Aquarium 6: Batfish

P.orbicularis adult

 Among the reef fish in the big tank are a small shoal of juvenile Orbiculate Batfish, Platax orbicularis. Superficially similar to the well known freshwater aquarium angelfish (which was originally described as a Platax), they belong to a group of mostly monochrome, large tropical marine fish commonly referred to as spadefishes. As juveniles they have very tall dorsal and anal fins, which combined with a circular, compressed body gives a triangular body shape. As they mature their fins become proportionally smaller, with a more rectangular body shape. They are all fairly large fish, with some species growing to over 70cm

Orbiculate Batfish can grow to 50cm, although aquarium specimens are usually less. Given the size tank they are in, the ones at Bristol should reach typical size for the species. Batfish are unusually tolerant of changes in salinity compared to many marine fish, and are often found in mangroves. In fact, the brown juveniles of many species strongly resemble dead leaves drifting in the water, and even move in a similar fashion, which is presumably camouflage. Other species may mimic toxic flatworms or crinoids. Adults usually spend more time in open water and greater depth, especially around shipwrecks. Orbiculate Batfish range from the Red Sea across to northern Australian waters. Other species have the distribution centred in the western Pacific.

P.orbicularis juvenile

Some marine fish are dietary specialists but Platax species are more omnivorous, with a strong preference for various algae. They may be quite important as grazers on reefs and in lagoons, preventing overgrowth of corals. Given their shape and comparatively slow movement, most of their food is obtained from the substrate rather than pursuing it in open water.

P.pinnatus juvenile

Batfish spawn in the open water, but there does not appear to be much information available on the actual spawning behaviours. Each female can produce around 50,000 eggs per kilo of body weight at each spawning, with the individual eggs being just over 1mm across and floating in the plankton. Larvae hatch after 24 hours at around 4mm long, and feed on microscopic plankton until they metamorphose and settle down in a mangrove or lagoon.

As with many large and quick growing marine fish they are important food fish, and on a small scale they are farmed in Taiwan, Thailand and experimentally on Tahiti. On a sufficiently nutritious diet they grow quickly to 1kg body weigh in 12 months, and can reach breeding size within 2 years. One drawback is that they are sensitive to disease in captivity

Next time, another unusually shaped fish, the Shotsilk goby.

Images from wikipedia