Zooplankton of the Atlantic and Gulf Coasts: A Guide to Their Identification and Ecology

Hydrozoans

HYDROZOANS 85 HYDROZOANS Hydromedusae Hydromedusae are roughly umbrella or bell shaped, transparent, and usually only a few millimeters in diameter. They are primarily marine to mesohaline; few occur in low salinities . These small, delicate creatures are often mangled in plankton samples. If alive, their pulsing movement is unmistakable. Many species occur, but most are planktonic for only a brief period each year. Only the most common are treated here. Hydromedusae represent the sexual medusoid stage in the life cycle of most hydroids. Hydromedusae are solitary individuals that shed either sperm or eggs into the water. The resulting planula larvae settle to develop into the benthic polyp form. An exception is Liriope where the planulae transform directly into the medusa stage. In some groups, the planula develops into a polyp-like actinula larva (Fig. 10) that then transforms into either a medusa or a polyp, depending on the species. Release of hydromedusae seems largely controlled by temperature. Hydromedusae may vary in abundance from year to year, but they are usually present at the same times each year and for the same duration. Contractions of the bell propel water backward and the hydromedusa forward. In nearly all species of hydromedusae, the opening across the bell may be partially constricted by a thin velum that acts as a nozzle to increase the velocity of flow and helps direct the water, resulting in turns and maneuvers. Swimming in species with a shallow bell and poorly developed velum is slower. Pigmented ocelli (light receptors) and statocysts (gravity receptors ) on the margin of the bell help determine orientation. Hydromedusae use their stinging capsules (nematocysts) to capture a variety of zooplankton and can be categorized as either planula actinula 100 µm 100 µm Fig. 10. The planktonic stages of hydroids include the planula larva, the actinula stage (in some species), and the hydromedusa (Fig. 11). The hydromedusae mature into separate sexes and shed their gametes into the plankton where fertilization occurs. The resulting planula larva settles to the bottom to initiate a new polyp. The actinula, when present, occurs at different stages in the life cycle for different species. 86 IDENTIFICATION AND BIOLOGY OF COMMON ZOOPLANKTON ambush or cruising predators. Ambush feeders hang motionless in the water and wait for mobile prey, especially small crustaceans and fish larvae, to swim into their long tentacles. In contrast, cruising predators swim more constantly and rely on the vortices created by swimming pulses to draw smaller and less motile prey into their short tentacles. Ciliated larvae, invertebrate and fish eggs, other jellyfishes, and larvaceans often fall prey to the cruising predators. Siphonophores Siphonophores in our area are entirely planktonic hydrozoans. Most are animals of the open ocean, but a few occur in nearshore waters in high salinities. These are complex, colonial animals made of individuals (zooids), including both polyp and medusoid forms. Although the zooids are variously modified for locomotion, feeding (gastrozooids), reproduction (gonozooids), prey capture and defense (dactylozooids), or flotation (pneumatophores ) the colony functions as a single animal. The colony grows by asexual budding of individual zooids, but details vary, depending on the group. Colonies contain both male and female zooids and reproduce sexually. The swimming bells (nectophores) of most siphonophores contract to provide propulsion. In Nanomia, a gas float at the head of the colony can inflate or deflate to adjust vertical position. Physalia, the Portuguese man-ofwar , is the largest and most famous animal in this group. Although Physalia lacks any form of active locomotion, its sail-like float is ideally suited for wind-assisted propulsion. Wind and currents may take these high-seas sailors thousands of kilometers. Siphonophores trail tentacles arrayed with batteries of stinging nematocysts from their feeding polyps. The common nearshore siphonophores spread their tentacles and feed on small crustaceans while drifting passively. The Portuguese man-of-war (Physalia) trails long tentacles for feeding. When food is captured, the tentacles contract and bring food, primarily small fish, to the mouth of feeding polyps for digestion. Sea turtles, jellyfishes (scyphomedusae), and some comb jellies eat siphonophores. Porpitids Porpitids, once known as “chondrophores,” or “capitate hydrozoans,” are widespread from tropical to temperate waters where they float at the surface. Like the siphonophores, porpitids are complex colonies composed of specialized polyps, or zooids, each modified for a specific job. Some polyps form a central, gas-filled disc for floatation. Tentacle-like dactylozooids suspended from beneath the margin capture small zooplankton. Reproductive zooids or gonozooids release tiny medusae that give rise to new colonies. A large central gastrozooid...