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The seston represents a highly dilute food source to potential consumers. Consequently, 3 general properties may be expected of sessile filter feeders: (1) high retention efficiency; (2) possession of a low-energy pump system in active filter feeders; and (3) consumption of seston fractions in proportion to availability (i.e. opportunistic feeding). Whereas the first 2 properties have been well documented, until recently little evidence had been adduced for the third. We assessed whether a pattern across particular studies exists that could provide evidence of opportunistic feeding. Recent studies of the seasonal variation of seston composition and of natural feeding of species representing 3 contrasting feeding modes were reviewed. For the first time, it was possible to demonstrate in the field that sessile filter feeders consume a broad spectrum of seston fractions in proportion to their availability. We conclude that such opportunistic feeding, within phylogenetically determined broad limits, is a general property of sessile filter feeders, appropriate for exploiting the ubiquitous but dilute and dynamic seston.