In lieu of an abstract, here is a brief excerpt of the content:

MOLLUSCS Gastropods, Bivalves, and Cephalopods Many of the familiar coastal and estuarine gastropods (snails) and bivalves (clams, oysters , and scallops) have planktonic larvae that are often abundant in nearshore plankton samples. Less familiar are the holoplanktonic gastropods seen nearshore when pulses of offshore ocean water come ashore. Although most cephalopods (squid) are fast enough to escape plankton nets, juvenile squid appear sporadically in estuarine and nearshore plankton collections. MOLLUSCAN LARVAE Trochophore and veliger larvae are typical of bivalves and snails and common in the plankton (Fig. 26). Although the molluscan and polychaete trochophores are quite similar, recent evidence suggests that the similarity may be more due to coincidence than common ancestry. Trochophore larvae spin as the band of cilia on the anterior end propels them through water. The trochophore usually transforms into the veliger stage in less than 24 hours. The veliger is a uniquely molluscan larval type, characterized by two (sometimes four) ciliated lobes (the velum) that provide locomotion. When feeding, the cilia and mucus of the velum catch small particles. Veligers soon develop eyes and statocysts, both useful in vertical positioning. At the appearance of the foot, the larvae (now called pediveligers ) often alternate between swimming and crawling as they investigate the substrate before settlement and metamorphosis. Comb jellies, jellyfishes, and many larval fishes eat veligers. Gastropods (snails) show a variety of reproductive modes. In some snails, including whelks (Busycon), mating and internal fertilization lead to production of elaborate egg cases from which tiny snails emerge, thus eliminating planktonic larval stages. In other snails, the trochophore stage takes place within egg cases or gelatinous egg masses, and larvae first appear in the plankton as veligers. For example, veliger larvae of moon snails (Polynices) emerge from the sandy, collar-shaped masses often seen on beaches. Gastropod larvae are episodic and may be among the most abundant zooplankton in some nearshore samples. Bivalves typically shed their gametes into the water where fertilization and complete larval development ensue. The majority of trochophores in the plankton are those of bi- MOLLUSCS 279 valve molluscs. In less than a day, the trochophores transform into veligers with a shell and velum. The first shelled larva is called the straight hinge larva. As the larva grows into the “umbo stage,” the umbo gradually obscures the hinge. The first larval foods are small pico- and nanoplankton (0.5–3 μm) and even bacteria. As the larvae grow, they may shift to slightly larger food (3–10 μm). The final veliger stage (the pediveliger) is often epibenthic, alternating between swimming and searching for a substrate. In some species, habitat selection involves a complex set of behaviors involving both physical and chemical testing. The newly developed foot is used in locomotion as the larva crawls on the bottom. If a suitable substrate is not found, metamorphosis may be delayed as searching continues. After metamorphosis, many bivalves reenter the plankton during a secondary planktonic dispersal phase before final settlement. Bivalve larvae are common in the nearshore plankton , especially in the spring and early summer. HOLOPLANKTONIC SNAILS: PTEROPODS (SEA BUTTERFLIES) The pteropod gastropods, sometimes called sea butterflies, include both shelled (Thecosomata ) and shell-less (Gymnosomata) orders of holoplanktonic snails. The traditional term pteropod remains a convenient informal grouping for Thecosomata and Gymnosomata. Adaptations to planktonic life include loss or reduction of the shell to retard sinking and the conversion of a portion of the foot and body wall into lateral “wings” for swimming. Pteropods literally flap themselves through the water. The shelled thecosomes use a large external mucous net to trap phytoplankton and protozooplankton, including tintinnids and forams. The shell-less gymnosomes are specialized predators, often feeding on their thecoveliger umbo of shell foot velum trochophore apical tuft apical plate blastophore digestive system 100 µm 100 µm Fig. 26. Typical molluscan larval stages. A ciliated molluscan trochophore larva and a bivalve veliger. 280 IDENTIFICATION AND BIOLOGY OF COMMON ZOOPLANKTON some relatives. Fishes, baleen whales, and seabirds also eat thecosomes. Sea butterflies are widespread in the open oceans but are rare nearshore, except when brought shoreward in oceanic water masses. CEPHALOPODS (SQUID AND OCTOPUS) The first posthatching stage of cephalopods is called the paralarva, which is often found in near-surface waters during the day. Occasionally, newly hatched individuals of commercial squid (Illex or Doryteuthis) are captured within a few kilometers of shore. The only squid found regularly in nearshore areas is the brief squid (Lolliguncula brevis), common along the Southeast and the Gulf Coasts and estuaries. Although the brief squid...


Additional Information

Related ISBN
MARC Record
Launched on MUSE
Open Access
Back To Top

This website uses cookies to ensure you get the best experience on our website. Without cookies your experience may not be seamless.