Comparison of the Development and Function of the Swimbladder of Brevoortia tyrannus (Clupeidae) and Leiostomus xanthurus (Sciaenidae)

The swimbladders of physostomous Brevoorlia tyrannus (Atlantic menhaden) an d physoclistous Leiostomus xanthurus (spot) develop as a simple evagination of the larval foregut, The swimbladder of L, xanthurus develops earlier (a t yolk and oil globule depletion when larvae are two days posthatch and sim ilar to2 mm notochord length) than it does in B. tyrannus (well after yolk and oil depletion when larvae are 12 days and similar to8 mm), The Larvae o f each species initially inflate the swimbladder by ascending to the surfac e 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 alon g with expulsion through the anus and mouth. This capacity of swimbladder v olume modulation and buoyancy compensation corresponds with the average ver tical distribution of larvae in the sea, where larval B. tyrannus occupy th e upper water column. The pneumatic duct of L. xanthurus atrophies after 20 days and has no discernable lumen after 40 days, the beginning of metamorp hosis when larvae are similar to7 mm standard length, The mucosal epitheliu m of the swimbladder of L, xanthurus acquires cytological characteristics o f 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 providin g neutral buoyancy with low amplitude diel depth changes in swimbladder vol ume. In the sea, larval L, xanthurus occupy primarily mid- and deep depths.