MARINE ECOLOGY PROGRESS SERIES Published December 31 Mar Ecol Prog Ser NOTE Black gut phenomenon in cardinal fishes (Apogonidae,Teleostei) 'Dept of Zoolog)!, George S. Wise Faculty of Life Sciences, Tel-Aviv University, Ramat Aviv. 69978 Israel *J.L.B. Smith Institute of Ichthyology, PB 1015, Grahamstown 6140, South Africa ABSTRACT: A study of 78 species of cardinal fishes (Apoyo- cardinal fish. Lachneratus phasmaticus, mention its nidae) revealed that 22 of them had black guts, 5 species had 'blackish alimentary canal' as an identifying character. partly black guts and, in 51 species, the digestive tube was To study the distribution of this phenomenon in car- unpigmented or had dispersed melanophores in the external tunic. The black plgrnentation is caused by melanization of dinal fishes (Apogonidae),we investigated 78 species the submucosal connective tissue which is s~tuatedbetween of this family. the musculans and the basal lamina of the Internal epithe- Methods. The material for this study included cardi- lium. This phenomenon was previously observed in moray nal fishes preserved in collections of the Department of eels and some pelag~cfish. In nocturnal predators, it appears Zoology, Tel-Aviv University, Israel, as well as in the to serve to conceal bioluminescent prey in the stomach cavity. collection of the J.L.B.Smith Institute of Ichthyology, KEY WORDS: Apogonids - Melanization of guts Grahamstown, South Africa. In addition, 18 species of live cardinal fishes were collected in the vicinity of Eilat, Gulf of Aqaba. For daytime collection we used quinaldin anesthetic, which we sprayed into the rock The occurrence of highly pigmented tissue in the crevices used as hideouts by these fishes while, at abdominal cavity is one of the specific morphological night, underwater lights and hand nets were used. The characteristics of teleost fishes. This usually occurs in collected fishes were sacrificed by an overdose of the form of melanization of the external gut tunic, the MS222, and their alimentary canals separated and extension of the somatopleural mesoderm that stains fixed for light and electron microscopy (for methods the abdominal cavity (Kent 1992). It has recently been see Fishelson 1994). In total, microsections from 20 fish demonstrated that in some species of moray eels, espe- were prepared. Macro cross-sections using a razor cially from deeper waters, dense melanization occurs blade were studied in other fishes. within the connective tissue layer (tunlca submucosa) Results. Species of the genera Apogonlchthys, Cheilo- between the muscularis and the m.ucotic epithelium dipterus, Foa, Siphamia and Sphaeramia did not pos- of the gut-wall mucosa, producing partially or entirely sess melanized guts, while species wlth melanized black alimentary canals (Bohlke 1989, Fishelson 1994). guts were found in the genera Rhabdarnia, Archamia A similar phenomenon was also discovered In Antarctic and Apogon. In the latter genus, 25 species possessed nototheniid fishes (Eastman & De Vries 1997). The black alimentary canals while 22 species had normal extent to which these black layers are developed dif- unpigmented guts (Table 1). It should be noted that in fers among the various species of moray eels (Fishelson some of the cardinal fishes, such as Archamia mosam- 1994). In some species the entire gut is black, and in biquensls and Rhabdamia cypselurus, melanophores others only the anterior parts. While studylng the bio- were found in the peritoneal lining enveloping the gut, logy of cardinal fishes in the Gulf of Aqaba, Red Sea giving it a dark, speckled appearance In Apogon (Flshelson 1970, 1977), we observed for the first time smithi and A. truncatus this peritoneal cover varied that this family of fishes, too, features species with from black to pale and was unpigmented. In the spe- melanization of the alimentary canal. Fraser & Struh- cies with black guts, the melanized connective tissue saker (1991), describing a new genus and species of of the submucosa extended from the esophagus to the end of the hindgut (Fig. lA, B), while in some species only part of the gut was melanized (Fig 1C). In Apogon leptacanthus the hindgut was not pigmented 0 Inter-Research 1997 Resale of full drbde not permitted 296 hiar Ecol Prog Ser 161. 295-298, 1997 Table 1 The studl~dcard~nal f~shes Color of guts L4 = unpigmented, B = black, PB = partly black, S = sperkled by pentonral cells Apogon ang~lstatus Apogon Ilneatus Archamia lineold ta Apogon abrogrdrnnla Apogon maculiferus Archamid nlozarn biquensls Apogon apogonldes Apogon menesen~us Cheilodipterus alleni Apogon a ureus Apogon nlultltdcniatus Cheilodipterus artus Apoyon carindtus Apogon natalensis Ch~~llodipterusintermedius Apogon roccineus Apoyon nlger Cheilodlpterus isostigni us Apogon cookii Apogon nlgnpes Cheilodlpterus lachner! Apogon crassiceps Apogon nlgriplnnis Chejlod~pterus ara bicus Apogon cyanosoma Apogon n~grofasciatus Cheilodlpterus macrodon Apogon darnleyctnsis Apogon nititdus Cheilodipterus nigrotaeniatus Apogon doderleini Apogon novaeguinae Cheilodipterus novemslriatus Apogon doryssa Apogon novemfasciatus Cheilodipterus pardzonatus Apogon erythrinrls Apogon sava yensis Cheilodipterus persicus Apogon evermannl Apogon sernlornatus Cheilodlpterus pygmajos Apogon exostigma Apogon snx th~ Cheilodipterus quinquel~neatus Apogon fasciatus Apogon stnatus Cheilod~pterus singapurens~s Apogon flagelliferus Apogon taenlatus Cheilodipterus zonatc~s Apogon fleuneu Apogon taeniophortrs Foa brach ygramma Apogon fraenatus Apogon thermalis Fowleria a bocellata Apogon guamensls Apogon timrorensis Fo wleria isostigma Apogon hungi .4pogon Irt~ncatrrs Fowleria variega ta Apogon imberbls Apogon wllsoni Rhabdamia cypselurus Apogon kallopterus Apogonichth ys ocellatus Sipharnla mossam bica Apogon kalosoma Apogonichthys perdix Sipham fa permutata Apogon la teralis Archamia dispil us Sphaeramia orbicularis Apogon leptacanthus Archamia fnca ta and neither was the caeca in Apogon striatus. In all venting the predator from becoming luminous, and black guts the melanin tissue was situated between the thereby becoming exposed to predators in its turn, muscularis within the submucosa and the mucosal until the shining prey has been digested. epithelium (Fig. 2A, R), and frequently enveloped iso- Herring (1967), and recently Eastman & De Vries lated bundles of muscles and nerves. At the base of the (1997), suggested that the black peritoneum on the mucotic epithelia1 folds of the gut, this melanized guts in some planktonic fish-larvae and oceanic fishes tissue extended between the basal membranes of the also serves the same purpose. A similar explanation juxtaposed epithelia into finger-like villi (Figs. 3 & 4). could hold true for the cardlnal fishes with black guts, Micrographs of this black tissue revealed that the despite being shallow water fish, assuming that they melanin granules were irregularly rounded in form, also prey on luminesccnt organisms. Underwater ob- 0.3 to 1.4 pm in diameter, situa.ted within melano- servations show that some species of cardinal fish are phores of 20 to 24 pm and often formed aggregates crepuscular In their foraging behavior; others are noc- (Figs. 5 & 6). Some 6 to 7 layers of such cell aggrega- turnal foragers resting in the deeper coral-reef crevices tions were generally found densely packed between during the day (Allen 1975, Vivien 1975).They include the other elements of this connective layer, as was also the species Apogon cyanosoma, A. cookii, A. angusta- observed in the moray eels (Fishelson 1994).The thick- tus, Archamia fucata and A. lineolata (Fishelson pers. ness of thls bla.ck coating varied from 160 to 240 pm in obs.), all of which possess black guts. At nlght, bio- different parts of the alimentary canal, with the thick- luminescent planktonic oryanlsms are plentiful, in- est layers in the esophagus and stomach. cluding the dense population of Noctiluca, shining This unique pigmentation of the gut (and not of the euphausiids and mysids, and the lantern fish Photoble- peritoneum) which occurred in more then 40 % of car- pharon palpebratus. On the other hand, species with dinal fish, compared with the pale gut of the other unpigmented guts, such as those of the genus studied species, raises the same questions posed by Cheilodipter-us and some other cardinal fish, are cre- Fishelson (1994) for moray eels: Why does such black- puscular or diurnal, and were observed feeding during ening of the submucosa occur only in some species of the late afternoon (Vivien 1975, Fishelson pers. obs.), both morays and apogonidsT Why does the same genus but disappeared at night. Does this behavior mark the feature species both with and without melanized guts? division between the species with black or white ali- In morays the black pigmentation is typical of deep-sea mentary canals? The next stage of th.e study should forms, and it has been postulated that these fish may examine the diets of the various cardinal fishes In order specialize on bioluminescent prey, and that the black to determine the reason for this type of melanization of wall of the alimentary canal thus provides a shield pre- the alimentary canal in selected species. Fishelson et al.: Black guts in cardinal fishes 297 Figs. 1 to 6. Black gut phenomena in cardinal fishes. Fiq.P. 1. Guts of apogonids. (A) Apoyon doderlejni; (B) A. taeniophorus; (C)A, novaeguinae (stomach partly melan~zed)(x4). X2 Macro cross-section of stomachs: (A) Archamla dispilus; (B) Apogen everrnanni (x8) Q.3. Apogon cyanosorna The