Superfast muscles power high-frequency motions such as sound production and visual tracking. As a class, these muscles also generate low forces. Using the toadfish swimbladder muscle, the fastest known vertebrate muscle, we examined the crossbridge kinetic rates responsible for high contraction rates and how these might affect force generation. Swimbladder fibers have evolved a 10-fold faster crossbridge detachment rate than fast-twitch locomotory fibers, but surprisingly the crossbridge attachment rate has remained unchanged. These kinetics result in very few crossbridges being attached during contraction of superfast fibers (only Ϸ1͞6 of that in locomotory fibers) and thus low force. This imbalance between attachment and detachment rates is likely to be a general mechanism that imposes a tradeoff of force for speed in all superfast fibers.The superfast fiber type is found where high-frequency contractions are required, such as in vertebrate eye muscles and in both vertebrate and invertebrate synchronous soundproducing muscles. These muscles have a series of modifications for speed, including a large volume of sarcoplasmic reticulum (SR) (1-7) to produce very rapid calcium transients (8) and low-affinity troponin to speed myofilament deactivation after [Ca 2ϩ

SynopsisHydrophone recordings were made to document the daily and seasonal cycles of weakfish, Cynoscion regalis (Sciaenidae), sound production in the Delaware Bay . Recordings were made at three stations perpendicular to the shore and rated qualitatively on a scale of 0-4 . `Drumming' (sonic muscles, males only) and `chattering' (pharyngeal stridulation, both sexes) were tallied separately . Weakfish were collected from the field throughout the spring and summer to monitor the reproductive status of the population . A clear seasonal trend was noted in both drumming and chattering activities . Drumming activity rose abruptly in mid-May to maximal levels, which were maintained through late June, then dropped off in July and ceased in August . Chattering activity began abruptly in late June, then rose through July and remained high in August . Seasonal drumming activity was more intense at the inshore station . On a daily scale, drumming activity was at its lowest level during the early to mid-morning, then increased to maximal levels in the early evening and remained high until the following morning . There was no daily trend in chattering activity . Daily drumming activity was also most intense at the inshore station . Male and female reproductive readiness, determined by gonadosomatic indices, sperm motility and plasma androgen levels, peaked within the period of maximal seasonal drumming activity .

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