Ocean Drifters

The larva of Luidia Sarsi, magnification x 80. The developing yellow-orange juvenile starfish will detach from the translucent larval body and sink to the seabed. The larval body may survive for a month or more before it dies. Adults have the remarkable ability to regenerate lost arms—or even grow a whole new individual from a single arm.

An annelid metatrochophore larva, magnification x 160. This polychaete worm larva has three bands of beating cilia that generate a current to help it feed and propel it, corkscrew-like, through the water. As it ages, it will lose the bands, its segmentation will be more pronounced, and bristlelike chaetae will grow and help with locomotion.

The larva of a sea cucumber, magnification x 340. The convoluted folds of this larva are finged with beating cilia that are used for locomotion and to generate a current that draws food particles toward the mouth.

The echinopluteus larva of the heart urchin, Echinocardium cordatum, magnification x 200. Irregular urchins, like the one shown here as a larva, don’t display the radial symmetry of regular urchins (think Koosh ball). Adults burrow into mud or soft sediments, where they feed on dead and decaying organic matter and play a key role in turning over sediments and recycling nutrients.

The sea angel Clione limacina, magnification x 20. These sea angels—planktonic, predatory sea slugs—are found from the Arctic to the Antarctic. The juveniles pictured here are just 5 millimeters long, while adults can reach 5 centimeters. They swim by flapping the two winglike appendages, and use rapid bursts of speed to catch their zooplankton prey.

The copepod Acartia clausi, magnification x 100. Copepods have a range of predators including other zooplankton, as well as jellyfish, fish, whales, and seabirds. To help avoid being eaten, many plankton have evolved defense mechanisms which may include body armor, such as spiny extensions or bristles; flashes of light; or escape strategies.

Dorsal and ventral (showing the red eye spot) views of a facerotecan nauplius, magnification x 300. This nauplius larva is one of the mysteries of the plankton. Some evidence suggests that facetotectans are related to the barnacles. A recent study of facetotectan larvae collected at just one site in coastal waters off Japan revealed more than 40 different species, which is an amazing biodiversity for this obscure group.

A late-stage brachiolaria larva of a starfish, magnification x 180. Many adult starfish are important hunters on the sea bed where they feed on mussels, crabs, and even other starfish species. Most adults also have an unusual way of feeding that involves turning their stomach completely inside out. After capturing their prey, such as a mussel, the starfish will first pry open the shell using its powerful tube feet, which are situated in rows along each arm. Next, it will evert its stomach through its mouth to envelope the prey, which is then digested externally. After the stomach lining has absorbed the liquified prey, it is drawn back through the mouth and into the starfish's body.

The doliolid Doliolum nationalis, magnification x 100. Eight ring-shaped muscles help adults of this species to both swim and eat. By contracting them, it draws water in through the buccal siphon at the top and expels it from the atrial siphon at the bottom. A mucus sheet behind the gills traps food particles from this stream of water before it leaves the body.

The chain-forming centric diatom Eucampia zodiacus, magnification x 300. This chain-forming diatom grows well at low temperatures. In Japanese waters, the autumn and winter blooms of this species reduce the production of nori, the seaweed that’s an important component of sushi, costing the country’s aquaculture industry an estimated $13 million each year.

The mitraria larva of an owenid polychaete, magnification x 170.

Coscinodiscus concinnus, magnification x 280. A microalgae and member of the phytoplankton group known as diatoms, this species can occur in such vast numbers that when they bloom that they can color the sea surface and are even visible from space.