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LOWER CHORDATES Larvaceans, Sea Squirts, Salps, Doliolids, and Lancelets Phylum Urochordata contains the benthic sea squirts (or ascidians), most of which have planktonic “tadpole” larvae. The phylum also contains some holoplanktonic groups (larvaceans , salps, and doliolids), all highly modified for planktonic life and bearing little resemblance to the more familiar sea squirts. The planktonic urochordates are filter feeders that pump water through a mucous sieve to catch small, suspended particles. They can form an important link in the food web by transferring energy contained in bacteria and nanoplankton to higher trophic levels. LARVACEANS (APPENDICULARIA) Larvaceans are one of the two types of holoplanktonic urochordates. Their name is derived from their resemblance to the tadpole larvae of sea squirts. These small (1–5 mm) urochordates are common in coastal waters where they are often abundant. There is no definitive head. The body consists of an amorphous-looking trunk and a characteristic, curved tail. The most remarkable feature of larvaceans is their external gelatinous “house” containing a series of filters (Fig. 28). The empty mucous houses contribute to a particular form of detritus known as marine snow, an important part of coastal and offshore food webs. Planktivorous fishes (including many fish larvae), arrow worms, and jellyfishes prey on larvaceans. SALPS AND DOLIOLIDS (THALIACEA) Salps and doliolids represent the other group of holoplanktonic urochordates. They are hollow , barrel-shaped, gelatinous animals common in the open sea. Both salps and doliolids are nonselective feeders on particles ranging in size from bacteria to diatoms. The filter is composed of two mucous nets secreted by the endostyle, one fine and one of coarser mesh. Water enters the oral opening or siphon, passes through the mucous filters, and then exits the atrial opening at the opposite end of the barrel. The fine net with its trapped food particles is ingested, and another is secreted. Although somewhat larger particles are ingested, particles 1–4 µm are removed with maximum efficiency. Most other suspension feeders cannot remove such small cells, so salps and doliolids, along with larvaceans, constitute a LOWER CHORDATES 303 key link between nanoplankton and larger consumers. In salps, peristaltic contractions of the muscular bands that encircle their bodies (Fig. 29) produce a water current through the barrel and its filter and provide propulsion. In contrast, doliolids use cilia to provide both the feeding current and limited locomotion. Both salps and doliolids have complex life histories, with alternation of sexual and asexual generations. Asexual budding results in rapid colony formation. In addition, salps and doliolids have some of the fastest generation times recorded for metazoans, resulting in massive “swarms” that can extend for hundreds of kilometers when food is plentiful. An individual salp or “zooid” initiates asexual reproduction by producing a stolon (Fig. 29) that gives rise to a chain or colony of many individuals. Members of the colony reproduce sexually with internal fertilization that leads to development inside the parental individual until young zooids are released through the atrial opening. This entire cycle may take only 50 hours. The doliolid life cycle is even more complex, with considerable specialization in the “zooids” not seen in salps. Doliolids produce a short-lived tadpole larva. ASCIDIAN TADPOLE LARVAE (UROCHORDATA) Several features of the swimming tadpole larva suggest an evolutionary link between sea squirts and the vertebrates. The nonfeeding larvae seem specifically adapted for dispersal; statocysts and ocelli are common. Initial photopositive behavior followed by photonegativity before settlement occurs in the lab. Chemical cues facilitate settlement on or near adults of the same species. At settlement, adhesive papillae cement the tadpole in place. A complex metamorphosis into the sessile adult form follows promptly. Most solitary sea squirts shed their gametes, leading to external fertilization and a planktonic phase of sevFig . 28. Larvaceans, such as Oikopleura, secrete complex gelatinous houses. The tail propels the “house” through the water and simultaneously creates a water current through the house. As water flows through the fine filter, small particles become trapped in mucus and are eventually ingested. The houses are disposable and are abandoned when the filters become clogged or if the larvacean is threatened. coarse prefilters at intake "house" escape opening for animal's exit head tail excurrent opening fine food gathering filters mouth 304 IDENTIFICATION AND BIOLOGY OF COMMON ZOOPLANKTON eral weeks. In contrast, most colonial sea squirts brood their young, and the larval phase lasts only a day or two. Two common genera, Molgula and Styela, release nonswimming larvae that settle after a brief dispersal phase. Ascidian tadpole larvae can...


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