A Possible Relation between the Occurrence of a Dendritic Organ 
and the Distribution of the Plotosidae (Cypriniformes) 
W. J. R. Lanzing 1 
ABSTRACT: Three marine species 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 inhabitants, some 
of which are indigenous to both Australia and New 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 catfish Plotosus 
anguillaris , Bloch (1794) and LaCepede 
(1803) mention the presence of a peculiar 
external structure situated posterior to the vent 
and between the pelvic fins. Cuvier and Valen- 
ciennes (1840) observed that this structure 
had no connection with the urogenital system, 
but was attached to the last abdominal verte- 
bra by means of a long tendon. According to 
Brock (1887) and Hirota (1895) this so-called 
dendritic organ consists of numerous well- 
vascularized epithelial folds. Weber and de 
Beaufort (1913), Taylor (1964), and Munro 
(1966) used the presence of this organ as a 
criterion in their keys to the family Plotosidae. 
However, apart from its use in taxonomy this 
structure has attracted very little attention. 
Recently, electron microscope studies by van 
Lennep and Lanzing (1966) have shown that 
the dendritic organ of Plotosus anguillaris 
(Bloch), Cnidoglanis macrocephalus (VaL), 
and Eur isthmus lepturus (Gunther) possesses 
two main cell types: principal cells containing 
parallel groups of cytoplasmic tubules and many 
mitochondria, and clear cells containing an 
unusual three-dimensional network of cyto- 
plasmic tubules. Because of a similarity be- 
tween these cells and those occurring in salt 
glands of sharks and marine birds and the 
chloride cells in fish gills, the authors suggested 
that the plotosid dendritic organ is involved in 
salt transport. This assumption has led to a 
study of a possible relation between the occur- 
1 School of Biological Sciences, University of Syd- 
ney, Sydney, N.S.W., Australia. Manuscript received 
September 23, 1966. 
rence of a dendritic organ and the distribution 
of the Plotosidae in marine and freshwater en- 
vironments. 
DISTRIBUTION OF THE SILUROIDEI 
The siluroids of the Austral- Asian region 
are best represented in the area bounded by 
Thailand, Vietnam, and Indonesia. The follow- 
ing families occur in this region: Akysidae, 
Amblycipitidae, Bagridae, Chacidae, Clariidae, 
Heteropneustidae, Plotosidae, Schilbeidae, Si- 
luridae, Sisoridae (Bagariidae) , and Tachy- 
suridae (Ariidae). Except for the Tachysuridae 
and three species of Plotosidae none of these 
catfish are found east of the line of Wallace. 
In the Australian region, which includes Aus- 
tralia, New Guinea, and some adjacent islands 
(Darlington, 1957), only the Doiichthyidae, 
Plotosidae, and Tachysuridae occur. 
DISTRIBUTION OF THE PLOTOSIDAE 
All known species of Plotosidae are inhabi- 
tants of the Australian region, but three marine 
species have a much wider range (Fig. 1). 
Paraplotosus albilabris (C. et V.) occurs in 
Indonesia, Vietnam, and the Philippines 
(Suvatti, 1950; Herre, 1953; Kuronema, 1961). 
Plotosus canius Ham. Buch. is reported from 
East Africa as well as from Fiji (Fowler, 
1959), but does not seem to occur in China 
or Japan. On account of its wide range it is 
surprising that it has not yet been reported from 
Australia, although it is present in New Guinea 
(Munro, 1958). Plotosus anguillaris (Bloch) 
is distributed over a vast area. The western 
498 
