Pacific Plate Biogeography, with Special Reference to Shorefishes

NUMBER 367 29
stricted to the Indo-West Pacific; one (2.23 m) in
the eastern Atlantic and Indo-West Pacific; three
(1.0-1.5 m) only in the western Pacific; one (1.72
m) only in the western Indian Ocean; and one
(1.34 m) in the western Atlantic, eastern Pacific,
and western Pacific.
From the information presented above, it is
apparent that size alone is insufficient to account
Bombay, India, east and south to Java and north
to Tinghai, Chekiang Province, China, and Japan
(and perhaps to the Bonin Islands; see Zama
and Fujita, 1977, as S. wahlbeemi). It is also recorded
from Sarawak, but not the Philippines.
Loxodon is also monotypic, but with a wider Indo-
West Pacific distribution (Figure 13) than Scolio-
don. Loxodon is the only one of the three genera
for the presence or absence of Carcharhinus species that occurs on islands (Seychelles, Mascarene Pla-
on the Pacific Plate (although all four of the teau, including, perhaps, Mauritius) remote from
worldwide species are large). The large propor- continents. Springer (1964) reported extreme and
tion of the species that reach the margin of, but discontinuous variation in numbers of precaudal
do not extend out on, the Plate, agrees with my vertebrae in different populations of Loxodon, in-
findings on other elasmobranch groups, which, in dicating that a species complex might be in-
general, tend to be absent from the Pacific Plate. volved. The Seychelles and Mascarene Plateau
To avoid the possible criticism that there are are continental fragments (Scrutton, 1976), and
probably inadequate data available for large it is possible that the populations of Loxodon were
sharks on the Pacific Plate, it is noted that Stras- left behind to diverge (vertebrally) when the conburg
(1958) reported on longline collections that tinent, or continents (India, Africa), that were
covered much of the central Pacific during a four- associated with the fragments disappeared or
year period and produced over 6000 specimens of moved away.
sharks, but only 12 species, of which only three Rhizoprionodon, with seven species (three Indowere
Carcharhinus.
West Pacific, of which one also occurs in the
Triaenodon contains two species. Triaenodon obtu- eastern Atlantic; one eastern Pacific; and three
sus is known only from the holotype from India western Atlantic) is more widespread (Figure 13)
(Taniuchi, 1975). Triaenodon obesus, which attainsthan
Loxodon, occurring more or less circumtrop-
a length of 2.13 m, is found only near coral reefs, ically, but absent from the Pacific Plate.
often resting in caves, yet it ranges from the Red It is not hard to visualize the distributions of
Sea and east coast of Africa eastward to Panama Loxodon and Scoliodon as representative of levels of
(Randall, 1977). Randall (1977) tagged 124 T. increasing restriction in distribution from what
obesus at Johnston Island and recovered seven were previously widespread distributions similar
after 16 days to two years; none had moved more to that found for Rhizoprionodon; especially as all
than 2.9 km from their tagging site (tagging three genera occur together over much of their
information in the abstract to Randall's paper Indo-West Pacific distribution. Bearing on this
expands on tagging data reported in the body of are the distributions of the two subgenera of
the paper). One may wonder, as did Randall, Rhizoprionodon. The type subgenus has the same
how such a sluggish, reef-associated shark was overall distribution as the genus, whereas the
able to cross the Pacific. A possible answer to this other subgenus, Protozygaena, has a disjunct distri-
question is discussed in the section on the geologbution, occurring from India to Queensland in
ical history of the Pacific and Philippine plates. the western Pacific, and from Panama to southern
Scoliodon, Loxodon, and Rhizoprionodon comprise Brazil in the western Atlantic. If the distribution
small, shallow-dwelling sharks with maximum of the subgenus Protozygaena is segregated, the
total lengths of about 350-1000 mm. The species order of increasing distributional restriction is as
are of commercial importance in Asia. The genera follows: Rhizoprionodon, Protozygaena, Loxodon, Sco-
were revised by Springer (1964). Scoliodon is monliodon.otypic and its distribution is coastal, from about Galeocerdo (tiger shark), monotypic, is a large