of numerous well­ Amblycipitidae, Bagridae, Chacidae, Clariidae, vascularized epithelial folds. Weber and de Heteropneustidae, Plotosidae, Schilbeidae, Si­ Beaufort (1913), Taylor ( 1964) , and Munro luridae, Sisoridae (Bagariidae) , and Tachy­ ( 1966) used the presence of this organ as a suridae (Ariidae) . Except for the Tachysuridae criterion in their keys to the family Plotosidae. and three species of Plotosidae none of these However, apart from its use in this catfish are found east of the line of Wallace. structure has attracted very little attention. In the Australian region, which includes Aus­ Recently, electron microscope studies by van tralia, N ew Guinea, and some adjacent islands Lennep and Lanzing (1 966) have shown that (Darlington, 1957), only the Doiichthyidae, the dendritic organ of Plotosus anguillaris Plotosidae, and Tachysuridae occur. (Bloch), Cnidoglanis macrocephalus (Val.) , and Euristbmus leptum s (Gunther) possesses DISTRIB UTION OF THE PLOTOSIDAE two main cell types: principal cells containing All known species of Plotosidae are inhabi­ parallel groups of cytoplasmic tubules and many tants of the Australian region, but three marine mitochondria, and clear cells containing an species have a much wider range (Fig. 1) . unusual three-dimensional network of cyto­ Paraplotosus albilabris (c. et V.) occurs in plasmic tubules. Because of a similarity be­ Indonesia, Vietnam, and the Philippines tween these cells and those occurring in salt (Suvatti, 1950; Herre, 1953 ; Kuronema, 1961). glands of sharks and marine birds and the Plotosus canius Ham. Buch. is reported from chlor ide cells in fish gills, the authors suggested East Africa as well as from Fiji (Fowler, . that the plotosid dendritic organ is involved in 1959), but does not seem to occur in China salt transport. This assumption has led to a or Japan. On account of its wide range it is study of a possible relation between the occur- surprisi ng that it has not yet been repor ted from Australia, although it is present in New Guinea 1 School of Biological Sciences, University of Syd­ ney,Sydney, N .S.W ., Australia. Manuscript received (Munro, 1958). Plotosus angt,illaris (Bloch) September 23, 1966. is distributed over a vast area. The western 498 Distribution of the Plotosidae-LANZING 499

. :. HAWA II " " MARIANAS " .

., F1J 1 .... .,SOCI ETY. .. , ......

FIG. 1. Distribution of the Plotosidae. - - , Principal range of the Plotosidae; ... , range of Plotosus anguillaris; /// , range of Paraplotosus albilabris. For letter symbols see footnote for Table 1. limit of its range is formed by the Southwest shows that several freshwater plotosids are Asiatic Barrier (Ekman, 1953) , as it has been common to both Australia and New Guinea found in the Red Sea and the Suez canal (e.g., Porocbilus obbesi W eber) . Other spe­ (F owler, 1956) . Although present along the cies are exclusively New Guinean, e.g., N eo­ East African coast, it does not reach Cape silurus gjellerupi (Weber) , or Australian, e.g., Town (Smith, 1949) . It occurs in Korea 'I'andanus tandanus. Munro (1964) recently (Mori , 1952) and Japan (Okada, 1955) as drew attention to the existence of differences well as in the Society Islands; the Eastern between the ichthyological fauna of northern Pacific Barrier (Ekman, 1953) apparently New Guinea (Gaimardian fluvifaunula) and forms the eastern limit of its range. Plotosus that of southern New Guinea (Riechian fluvi­ anguillaris is not recorded from the Marshall faunula) . Apparently, some plotosids like and Marianas Islands (Schulz, 1953) , which N eosilurus gjellerupi and N . ater sepik ensis ichthyologically are more related to the Hawai­ (Whitley) live in the Gaimardian area, whereas ian region. N . brevidorsalis (Gunther) and N . ater ater The distribution of the other plotosids, how­ (Perugia) are confined to the Riechian area. ever, is strictly limited to the Australian region. Of the marine plotosids only Cnidoglanis THEPRESENCE OF A DENDRITIC ORGAN macrocephalus is found all along the coast of AMONG THE PLOTOSIDAE mainland Australia, Tasmania, and New Guinea ; the other marine species occur only Table 1 lists two groups of Plotosidae : the in the northern half of Australia and in New species in group A possess a dendritic organ, Guinea. Most of the freshwater plotosids are whereas this organ is lacking in the plotosids indigenous to the Leichhardtian fluvifaunula of group B.The species of group A occupy a (see map by Whitley, 1959), although Tan­ marine or estuarine habitat, but thus far 010­ danus tandanss Mitchell, for instance, is con­ plotosus mariae W eber and Plotosus papuensis fined to the Mitchellian fluvifaunul a. Table 1 Weber have been found only in fresh water 500 PACIFIC SCIENCE, Vol. XXI , October 1967

TABLE 1

D ISTRIBUTI ON AN D H ABITATS OF THE P LOTOSIDAE *

SPECIES AU STRALIA N EW GUI NE A HA BITAT Group A (de ndritic organ present) 1. Cnidoglanis macrocephalus (Val.) + + marine 2. C. microceps (Rich.) + marine 3. C. muelleri (Klunzer) + marine 4. Euristbmus lepturus (Gunther) + + marine 5. E. nudiceps (Gunther) + + marine 6. Oloplotosus marlae Weber + R fresh water 7. Paraplotosus albilabris (Val.) + + marine 8.Plotosus anguillaris (Bloch ) + + marine 9. P. canius H am. Buch. + marine 10 . P. papuensis W eber + R fresh water Group B (n o dendri tic organ ) 1. A nodontiglanis dahli Rendahl +L fresh water 2. Neosilurus argenteus (Zietz) +S fresh water 3. N . ater aier ( Perugia) + L + R fr esh water N . ater sepihensis (Whitley) + G fresh water 4. N. bartoni Regan +R fresh water 5. N . brevidorsalis (Gunther) + L, ] + R fresh water 6. N . equ in us (Weber) + R fresh water 7. N . gjellerupi (Weber) +G fresh water 8.N . glencoensis (Rendahl ) + L fresh water 9. N . hyrtlii Steindachner +L,] fresh water 10. N . idenburgi (Nichols) +G fresh water 11. N . merauhen sis (Weber) + R fresh water 12. N . mortoni Whitley +L fresh water 13. N. novaeguineae niger (N ichols) +G fresh water N . novaegui neae nooaeguineae (W eber) + R fresh water 14. N . perugiae (Ogilby) + R fresh water 15.N . rendahli (Whitley) +L fresh water 16. Parocbilus obbesi W eber + L + R fresh water 17. T andanus bostocki Whitley + Gr fresh water 18. T. tandanus Mitchell + M fresh water • Symbols used: +. present; - , absent; G . Gaimardian lIuvifaunula; Gr, Greyian; J, Jardinian; L, Leichh ardtian; M, Mitchell ian; R. Riechian; S, Sturtian .

(W eber and de Beaufort, 1913). All the spe­ length. Since some of the gonads were either cies of group B are freshwater inhabitants. in a fully mature or in a spent condition, these Examinations carried out on adult and juve­ measurements must represent the size of adult nile individuals of Plotosus angllillarisJ Cnido­ catfish of this species. Comparable figures have glanis macrocephalus, and Euristbm us leptur«: been reported by Delsman and H ardenberg showed that the dend ritic organ is present and (1934) , 300 mm; Okada ( 1955) , 250 mm; equally developed in both sexes, and also that and Fowler (19 59),460 mm. A much higher it is already conspicuous in juvenile catfish figure for maximum size (3 0 inches) is given ranging in size between 45 and 56 mm. Re­ by Smith (1949) , and is quoted by Munro cently, van Lennep (unpublished ) found that , ( 1954) and Fowler ( 1956) , but probably is in comparison with adults, the dendritic organ erroneous. of juvenile catfish contains only a relatively small number of fully developed glandular DISCUSSION cells. The mean length of nine adult Plotosus More than half of the 28 species of Ploto­ anguillaris was 322 mm (286- 361 mm ) total sidae are freshwater inhabitants. This makes Distribution of the Plotosidae-LANZING 501

the Plotosidae a predominantly freshwater raphy must remain speculative, it is suggested family rather than a chiefly marine family, as that the dendritic organ was developed while is often implied in the literature (Berg, 1957 ; plotosid ancestors in the Southeast Asian region Darlington, 1957; Nikolsky, 1961; Sterba, invaded the sea. This invasion would be differ­ 1963) . ent from that of the Tachysuridae, which were It appears that 25 species are found in the able to cope with osmotic stresses by means of Australian region. Three marine species occupy mechanisms similar to those used by other old a much larger area covering most of the Indian teleost families, such as the Salmonidae. Per­ and W est Pacific oceans. Sterba's map (1963) haps because of tachysurid competition, the of the distribution of the Plotosidae therefore plotosid ancestors became firmly settled only in actually shows the range of one species, namely the Australian region. From them would have Plotosus anguillaris. It is of intere. t that among evolved, on the one hand, the freshwater spe­ the non-plotosid siluroids only T acbvsurus thal­ cies which lost the dend ritic organ in the lasinus (Rueppel) has a range as wide as that process. Some of the marine species, on the of Plotosus canias, except that the former is other hand, managed to disperse radially along also reported from Japan (Matsubara, 1955). the edges of the Ind ian and West Pacific As yet no physiological work has been car­ ocean basins. ried out with regard to the function of the dendritic organ. The available evidence indi­ I am grateful to Mr. 1. S. R. Munro (CSI. cates that: (a) its structure resembles that of R.O. Marine Laboratory, Cronulla) for his ad­ salt-secreting glands in other vertebrates, (b) vice and for putting at my disposal the manu­ it is present in both juvenile and adult ploto­ script for his forthcoming book on the fishes of sids, (c) there exist no sexual differences, and N ew Guinea. (d) it is not present in freshwater plotosids other than Oloplotosus mariae and Plotosus REFERENCES papttensis. H ardenberg and Delsman ( 1934) suggested that the dendritic organ is involved BERG, L. S. 1957. Classification of Fishes Both in reproduction, but produced no evidence in Recent and Fossil. Edward Bros. Ann Arbor, suppor t of this claim. A possible respiratory Michigan. function merits consideration since other cat­ BLO CH, M. E. 1794. N aturgeschichte der Aus­ fish (Clariidae, Heteropneustidae) possess ac­ landischen Fische, vol. 8.Schlesinger, Berlin. cessory respiratory organs. These, however, are BROCK, J. 1887. Uber Anh angsgebilde des in connection with the branchial chambers and Urogenitalapparates von Kn ochenfische. Z. are structurally different from the dendritic wissen. Zool. 45 :532. organ. Furthermore, there seems to be no rea­ CUVIER, G., and A. VALENCIENNES. 1840. H is­ son why only marine plotosids should require toire Naturelle des Poissons. XV. Levrault, an accessory respiratory organ. Both marine Paris. and freshwater plotosids enter muddy environ­ DARLINGTON, P. J. 1957. Zoogeography. The ments that could contain oxygen-deficient water. Geographical D istribution of Anim als. John A salt-excretory function seems to be the most W iley & Sons, New York. likely, although, admittedly, the presence of a DELSMAN, H . c., and J. D. F. HARDENBERG. dendritic organ in Oloplotosus mariae and 1934. De Indische Zeevissen en Zeevisscherij. Plotosus papuem is does not fit in with this Visser and Co., Batavia. theory. Too little is known about these two EKMAN, S. 1953. Zoogeography of the Sea. species to venture any explanation. Sidgwick and Jackson, London . According to Darlington (1957:46) marine FOWLER, H. W . 1934. The fishes of Oceania, plotosids may have invaded the Australian Suppl. 2.Mem. B. P. Bishop Mus. 10 (6) . region and , after entering a freshwater habitat, H onolulu. reached the end of a complicated line of teleost - -- 1938a. The Fishes of the George Van­ evolution. derbilt South Pacific Expedition. Acad. Nat. Although evolutionary aspects of zoogeog- Sci. Philadelph ia, Monogr. 2. 502 PACIFIC SCIEN CE, Vol. XXI, October 1967

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