The swimbladders of physostomous Brevoortia tyrannus (Atlantic menhaden) and physoclistous Leiostomus xanthurus (spot) develop as a simple evagination of the larval foregut. The swimbladder of L. xanthurus develops earlier (at yolk and oil globule depletion when larvae are two days posthatch and ∼2 mm notochord length) than it does in B. tyrannus (well after yolk and oil depletion when larvae are 12 days and ∼8 mm). The larvae of each species initially inflate the swimbladder by ascending to the surface and forcing atmospheric gas through a pneumatic duct into the swimbladder lumen. Larval B. tyrannus modulate swimbladder volume and buoyancy within a diel cycle by inflation with atmospheric gas passed through a persistent pneumatic duct and deflation by diffusion through the swimbladder wall along with expulsion through the anus and mouth. This capacity of swimbladder volume modulation and buoyancy compensation corresponds with the average vertical distribution of larvae in the sea, where larval B. tyrannus occupy the upper water column. The pneumatic duct of L. xanthurus atrophies after 20 days and has no discernable lumen after 40 days, the beginning of metamorphosis when larvae are ∼7 mm standard length. The mucosal epithelium of the swimbladder of L. xanthurus acquires cytological characteristics of adult gas-gland tissue soon after initial inflation when larvae gain the capability of gas secretion. A plexiform rete mirabile does not form until metamorphosis. The swimbladder of larval L. xanthurus functions by providing neutral buoyancy with low amplitude diel depth changes in swimbladder volume. In the sea, larval L. xanthurus occupy primarily mid- and deep depths.
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Vol. 2001 • No. 2