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DECAPODS Shrimps, Crabs, and Related Crustaceans Although some decapods are planktonic for their entire lives, most spend their adulthood on the bottom. Their planktonic larvae bear little resemblance to the benthic adults, a source of confusion for many decades. Even now, the larval stages of many decapod species have not been described. This is not surprising because the only definitive way to match adults with larvae has required rearing of wild-caught larvae through all larval stages in the laboratory or hatching of eggs from known species of adults to determine their successive larval stages. Only of late has matching of gene sequences offered an alternative approach. Larval distribution and biology are well studied for commercially important species, but information remains scarce for many others. Decapod life cycles typically involve mating followed by egg deposition and subsequent hatching of planktonic larval stages. Females use pheromones to attract males. Mating often occurs at the female’s maturation molt when she is in the “soft shell” state. Once mated, most penaeid shrimps, including the familiar commercial shrimps, release their eggs directly into the water, and a planktonic developmental sequence begins with the nauplius and protozoea stages. In contrast, females of other decapods attach their developing eggs to the setae of their abdominal appendages and brood them until zoea are released into the plankton. Each molt results in an increase in size and in progressive development of appendages, with the more posterior appendages developing last (Fig. 23). As noted in Figure 2, life cycles often include long-distance transport and dramatic changes in habitat during the larval period. The notes on individual species illustrate the complexity of these life histories. Although there are many variations in the sequences of developmental stages and in the names given to special stages in various decapod groups, the following stages are the most representative of decapods overall. Penaeid and sergestid shrimps have a free-living nauplius stage. This stage of development occurs within the egg in all other decapods. The only functional naupliar appendages, used for swimming, arise from segments (somites) of the head region. The nonfeeding decapod nauplius is usually found offshore and should not be confused with the nauplii of copepods or barnacles (see p. 52). The most characteristic larval stage of decapods is the zoea. The now functional thoracic appendages are used in swimming and feeding. Each successive molt gives rise to a sequence of progressively larger larval stages designated zoea I, zoea II, and so on, as the larvae develop. The total number and longevity of zoeal stages for a species may differ, I II 1 mm 1 mm III IV 1 mm 1 mm V VI 1 mm 1 mm VII 1 mm 1 2 3 4 VIII 1 mm 5 Fig. 23. The developmental sequence (stages I–VII) of the grass shrimp Palaemonetes sp. shows the incremental changes that occur in successive molts. In particular, note the addition of teeth on the rostrum (1), development of lateral spines on the fifth abdominal segment (2), the emergence of lateral tail fins (uropods) (3), the gradual development of swimmerets (pleopods ) on the abdomen as the relative size of the abdomen increases (4), and the appearance of claws on some legs (5). 220 IDENTIFICATION AND BIOLOGY OF COMMON ZOOPLANKTON depending on environmental or laboratory culture conditions. The final larval stage is a transitional stage from planktonic to benthic lifestyles and usually bears a much closer resemblance to the adult than to earlier stages. This stage is technically the decapodid stage for all decapods, but it is sometimes called the postlarval stage. In contrast to the shrimps, most crabs show a dramatic metamorphosis from the zoeal stage to final larval stage as seen in Figure 24. Pincers, or claws, are used by the increasingly carnivorous decapodids to capture prey. In the Brachyura (true crabs), the last larval, or decapodid, stage is called a megalopa or megalops and that of the spiny and slipper lobsters is the puerulus. In this section, we use the term: (1) nauplius for all prezoeal stages of all decapods that produce them, (2) zoea for all but the final stage of all decapods, (3) postlarva for final stages of penaeid shrimps and the American lobster, (4) megalopa for the final stage of all crabs, and (5) phyllosoma for the prepuerulus stage of the spiny and slipper lobsters. Many decapod larvae are accomplished swimmers. They respond to a variety of environmental stimuli and may undergo vertical migrations. Different groups and different stages...


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