number of genera such as Lohmanniella, Strombidium, and Laboea. Some
choreotrichs are facultative “mixotrophs”, using chloroplasts from ingested
phytoplankton to provide them with photosynthate. The complexity this imposes on
pelagic food webs will be considered in Chapter 9. Not all ciliates are naked; the
tintinnids found more abundantly in coastal regions have leathery, cone-shaped tests
that are often covered with cemented mineral particles. Naked flagellates are also
abundant but delicate, which together with their small size (many <10 μm) delayed
recognition of their importance, particularly as bacterivores. They are a complex
assemblage, most of which are “heterokont”, that is, having two flagellae of different
types. Flagellate size and form are highly varied (Fig. 6.2). All of these
“microheterotrophs”, together with mixotrophic phyto-flagellates, are the grazing
component in the microbial food web. It is typical for microheterotrophs to be the
preferred foods of mesozooplankton such as copepods (Gifford 1993), which were
long considered to be the principal pelagic herbivores.
Fig. 6.2 A range of cell types among heterotrophic flagellates, grouped as small,
medium, and large. Those with attachment filaments are indicated by a little
“holdfast” at the bottom. Those are commonly seen on mesozooplankton surfaces,
particularly crustacean exoskeletons. All scale bars are 10 μm.
(^) (After Sherr & Sherr 2000; sketches by Naja Vors.)