organisms smaller than the mesh can be attracted to it, then stick.
Box 7.1 Oceans are a trophically dilute medium
(^) Relatively rich water has chlorophyll concentrations from 1 to 20 μg liter−1. Take, for example, 2.0
μg Chl liter−1, a typical coastal or oceanic spring-bloom level. At carbon : chlorophyll = 60–200 (use
100), carbon : dry weight = 0.4, and dry weight : wet weight = 0.3, we have:
(^) = 1667 μg wet weight of food liter−1 = 1.7 mg/1025 g, which is <2 parts per million in coastal water
or oceanic blooms. In the vast oligotrophic ocean stretches, chlorophyll is less than 0.3 μg Chl liter−1
and food <0.3 ppm. These quantities of “food” are two-thirds seawater (at 1025 g liter−1).
Fig. 7.1 Retention efficiency of the mucous filtering screens of two salp species as a
function of particle size.
(^) (Data from Harbison & McAlister 1979.)
Appendicularians also feed by pumping water through a mucous mesh filter (Plate
6.14) that is part of their house structure, but the mechanics are subtler than in salps.
Observations of transparent mucous structures and the movement of tiny particles
within them are extremely difficult, but Flood (1991) provided a convincing
description of the feeding process in Oikopleura labradoriensis. The house itself has
walls about 1 mm thick, a rather random meshwork of hydrated mucopolysaccharide
fibrils carrying at some points clusters of bioluminescent granules. Entering water
passes through outer screens in which the mucous strands are arranged in a
rectangular grid that keeps particles larger than ∼13 μm out of the house. Funnel
passages carry the flow from the outer screens toward a chamber containing the trunk,
and from which the tail chamber extends to the back of the house, just wider than the
tail and just taller than the wave-like tail oscillations. It narrows at the back to an exit
spout. Passages along the house sides and extending upward carry water from the
back of the house toward entrance slots in an “organ-pipe” filtering apparatus. This is