Early Larvae of the Tarpon, <I>Megalops

136 BULLETINOFMARINESCIENCE,VOL.30, NO.I, 1980 BULLETINOFMARINESCIENCE,30(1): 136-141,1980 EARL Y LARVAE OF THE TARPON, MEGALOPS ATLANTICA VALENCIENNES (PISCES: ELOPIDAE), WITH NOTES ON SPAWNING IN THE GULF OF MEXICO AND THE YUCATAN CHANNEL David G. Smith ABSTRACT-A description is given of the early larval stages of the tarpon, emphasizing the pre-flexion and flexion stages, up to about 12 mm standard length. A 5.7 mm specimen retained part of the yolk sac and is the smallest tarpon larva known. At a length of about 10-11 mm flexion of the notochord occurs and dorsal and anal pterygiophores appear. Hypura]s and caudal-fin rays develop at flexion; dorsal- and anal-fin rays appear somewhat later, at about ]6 mm. Pelvic fins appear at about 2] mm. Maximum size before metamorphosis is about 28 mm SL. Spawning occurs in the spring and summer in the Yucatan Channel and the Gulf of Mexico. Spawning areas are located off Cozumel, off the west coast of Florida, and in the southwestern Gulf of Mexico. The duration of larval life is about 3 months. Although the life history of the tarpon, Mega/ops atlantica Valenciennes, is now fairly well known, some important gaps remain. Eggs are unknown and no larvae smaller than 9.4 mm have been described (Eldred, 1972). Neither spawning areas nor rates of larval growth have been determined. This report is based on a series of larvae collected in the Gulf of Mexico and the Yucatan Channel. Since it includes the smallest larvae yet known, it permits a more complete description than has previously been possible of the early larval stages. It also permits spec- ulation on spawning areas and length of larval life. MATERIAL AND METHODS Most of the present material was collected on a series of cruises conducted by the Marine Biomed- icallnstitute (MBI) to study leptocephali in the Gu]f of Mexico and the Yucatan Channel. The cruises were conducted seasonally (February, April, July-August, and November), and collections were made in the Yucatan Channel and the southern and western Gulf of Mexico (Fig. I). Supplementary collections were made in August at Cozumel and off Veracruz, Mexico. Collecting was done primarily beyond the 200-m curve with a 2-m conical-nylon plankton net. Sixty specimens of larval tarpon were collected, ranging in size from 5.8 to 22.3 mm. Eight additional specimens, 5.7 to 9.9 mm, from off the west coast of Florida were made available by Edward D. Houde of the University of Miami. They were collected in 197] and 1973 during a survey of larval fishes of that area. Measurements were made with a dial caliper. In measuring larvae with an unflexed notochord, a notochord length (NL) was used, that is from the tip of the snout to the posterior tip of the notochord. For larvae with a fully flexed notochord, standard length (SL) was used. DEVELOPMENT Eggs of the tarpon are still unknown, so neither the size of the egg nor the size of the larva at hatching can be determined. The eggs and larvae described by Breder (1944) as possibly those of the tarpon are now known to be those of the clupeid Harengula jaguana Poey (Fagetti et aI., 1975: 20). The smallest specimen in the present collection is 5.7 mm NL (Fig. 2a) and represents the smallest tarpon larva yet known. Part of the yolk sac remains anteriorly in two parts: a spherical mass under the throat and a smaller portion just behind it. The head is short and stubby in profile with a small gape. Two large teeth are present near the tip of the upper jaw on each side. The lower jaw protrudes beyond the upper; a single pair of short teeth is present at the tip. The gut has a noticeable swelling at its posterior end. The pectoral fin is present but SHORT PAPERS AND NOTE 137 Figure I. Map of Gulf of Mexico and adjacent northeastern Caribbean Sea. Closed circles represent stations where larvae of Megalops atlantica were collected. Open circles represent negative stations for July-August, 1977 MBI cruise. Dotted line represents 100 fm (183 m). unrayed. The dorsal fin-fold begins somewhere in the anterior half of the body, but the exact position is unclear. It is continuous with the caudal and anal fin- folds. The anal fin-fold begins immediately behind the anus. The notochord is unflexed, and no hypurals or dorsal- and anal-fin ray elements are present. Pig- ment is distributed extensively along the myosepta over the intestinal swelling and on the dorsal surface of the swelling itself. A few melanophores are present on the ventral margin of the myomeres anteriorly. On the head there is a mela- nophore ventrally just anterior to the yolk globule and two more on the lower Jaw. The yolk sac is lost at a length of approximately 6 mm. A 5.8 mm and a 6.1 mm specimen still had traces of yolk, but another 5.8 mm specimen and a 6.3 mm specimen had none. The notochord remains unflexed in specimens up to 10-11 mm. A 9.9 mm specimen had an unflexed notochord, but the next largest specimen, 11.7 mm, had a fully flexed notochord with about 9 hypural elements and about 19indistinct caudal rays. Wade (1962: 555) described an 11.7 mm specimen whose notochord had just begun to flex and which had five or six hypurals. He mentioned that a slightly smaller specimen (11.0 mm) was essentially similar. Hypural elements begin to form indistinctly at about 9 mm but do not become prominent until flexion occurs. Caudal rays begin to form at flexion. The dorsal and anal fins remain as an undifferentiated fin fold until flexion, when dorsal and anal pterygiophores begin to develop. Wade (1962: 555) reported eight dorsal pterygiophores in his 11.7 mm specimen, although his 11.0 mm specimen had none. Neither specimen had anal pterygiophores. My 11.7 mm specimen, which seems slightly more advanced than Wade's, had 7 dorsal and about 15 anal pterygiophores. Rays are somewhat slower to form, appearing finally at about 16-17 mm SL. 138 BULLETIN OF MARINE SCIENCE, VOL. 30. NO. I. 1980 A B c Figure 2. Larvae of Megalops atlantica: A, 5.7 mm NL, G 71 17 sta. 81; B, 6.3 mm NL MBI 24; C, 8.1 mm NL, MBI 701. Bar equals I mm. The pectoral fin is present from the beginning but remains unrayed until metamorphosis. The pelvic fins are the last to develop, appearing as tiny buds only in the more advanced specimens, about 21-22 mm SL. The head in the smallest larvae is short and blunt-snouted with two pairs of long teeth in the front of the upper jaw and one pair of smaller teeth at the tip of the lower jaw. Additional teeth appear with growth up to a maximum of six pairs in each jaw, reached at a size of about 16 mm. The teeth become smaller relative to the size of the head as the larva grows. The lower jaw protrudes well beyond the upper in the smallest larvae, but this difference becomes less marked with growth. Filaments appear on the gill arches at about 15-16 mm SL. The intestine is swollen at the posterior end, more abruptly so in smaller specimens. In larger specimens the gut turns downward just before the anus, and a slight constriction appears at that point. The air bladder appears as a dorsally directed protuberance near the midpoint of the gut; this only becomes prominent after flexion. Pigment in the smallest larvae is most conspicuous on the myosepta above the intestinal swelling. With growth the pigment here becomes restricted to the ven- SHORT PAPERS AND NOTE 139 tral half of the myomeres and eventually takes the form of short diagonal lines or spots. In the meantime pigment is developing near or slightly below the mid- lateral line on the anterior myosepta. With growth, the anterior lateral pigment becomes more conspicuous while the pigment over the intestinal swelling becomes less so. In larger specimens the lateral pigment appears as a series of short diagonal lines or spots from shortly behind the head to about the level of the anus, with no intensification over the intestinal swelling. Behind the anus pigment occurs on the lower extremities of the myosepta. Ventral pigment is in the form of short horizontally elongated spots along the ventral margin of the myomeres on each side, level with the dorsal surface of the gut. These spots occur sparsely in the youngest larvae but increase in number and conspicuousness with growth until they form a broken line from the throat nearly to the anus. Body proportions change little with growth prior to metamorphosis. Head length decreases from the preflexion to postflexion stages (14.3% NL vs. 12.8% SL for 24 specimens examined), and body depth increases linearly with growth (8-11% NL in the smallest specimens, 14-18% SL in the largest), but these are minor compared to the changes that occur during metamorphosis. The maximum size attained by the tarpon larva before metamorphosis is somewhat uncertain. The largest specimens examined here were about 22 mm SL. Wade's (1962) and Eldred's (1967) largest specimens were 21.3 mm and 22.0 mm, respectively, but both these authors recorded metamorphic larvae (Stage II of Wade) slightly larger than these (23.0 mm and 23.7 mm, respectively). Since metamorphosis is characterized by a decrease in length, the latter specimens must have been somewhat larger originally.

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