Rhinecanthus

Abstract: Double cones (DCs) are the most common cone types in fish, reptiles and birds. It has been suggested that DCs are used for achromatic tasks such as luminance, motion and polarization vision. Here we show that a reef fish Rhinecanthus aculeatus can discriminate colours on the basis of the difference between the signals of individual members of DCs. This is the first direct evidence that individual members of DCs are used in colour vision as independent spectral channels.

Abstract: Paternal care is predominant among telcost fishes with external fertilization. This study describes maternal care in a haremic coral-reef fish and discusses the possible factors leading to its evolution. Both sexes of the triggerfish Rhinecanthus aculeatus (Balistidae) maintained territories; some individuals for more than 8 years. Each male's territory overlapped 2-3 female territories. Pair-spawning occurred around sunrise. Only females cared for the demersal eggs until hatching, which occurred just after sunset on the day of the spawning. No predation was observed on eggs under the maternal care, but experimental removal of parental females decreased the hatching rate to nearly zero. Egg-guarding females foraged as frequently as males, but less than half of non-spawning days. Spawning occurred only in the periods of about 1 wk around the new or full moon, and individual females spawned up to three times in each period. Thus, the maternal care did not significantly affect the duration of the females' spawning intervals, while males would suffer mate loss if they performed parental care. In this situation, maternal care should be the evolutionarily stable strategy. Evolutionary transition from no care to maternal care and then to biparental care is suggested in the Balistidae.

Abstract: Suspected viral erythrocytic necrosis (VEN) was detected in blood films from an immature blackbar triggerfish, Rhinecanthus aculeatus, captured on a patch reef at Lizard Island, Great Barrier Reef in November 2005, probably the first record of such an infection from Australia. The fish was kept in captivity and sampled intermittently until December 2007. Giemsa-stained blood films showed initially ~18% of mature erythrocytes affected by the VEN-like condition, but accompanying erythroblasts appeared free from infection. Erythrocytes with VEN-like bodies were smooth or crenated in outline, while the inclusion bodies were intracytoplasmic, single or paired, round in outline, stained deep magenta and were between 0.5–1.7 μm across. Bodies associated with clouds of granular material, fine eosinophilic haloes, and distinct pink-stained comet-tails, were also observed. The DNA content of the VEN-like bodies was confirmed by their green fluorescence following acridine orange staining. Infection levels in this fish fell to 0.4% of mature erythrocytes by May 2006 and persisted at this level until December 2007, when the fish died after just over two years in captivity. Squashes of haematophagous, juvenile gnathiid isopods taken from this fish on initial capture, also contained eosinophilic VEN-like bodies within digesting erythrocytes, suggesting that these crustaceans may be vectors of the condition observed.

Abstract: The ability to produce sound has been known for decades in Balistidae. Sounds of many species have been recorded and a variety of sound-producing mechanisms have been proposed, including teeth stridulation, collision of the buccal teeth and movements of the fins. The best-supported hypothesis involves movements of the pectoral fin against the lateral part of the swimbladder, called a drumming membrane. In this study, we describe for the first time the sounds made by the blackbar triggerfish Rhinecanthus aculeatus, which are like short drum rolls with an average duration of 85 ms, 193 Hz dominant frequency and 136 dB SPL level at 3 cm distance. The sounds are a series of pulses that result from alternate sweeping movements of the right and left pectoral fins, which push a system of three scutes that are forced against the swimbladder wall. Pulses from each fin occur in consecutive pairs. High-speed videos indicate that each pulse consists of two cycles. The first part of each cycle corresponds to the inward buckling of the scutes, whereas the second part of the cycle corresponds to an apparent passive recoil of the scutes and swimbladder wall. This novel sound production mechanism is probably found in many members of Balistidae because these peculiar scutes occur in other species in the family. Comparison of sound characteristics from fishes of different sizes shows that dominant frequency decreases with size in juveniles but not in adults.