A wild-caught cyphonautes, likely of Membranipora, from a plankton sample taken in Coos Bay, Oregon A cyphonautes is a larva of an ectoproct or bryozoan. It is triangular in profile with a heavily-ciliated band called the corona at the base of the triangle and a sense organ at the apex (cypho-, bent; nautes, sailor). The beating of coronal cilia propels the cyphonautes through the water and contributes to a feeding current. The cyphonautes is very thin, i.e.
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In Membranipora, two zoids develop initially from the internal sac of a single settled cyphonautes. The relationship of the cyphonautes to other larval forms, either amongst the bryozoans or in related phyla, is a matter of debate. Because the cyphonautes features in the life cycles of some members in both the ctenostome and cheilostome bryozoans, it is inferred to be ancestral to the phylum. This leads to the inference that the many non-feeding coronate larvae must be derived by simplification of the cyphonautes, as sketched by Zimmer and Woollacott.
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The eggs of M. membranipora develop into planktonic cyphonautes larvae which is strongly triangular in outline and about 850μm x 600μm big.
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Like other bryozoans, Conopeum seurati is a suspension feeder, filtering phytoplankton from the water with its tentacles. Breeding takes place between June and October in Britain. The cyphonautes larvae are triangular in shape with brownish bivalve shells in their later developmental stages. These larvae are planktonic and drift with the currents for some time before settling and undergoing metamorphosis into ancestrulae to found new colonies.
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After substantial growth, an internal sac of undifferentiated tissue forms between the stomach and the wall of the exhalant chamber. In competent larvae the internal sac is the largest single organ in the body, and becomes quite convoluted. Once the cyphonautes selects a settlement site, metamorphosis begins with the rapid collapse of the triangle: the valves are pulled flat against the substrate and the internal sac spreads out beneath them. The corona and other larval organs are resorbed.
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By 1891 bryozoans (ectoprocts) were grouped with phoronids in a super-phylum called "Tentaculata". In the 1970s comparisons between phoronid larvae and the cyphonautes larva of some gymnolaete bryozoans produced suggestions that the bryozoans, most of which are colonial, evolved from a semi-colonial species of phoronid. Brachiopods were also assigned to the "Tentaculata", which were renamed Lophophorata as they all use a lophophore for filter feeding. The majority of scientists accept this, but Claus Nielsen thinks these similarities are superficial.
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These combined actions collect unicellular algae and other particles from the plankton. Muscles connected to the valves can quickly narrow the slender aperture at the base of the triangle, expelling most of the fluid volume in a sort of a sneeze that can flush undesirable material that enters the funnel. Combined with the action of long muscle fibers connected to the apical organ and corona, the entire body can transiently collapse between the valves. In feeding cyphonautes such as Membranipora's, rudiments of the adult body are not apparent in young larvae.
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Fossil of one of the paleocyphonautes described by Romero Romero has described a number of fossil species of horseshoe crabs, jellyfishes (of which there are very few good-quality fossil impressions from around the world) and a group of very unusual species that he grouped under the name of Paleocyphonates, which he described as a new subphylum. He proposed the hypothesis that these extinct, medium-size fossil impressions represented the adult stage of today existing larva cyphonautes and supported such hypothesis not only with paleontological information but with physiological and developmental biology data as well.
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However, in bryozoans the blastopore closes, and a new opening develops to create the mouth. Bryozoan larvae vary in form, but all have a band of cilia round the body which enables them to swim, a tuft of cilia at the top, and an adhesive sac that everts and anchors them when they settle on a surface. Some gymnolaemate species produce cyphonautes larvae which have little yolk but a well-developed mouth and gut, and live as plankton for a considerable time before settling. These larvae have triangular shells of chitin, with one corner at the top and the base open, forming a hood round the downward-facing mouth.
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In 2006 it was reported that the cilia of cyphonautes larvae use the same range of techniques as those of adults to capture food. Species that brood their embryos form larvae that are nourished by large yolks, have no gut and do not feed, and such larvae quickly settle on a surface. In all marine species the larvae produce cocoons in which they metamorphose completely after settling: the larva's epidermis becomes the lining of the coelom, and the internal tissues are converted to a food reserve that nourishes the developing zooid until it is ready to feed. The larvae of phylactolaemates produce multiple polypides, so that each new colony starts with several zooids.
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