THE ANATOMY OF THE FEEDING MECHANISM AND THE FOOD HABITS OF MICROSPATHODON CHRYSURUS (PISCES: POMACENTRIDAE)J ALEJANDRO CIARDELLF Institute of Marine Sciences, University of Miami ABSTRACT The morphology and action of the bones, ligaments and muscles involved in feeding is described. Tooth replacement is explained. The gastrointes- tinal tract is described. An analysis of the food habits is presented. The ecology of the juveniles and adults is discussed in terms of their feeding habits. INTRODUCTION The yellowtail demoiselle, Microspathodon chrysurus, is an abundant fish in the coral reef environment of southeastern Florida and the Florida Keys. It is widespread through the tropical western Atlantic, extending as far south as Brazil. Reports of its occurrence in Afro-Atlantic waters need confirmation. Microspathodon chrysurus is reputedly an algal browser (Longley, in Longley & Hildebrand, 1941: 186; Jordan & Evermann, 1898: 1565). This is true for the adults, but the juveniles are mainly carnivorous. The juveniles also differ sufficiently from the adults in body color that they were thought by Poey (1875: 102) to represent a distinct species which he named Pomacentrus niveatus. The adults are uniformly dark brown, generally with a yellow caudal fin. The conspicuous juveniles are violet black, with sky-blue spots located mainly dorsally on the body; their caudal fin is transparent. Observations by Walter A. Stark, 11, and Alan Roy Emery (personal communication) indicate that the young remove parasites from other fishes, at least occasionally. Its contribution to the biomass of shallow coral reefs and its seemingly specialized feeding make Microspathodon chrysurus an important subject for study. The anatomy of the feeding mechanism is described in detail, with comments on function, and the diet and ecology of the species are discussed. This study stems from a research program on food habits and feeding mechanisms of coral-reef fishes supported by the National Science Foundation (NSF-GB-1450), C. Richard Robins, principal investigator, and constitutes a technical report from the program. I am especially indebted to Dr. C. Richard Robins, for his advice and encouragement throughout the study and for his editing of the manuscript. I am also indebted to Dr. Frederick M. Bayer and to William N. Eschmeyer, for 1 Contribution No. 844 from the Institute of Marine Sciences, University of Miami. This paper is pan of a thesis which was submitted in partial fulfillment of the degree of Master of Science, Uni- versity of Miami. Cornl Gables, Florida. o Present address: Depto. de Investigaciones Jctiol6gicas y faunisticas, CVM, and Universidad de Carlagena, Cartagena, Colombia. 846 Bulletin of Marine Science [17(4) aid in the preparation of the illustrations and for the review of the manuscript. I thank Drs. Leonard J. Greenfield and C. P. Idyll for sug- gestions concerning the manuscript. I wish also to thank Dr. James C. Tyler of the Academy of Natural Sciences of Philadelphia, for the loan of West Indian specimens. In this study, I have received advice at various times, and help in collecting, from my fellow graduate students. I wish especially to thank Ray S. Birdsong, William P. Davis, and Henry A. Feddern. Special thanks are due to Gregorio Reyes-Vasquez and Charles D'Asaro for the identification of the gastrointestinal contents, and to Dr. Charles E. Lane for confirming the identification of the nematocysts. Dr. Reinaldo Pfaff advised me to come to the University of Miami to study Marine Science, and he has encouraged me throughout the study. I wish to thank Drs. German Covo and Luis H. Arraut of the University of Cartagena for their support, and my family for their encouragement. Through my first three years (1962-65) at the University of Miami, I was supported by the University of Miami which furnished my tuition, and by the University of Cartagena (Colombia) which provided living expenses and books. During the last year (1965-66), I was supported by scholarship No. 8284 from the Pan American Union. Finally, I would like to thank the Person-to-Person Committee of Coral Gables, Florida (Cartagena's sister city) which made it possible for me to study at the University of Miami. MATERIAL The specimens of Microspathodon chrysurus used in this study are all from the inshore waters of southern Florida and the Bahama Islands. All preserved material is in the collection of the Institute of Marine Sciences, University of Miami (UMML). Additional material from various regions of the western Atlantic was referred to, but was of no direct interest in this study and is not listed below. Preserved material.- The number of specimens and their range in standard length are given in parentheses. UMML 2639 (1, 38 mm) Alligator Reef, Monroe County, Florida; UMML 2933 (4, 37-49 mm) Alligator Reef, Monroe County, Florida; UMML 6435 (7, 19-52 mm) Alligator Reef, Monroe County, Florida; UMML 7234 (2, 34-45 mm) Ajax Reef, Dade County, Florida; UMML 1770 (8, 26-50 mm) S. side of Pigeon Cay, Bahama Islands. Cleared and stained material.-UMML 8877 (2 adults) Alligator Reef, Monroe County, Florida. Material for dissection.-Material for dissection preserved in Vin d'Alsace was from field station CRR-F-374, at Ajax Reef, Dade County, Florida. 1967] Ciardelli: Feeding Mechanisms of Microspathodon chrysurus 847 Some of this material was destroyed in the course of this study, but the remainder is catalogued as UMML 19841. Non-preserved material.-Freshly-collected, non-preserved material from the large populations of Microspathodon chrysurus at Ajax, Triumph, and Alligator reefs in Dade and Monroe counties, Florida, was used to supple- ment the preserved material, particularly in connection with muscle attach- ments, bone motion, and analysis of stomach contents. METHODS The illustrations of bones are based largely on the head bones of two large adults, more than 90 mm in standard length, collected at Alligator Reef. The bones of these adults were compared with those of two smaller specimens less than 50 mm in standard length to find morphological differ- ences, but none of importance was found. The material was photographed and the negatives projected onto glass and the image copied. All the bones illustrated are from the left side of the head. A 2-mm scale in each illustration indicates size. The illustrations of the teeth were made by the use of a dissecting microscope with camera lucida. Illustrations of the muscles were made by the use of a dissecting microscope with camera lucida, and by using a Goodkin enlarger. Illustra- tions of the gastrointestinal tract and of the mouth are based on photo- graphs, from which outlines were traced. The stomach and the intestine were removed to facilitate the study of their contents, by means of both dissecting and compound microscope. All measurements were taken with calipers. Measurements of standard length were recorded to the nearest millimeter. FOOD ANALYSIS Analysis of gastrointestinal contents shows a change in the quality of food ingested by juveniles and adults. Such a change of diet is common in fishes. Hunt & Carbine (1950: 71) recorded as many as three changes in the diet of young pikes. Vasisht (1959) showed a change from carnivo- rous to omnivorous diet for some commercial fishes of the Punjab region, and Robins & Deubler (1955: 14) showed changes in food with age and season for the burbot, Lota Iota lacustris. Gregorio Reyes-Vasquez and Charles N. D'Asaro identified the food items from the stomach and intestines of Microspathodon chrysurus. These contents are analyzed below. I. Diatoms: a. Planktonic: Amphiprora, Asterionel/a, Coscinodiscus, Cymbel/a, Diploneis, Eunotia, Leptocylindrus, Melosira, Navicula, Nitzschia, Pleurosigma, Rhabdonema, Skeletonema, Synedra, Triceratium, Thalassiotrix, Tropidoneis. 848 Bulletin of Marine Science [17(4) TABLE 1 FREQUENCY OF OCCURRENCE OF FOOD ITEMS IN GASTROINTESTINAL CONTENTS OF Microspathodon chrysurus BY SIZE CLASS1 Standard length (mm) 40 40-80 80 Sample size (Nos. of fish) 15 14 13 Diatoms a. Pelagic 0(0) 7 (50) 10(77) b. Epontic 0(0) 6 (43) 9 (69) Chlorophyceae 4 (27) 1 (7) 4 (30) Cyanophyceae 2 (13) 3 (21) 9 (69) Rhodophyceae 2 (13) 3 (21) 5 (38) Dinoflagellates 1 (7) 5 (36) 4 (30) Zooxanthellae 6 (40) 7 (50) 0(0) Nematocysts 10 (67) 8 (57) 0(0) Unidentified animal food 2 (13) 2 (14) 8 (61) Sand, spicules, etc. 2 (13) 3 (21) 5 (38) 1 Numbers indicate number of individuals in which the food item was found (per cent frequency of occurrence is given parenthetically). Although the Zooxanthellae are dinoflagellates, they were sep- arated as a food item because of their symbiotic relations with corals. b. Epontic: Amphora, Climaeosphenia, Grammatophora, lsthmia, Lie- mophora, Mastogloia, Striatella, Surirella, Tropidoneis. The term epontic (epi = on, ontos = being) is used here as defined by Wood (1965: 4). 2. Chlorophytes: unicellular and filamentous. 3. Cyanophytes: filamentous. 4. Rhodophytes: filamentous. 5. Dinoflagellates: Great numbers of Zooxanthellae were found in the juveniles. 6. Animal food: mainly copepods and some red gelatinous matter in the adults, and nematocysts in the juveniles. 7. Sand, Foraminifera, sponge spicules, teeth of Sagitta. As is shown by Table 1, the main materials found in the gut of juveniles are nematocysts and Zooxanthellae. This leads the author to believe that their principal food is the hydrozoan Millepora, though anthozoans may be ingested (see section on ecology, below). The adults eat large amounts of blue-green algae (cyanophytes), showing some selectivity in the choice of their food. However, if there is any other kind of food available in large amounts they will ingest it. This is shown by the fact that some specimens contained large quantities of planktonic diatoms of the genus Asterionella, which they probably ingested during a planktonic bloom of this diatom. Two specimens were found with their stomachs full of a gelatinous matter of animal origin.