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CTENOPHORES Comb Jellies and Sea Walnuts Ctenophores are among the most common macrozooplankton in estuarine and coastal waters in summer. The marine Phylum Ctenophora (pronounced with a silent “c,” as “teen-ofour -ah” or “ten-o-four-a”) has more than 150 species, but only several species are common in nearshore waters in our area. Ctenophores share many features with the Cnidaria, including radial (or biradial) symmetry and a gelatinous body with simple tissues. Unlike the cnidarians, these relatively large (up to 10 cm), wholly planktonic predators lack stinging cells and are harmless. Highly iridescent in sunlight and sometimes bioluminescent at night if disturbed, ctenophores are beautiful animals. Nearshore ctenophores in our geographical range have a planktonic life cycle. Unlike most cnidarian jellyfishes, there is no sessile polyp phase. Fertilized eggs develop into small ciliated larvae and then into cydippid “larvae” that resemble the adult in basic body plan. The most common ctenophores in nearshore waters belong to two classes. Class Tentaculata has tentacles for at least part of the life cycle (Fig. 15). Pleurobrachia and Mnemiopsis belong to this group. Class Nuda, which includes Beröe, completely lacks tentacles. Eight linear arrays of cilia (ctene, or comb rows) propel ctenophores through the water with a gliding motion quite unlike the muscular pulsing of cnidarian medusae. The extra long cilia (the longest known) beat in a synchronized fashion. Ctenophores may be the largest animals to rely on cilia for locomotion. Contact with predatory jellyfishes prompts surprisingly effective escape responses. While feeding, Mnemiopsis and Pleurobrachia hover or swim slowly. In contrast, the predaceous Beröe is a rapid and highly maneuverable swimmer. The three ctenophores treated here employ different modes of feeding. Pleurobrachia and larval Mnemiopsis feed with a pair of long, trailing tentacles (Fig. 15). The tentacles have sticky colloblast cells to entangle actively swimming prey. Larger Mnemiopsis lack tentacles. As they swim, they catch copepods, fish eggs, and mollusc larvae on the sticky mucus lining of the preoral lobes or on the numerous small tentacles that line the inner surface of the lobes as water is pumped past. The lobes close rapidly to catch prey and then reopen . Mnemiopsis often occurs in high densities and can have a major impact on meszooplankton abundance, sometimes affecting entire food webs. Beröe ovata feeds exclusively on other ctenophores, especially Mnemiopsis. When these voracious predators sense prey, they increase speed in a search behavior that often involves swimming in circles or helices. Smaller prey is simply engulfed whole. Beröe uses its long macrocilia on the edges of the CTENOPHORES 125 mouth to slice through the soft tissue of larger prey from which it removes large bites. Cydippid stage ctenophores eat copepod nauplii. Adult ctenophores fall prey to sea nettles Chrysaora quinquecirrha in brackish waters and to spadefish, butterfish, and harvestfish along the coast. Several parasites or symbionts regularly occur on local ctenophores, including the hyperiid amphipod Hyperoche medusarum and the larvae of the burrowing anemone Edwardsia leidyi. The North American ctenophores Mnemiopsis leidyi and Beröe have been introduced to European waters. Mnemiopsis was first noted in the Black Sea in 1982 where it multiplied to become a dominant planktivore that disrupted the local food web and threatened a fishery’s collapse until a subsequent introduction of the predaceous Beröe ovata began to control its numbers. Mnemiopsis then spread to the Sea of Azov and reached the Caspian and Mediterranean Seas by 1990. In 2006, Mnemiopsis appeared in the Baltic and North Seas. DNA analyses suggest that the Black Sea population originated from the Gulf of Mexico and that the northern European populations came from the northwestern Atlantic. Ctenophores inadvertently included in the ballast water routinely carried in transport vessels may have facilitated these invasions. A. B. oral lobe comb row mouth infundibulum tentacles mouth 10 mm 1 mm tentacle sheath Fig. 15. A, Cydippid stage (about 9 mm). B, Adult (5–6 cm) of Mnemiopsis sp., showing the comb rows of cilia and feeding structures. The cydippid feeds using its two long tentacles. When the tentacles are retracted into their sheaths, prey are brought into contact with the mouth for ingestion. The adult (B) lacks the long tentacles. It cruises through the water with its oral lobes spread wide. Prey are caught on the sticky surface of the inside of the lobes or on the short tentacles on the interior. 126 IDENTIFICATION AND BIOLOGY OF COMMON ZOOPLANKTON IDENTIFICATION HINTS The ciliated bands separate ctenophores from other phyla. Each of...


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