over the range examined (table 7). The num- 

 ber of rakers progressively decreases on the 

 second, third, fourth, and fifth arches. 



Table 7. — Gill-raker counts of young Atlantic 

 menhaden 



The fold of mucous membrane, which forms 

 a groove along the bone of the gill rakers in 

 adult Atlantic menhaden (Peck, 1894), appears 

 as a transparent ridge in larvae (fig. 8). In 

 prejuveniles the ridge becomes a distinct pig- 

 mented groove with an opening at the juncture 

 of each branch of the gill arch (fig. 10). The 

 function of this structure is not clear, but it 

 may aid channeling food organisms posteriorly. 



Pharyngeal pockets, described in clupeids by 

 Hyrtl (1855) and in Brevoortia by Monod 

 (1961), are first evident in larvae 25 mm. long. 

 These structures appeared as two slight swell- 

 ings, one on each side of the longitudinal plane, 

 above the pharynx and anterior to the fifth gill 

 arch. In prejuveniles the pockets become pig- 

 mented and kidney-shaped; their size increases 

 as the length of the fish increases. Lagler and 

 Kraatz (1945) postulated that the pharyngeal 

 organ in the gizzard shad, Dorosoma cepedi- 

 anum, is accessory to the digestive tract, rather 

 than to the respiratory system as suggested 

 by Hyrtl (1855). The presence of fresh food 

 organisms in these structures in Atlantic men- 

 haden suggests that the phai-yngeal pockets 

 are part of the digestive system and may ac- 

 cumulate the plankton prior to swallowing. 



The change from a zooplankton to a phy- 

 toplankton diet in young Atlantic menhaden is 

 thus associated with developmental changes in 

 the gill-raker complex. Peck (1894) described 



the gill rakers of adults as being ". . . so com- 

 plete as to render the whole pharyngeal cavity 

 capable of filtering large quantities of water 

 . . ." The gill rakers of larvae, however, are 

 rudimentary and have no obvious function. So 

 the incorporation of phytoplankton into the 

 diet of prejuveniles and juveniles seems to be 

 largely a function of the straining capacity of 

 the gill rakers. 



Alimentary Tract 



Like the gill arch complex, the alimentary 

 tract undergoes remarkable changes during de- 

 velopment. In larvae the tract is nearly a 

 straight tube connecting the pharynx and the 

 anus. At about one-third its length, a constric- 

 tion marks the junction of the stomach and in- 

 testine. The length of the anterior tract 

 changes relatively little as the fish grows 

 longer, but the posterior tract (intestine) 

 changes markedly (fig. 11). When linear rela- 

 tions are assumed, the regression lines in figure 

 11 show that the length of the alimentary tract 

 in larvae increases by about 0.3 mm. for each 

 1-mm. increase in length of fish; but in pre- 

 juveniles and juveniles, the length of the tract 

 increases about 10 mm. for each 1-mm. increase 

 in fish length. 



In prejuveniles the stomach folds on itself; 

 a blind sac develops posteriorly, forming the 

 tail of a Y, with the pneumatic duct leading 

 from its tip; and the lower part of the stomach 

 (gizzard) enlarges and develops a muscular 

 wall. Pyloric caeca are first evident in larvae 

 28 mm. long. Their number is variable and 

 reaches a maximum in juveniles (table 8). In 

 recently metamorphosed juveniles, the intes- 

 tine folds on itself and forms two or three 

 coils, the pyloric caeca lengthen, and fat is de- 

 posited among the caeca and coils of the in- 

 testine (fig. 12). 



The simple straight gut of Atlantic men- 

 haden larvae is characteristic of early clupeid 

 development (Harder, 1958, 1960) and gen- 

 erally associated with a carnivorous diet in 

 fishes (Harrington, 1957; Nikolsky, 1963). The 

 change in food habits during metamorphosis of 

 young menhaden is linked with changes in the 

 digestive tract. Development of the gizzardlike 

 pyloric stomach probably aids in the crushing 

 of diatoms, while development of pyloric caeca 



FOOD OF YOUNG ATLANTIC MENHADEN IN RELATION TO METAMORPHOSIS 



503 



