Mucous Envelope Formation in Two Species of Hawaiian Parrotfishes (Genus

Some parrotfishes have developed a unique capacity to form a mucous envelope at night. Scams dubius and S. perspicillams are two Hawaiian species that exhibit envelope-building behavior. Laboratory experiments indicate that envelope formation is promoted by darkness, and is inhibited by constant light. The completed envelope is a transparent, mucous cocoon surrounding the fish. A mass of glandular tissue was found in the buccal cavity of S. dabius and S. perspicillatttS. It is suggested that this tissue is the envelope-producing gland. THE PARROTFISHES are common inhabitants of tropical coral reefs. Several species inhabiting Bermudan reefs have been noted for their ability to form a large mucous envelope at night (Wi nn, 1955; Winn and Bardach, 1959,1960) . The structure was described as a "thin transparent and gelatinous mucoid substance which starts as a fold at the mouth and progresses backwards in folds to surround the body" (Winn, 1955). It was suggested that the envelope funct ions as a protective device which serves to reduce pred ation by nocturnal organisms (Winn and Bardach, 1959). This paper describes envelope formation in two species of Hawaiian parrotfishes, Scams dnbius and S. perspicillatus. The study concerns envelope formation under light and dark conditions, and provides a description of the location and general morphology of the mucus-producing gland. MATERIALS AND METHODS Laboratory specimens were trapped or netted on the coral reefs in Kaneohe Bay, Oahu . The fish were maintained in wire cages that were suspended in the saltwater lagoon next to the laboratory. Only vigorous and apparently healthy individuals were used for the experiments. 1 Contribut ion N o. 353 of the Hawaii Institute of Marine Biology, Un iversity of Hawaii. Manuscript received Apr il 9, 1970. 2 D epartment of Zoology, University of Hawaii, Honolul u, Hawaii. Present address: De partment of Zoology, Uni versity of Californ ia, Berkeley, Californi a 94720 The first laboratory tests examined the effects of light and dark on mucous-envelope formation. Six lO-gallon aquaria were equipped with sub-sand filters and prepared with a 4to 5-cm substrate of coarse sand. Three of these aquaria were maintained in constant artificial light, with two 25-watt incandescent lamps positioned 7 cm above the water surface. These aquaria were isolated on all sides by cardboard shields, and observations were made through a small, movable viewing port. Three similar aquaria were provided with a light-tight covering of black cardboard. These aquaria had the viewing ports covered by red plastic filters to prevent any interference with envelope build ing by the external lighting. A one-cell penlight was used to facilitate visual identification of the mucous envelope. One to four individuals were introduced to each aquarium for trials that had a maximum duration of 24 hours. All observations occurred at 30-minute intervals, and the presence or absence of a mucous envelope was recorded. In a second experiment, the behavior of S. dubius was recorded before, during, and after envelope formation. One fish was placed in each 10-gallon aquarium and the room lights were turned off. Observations were made at 15minute intervals with the aid of a one-cell penlight. The location, posture, and progress in envelope building were recorded for each fish. The lights were turned on after envelope formation was complete, and the behavior and external respiratory rate of all specimens were recorded. Insoluble carmine would adhere to the outer