and the larvae produced grow into filtering barrels with budding stolons that generate
new colonies. Both salps and pyrosomes, at least some of them sometimes, execute
extensive diel vertical migrations.
Appendicularia
(^) Appendicularia (Plate 6.13) resemble the larvae of the benthic sea squirts (tunicates),
and thus have also been called Larvacea. The body proper is a small pharynx below a
bulb of digestive gland and gonad. This basic layout resembles a sea squirt adult. The
resemblance to tunicate larvae is the long tail with its central notochord and adjacent
muscles, which in appendicularia persists through adulthood. From a patch of
secretory cells, the oikoplast, located near the mouth, the animal forms an elaborate
“house” of mucus (Plate 6.14), typically the size of a walnut but in a few species
much larger, even 1 m in diameter, which acts as a feeding filter. Water is pumped
through the house by oscillations of the flat, muscular tail. The house is an elaborate
apparatus with prefilters, filters, channels for excurrent flows, and more, the function
of which is described in Chapter 7. Appendicularian houses, both occupied and
abandoned, are at times abundant in the ocean. Their mucous surfaces are important
accumulators of particulate matter, providing a habitat for small crustaceans, most
prominently the copepod genus Oncea, that crawl over them and harvest delectable
bits. The houses make up part of the flocculent detrital matter called “submarine
snow”. The most frequently encountered appendicularian genus is Oikopleura. Like
the other planktonic urochordates, they are sequential hermaphrodites.
Vertebrata
(^) This phylum includes the fish. Fish larvae, or ichthyoplankton, are meroplankton.
Virtually all fish larvae look like little fish; most are a few millimeters long or a little
larger. Some hatch before fully absorbing their yolk sac, which remains as a large
ventral bulge. Such larvae do not need to feed immediately, but survival chances are
often better if feeding can begin before the yolk is fully absorbed. Other species are
more precocious and hatch ready to swim and hunt. Most fish larvae have relatively
large eyes. There are few adult fish that could be classed as plankton, except perhaps
the deep-sea family Gonostomatidae (“bristle-mouths”).
Chaetognatha
(^) This is an exclusively marine, epibenthic (the Spadellidae) and planktonic phylum
commonly called arrow worms (Fig. 6.7).
Fig. 6.7 Chaetognatha: several genera and several body-form variants. The 10 mm
scale bar applies to Pseudosagitta lyra, the 3 mm bar to all the others