Distribution of the Plotosidae — Lanzing 
the Plotosidae a predominantly freshwater 
family rather than a chiefly marine family, as 
is often implied in the literature (Berg, 1957; 
Darlington, 1957; Nikolsky, 1961; Sterba, 
1963). 
It appears that 25 species are found in the 
Australian region. Three marine species occupy 
a much larger area covering most of the Indian 
and West Pacific oceans. Sterba’s map (1963) 
of the distribution of the Plotosidae therefore 
actually shows the range of one species, namely 
Plotosus anguillaris. It is of interest that among 
the non-plotosid siluroids only T achy sums th al- 
ias inus (Rueppel) has a range as wide as that 
of Plotosus canius , except that the former is 
also reported from Japan (Matsubara, 1955). 
As yet no physiological work has been car- 
ried out with regard to the function of the 
dendritic organ. The available evidence indi- 
cates that: (a) its structure resembles that of 
salt-secreting glands in other vertebrates, (b) 
it is present in both juvenile and adult ploto- 
sids, (c) there exist no sexual differences, and 
(d) it is not present in freshwater plotosids 
other than Oloplotosus mariae and Plotosus 
pap Mens is. Hardenberg and Delsman (1934) 
suggested that the dendritic organ is involved 
in reproduction, but produced no evidence in 
support of this claim. A possible respiratory 
function merits consideration since other cat- 
fish (Clariidae, Heteropneustidae) possess ac- 
cessory respiratory organs. These, however, are 
in connection with the branchial chambers and 
are structurally different from the dendritic 
organ. Furthermore, there seems to be no rea- 
son why only marine plotosids should require 
an accessory respiratory organ. Both marine 
and freshwater plotosids enter muddy environ- 
ments that could contain oxygen-deficient water. 
A salt-excretory function seems to be the most 
likely, although, admittedly, the presence of a 
dendritic organ in Oloplotosus mariae and 
Plotosus papuensis does not fit in with this 
theory. Too little is known about these two 
species to venture any explanation. 
According to Darlington (1957:46) marine 
plotosids may have invaded the Australian 
region and, after entering a freshwater habitat, 
reached the end of a complicated line of teleost 
evolution. 
Although evolutionary aspects of zoogeog- 
501 
raphy must remain speculative, it is suggested 
that the dendritic organ was developed while 
plotosid ancestors in the Southeast Asian region 
invaded the sea. This invasion would be differ- 
ent from that of the Tachysuridae, which were 
able to cope with osmotic stresses by means of 
mechanisms similar to those used by other old 
teleost families, such as the Salmonidae. Per- 
haps because of tachysurid competition, the 
plotosid ancestors became firmly settled only in 
the Australian region. From them would have 
evolved, on the one hand, the freshwater spe- 
cies which lost the dendritic organ in the 
process. Some of the marine species, on the 
other hand, managed to disperse radially along 
the edges of the Indian and West Pacific 
ocean basins. 
I am grateful to Mr. I. S. R. Munro (C.S.I. 
R.O. Marine Laboratory, Cronulla) for his ad- 
vice and for putting at my disposal the manu- 
script for his forthcoming book on the fishes of 
New Guinea. 
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Berg, L. S. 1957. Classification of Fishes Both 
Recent and Fossil. Edward Bros. Ann Arbor, 
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Bloch, M. E. 1794. Naturgeschichte der Aus- 
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Brock, J. 1887. tJber Anhangsgebilde des 
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Cuvier, G., and A. Valenciennes. 1840. His- 
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Darlington, P. J. 1957. Zoogeography. The 
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Ekman, S. 1953. Zoogeography of the Sea. 
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Fowler, H. W. 1934. The fishes of Oceania, 
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Honolulu. 
1938^. The Fishes of the George Van- 
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