All life in the open ocean ultimately depends on the growth of microscopic phytoplankton. But these tiny plants are too small to be eaten by larger marine animals. While small creatures eat the phytoplankton, larger ones are mixed feeders on plant and small animal plankton, or they are carnivores that prey on animal plankton. Moon jellies and salps are examples of mixed feeders, and arrow worms are carnivores.
The moon jellyfish Aurelia is a common summer plankton. They are easily identified by four purplish oval rings (testes or ovaries) on transparent, bell-shaped bodies, which can be up to 40 centimetres in diameter. The creatures propel themselves through the water by rhythmic contractions of the bell. They are also carried along by ocean currents in the same way that parachutes are carried by the wind.
They feed primarily on small floating plankton which are caught in mucus on the umbrella’s surface and propelled to the rim by tiny beating hairs. Four long oral arms or lips then lick the food particles into the mouth.
The jellyfish has two stages. The first is a floating stage that reproduces sexually. Floating males and females produce tiny larvae that settle on the sea floor. Here, they begin an asexual stage, reproducing by budding off juveniles which grow into floating jellies. This capacity to produce numerous floating forms can result in a dense aggregation of jellyfish close to shore. Luckily these jellyfish do not sting.
A flash in the dark
Like many creatures in the ocean, moon jellyfish are bioluminescent. They produce blue light from a pigment called luciferin. When luciferin combines with oxygen it produces an unstable chemical that emits a flash of light. Most bioluminescent creatures are found in the upper 1,000 metres of the open ocean. Other common producers of bioluminescence in New Zealand waters are krill and dinoflagellates.
Salps are semi-transparent, hollow, barrel-shaped animals that move through the water by contracting bands of muscles around their bodies. The salp Thalia democratica forms long chains (up to 10 metres), and has been recorded in very large numbers in New Zealand waters during summer. Salps have a reputation for growing to swarm proportions when they encounter plankton-rich waters.
When salps propel themselves along, they feed from the stream of water they create. They appear to vacuum up small organisms which include phytoplankton, bacteria and ciliates. One individual is capable of clearing food from up to 440 millilitres of water each day. One of the fastest-growing plankton, Thalia democratica can grow 10−20% of its body length in an hour, or almost double its weight in a day. Salps are eaten by fish, marine mammals and seabirds.
Arrow worms are found only in the ocean, and are carnivores. Sagitta tasmanica and Sagitta minima are two species of arrow worms most commonly found in New Zealand waters. Straight and thin, they range in length from 0.3 to 15 centimetres. They are perfectly transparent, except for two black eyes on the tops of their heads.
Arrow worms are important predators. They are capable of eating animals as large as themselves, and may consume up to one-third of their body weight in a day. They detect vibrations made by their prey and dart towards them. Arrow worms belong to a large group of invertebrates called Chaetognatha – the name means ‘hairy jaw’, and refers to the large moveable hooks around their mouths, used to catch food.