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Morton, B. (1991) Aspects of Predation by Tonna zonatum (PROSOBRANCHIA: TONNOIDEA) Feeding on Holothurians in Hong Kong. J. Moll. Stud. (1991), 57, 11-19.
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A laboratory-held specimen of Tonna zonatum was fed holothurian prey over a period of 21 weeks. At first, the Tonna consumed Holothuria cinerascens, H. insignis and H. fuscocinerea, but subsequently consumed only H. leucospilota. Prey size chosen was in the range 54-200 g total weight with approximately 1 holothurian being eaten each week by extension of the proboscis. Consumption time was 2-5 minutes and records of rejection of living prey showed that food is not cut up in the proboscis by the jaws or the radula but is engulfed whole. Holothurians reacted violently to the hunting Tonna, typically by taking in large quantities of water to increase in size, particularly in diameter. Holothuna fuscocinerea 'fled' the predator while H. scabra and H. leucospilota became buoyant and, writhing, effected simple 'swimming'. Calculations of consumption show that this adult specimen consumed ~2% (wet weight) or ~ 1 % (dry weight) of its body weight. day"'. This is much lower than figures for other predatory gastropods, i.e. ~5%.day"'. Analysis of faeces production, correlated with consumption, suggests a residence time in the gut of 4 days and an assimilation efficiency of 80.02%. This study and literature data suggest that the Tonnidae are highly specialized, exclusive, predators of holothurians.
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