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COPEPODS Copepods are the most abundant animals in most mesozooplankton collections and often outnumber all other animals combined. This diverse group of small crustaceans (adults mostly 0.5–5.0 mm) contains more than 200 families and more than 10,000 marine species . Because of their abundance, copepods are important links in virtually all marine and estuarine food webs. Of the 10 orders of copepods, calanoids, cyclopoids, poecilostomatoids , monstrilloids, and harpacticoids are most frequently encountered in estuarine and nearshore zooplankton samples, but parasitic siphonostomatoids appear occasionally. Temperate species show seasonal cycles (Fig. 18) and have a variety of adaptations for surviving unfavorable periods. Most nearshore calanoids produce resting or diapause eggs that sink to the bottom and remain viable for months until changing temperatures induce hatching and initiate the next generation. Harpacticoids may overwinter as quiescent females that respond to increasing day length in spring to release their nauplii. In yet another variation, cyclopoids and poecilostomatoids often overwinter in copepodid stages. Species that favor cooler seasons may produce resting eggs in response to rising temperatures. Copepods reproduce sexually, with rare exceptions. Ritualized behaviors and physical or chemical cues assist pairing. Males often grasp the female using specially modified antennae or legs while cementing a sperm packet to her genital region. A single copulation often fertilizes multiple clutches. Eggs may be shed singly into the water or enclosed in egg clusters attached to the female. The eggs hatch as nauplius larvae, often only about 100 μm long and with only three pairs of appendages used for both swimming and feeding. Larvae go through six naupliar stages (molts) before becoming copepodids (Fig. 19). The five copepodid stages increasingly resemble the adults, adding body segments (somites), pairs of legs, or both in each molt. Most free-living copepods have a maximum life span of six months to a little more than a year. Generation times from egg to egg are temperature dependent—usually two to six weeks. Under favorable conditions, some copepods can release successive clutches daily. Each copepod group has its own mode of swimming. As suspension feeders, larger calanoids cruise slowly using the second antennae and oral appendages. When threatened, they execute rapid evasive maneuvers, or jumps, using their thoracic “swimming” legs, sometimes in combination with flapping the abdomen. Smaller calanoids are more likely to swim in an irregular, zigzag motion. As they swim, the first antennae are usually folded against the body to reduce drag. When copepods move slowly or are at rest, the antennae are extended and act as hydrodynamic sensors to detect vibrations produced by prey COPEPODS 149 or predators. The extended antennae also act as underwater parachutes to retard sinking. Cyclopoids typically exhibit a “hop and sink” motion in which periods of slow sinking alternate with periodic bouts of upward swimming. Recent studies suggest that copepods can cover their hydrodynamic tracks to conceal their presence or, alternatively, create distinctive hydromechanical signals that attract mates. Most nearshore calanoids are both omnivorous particle grazers and opportunistic predators on microzooplankton. Suspension feeding seems to be the primary feeding mechanism in smaller calanoids, but many species also employ raptorial feeding to catch individual prey, including ciliates and copepod nauplii. Planktonic cyclopoids and poecilostomatoids are primarily raptorial carnivores as adults and often feed on larger prey, including fish larvae. Planktonic harpacticoids are enigmatic. The benthic forms are grazers, or browsers, on benthic microflora and fauna. At least some feed on microalgae in the water column and detritus and its associated microflora, but the feeding mechanism is unknown. The most studied feeding mechanism is suspension feeding, widely, but incorrectly, termed filter feeding. Sixty years after the classic treatments of feeding in Calanus, the mechanism of suspension feeding remains an area of active inquiry and debate. Anterior appendages beat to produce currents that pull small suspended particles, primarily phytoplankton cells, ciliates, and other small “microzooplankton,” toward a ventral feeding chamber. Contrary to the old assumption that the feeding appendages simply strain particles from this feeding current in a continuous motion, high-speed motion pictures show that at least some copepods detect and trap individual particles with directed movements of the feeding appendages. Some copepods feed effectively on a surprisingly broad range of food sizes (3 μm to more than 300 μm in Calanus) and can adjust to the size spectrum of particles in the water. New appreciation for forces affecting movement of small particles in a viscous water medium may revise our understanding of suspension feeding. Raptorial feeding is used to capture motile prey...


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