Appendicularia

The image has a black background and shows an appendicularian without its gelatinous house.
© Maria McGuinness

What are they?

Appendicularians are part of the gelatinous zooplankton community found commonly throughout the world. They are free-swimming, solitary tunicates which resemble tadpoles. Tunicates are marine invertebrates which possess a notochord, an early type of spinal cord, which puts them in the phylum Chordata, the same as us humans. In fact, appendicularians are more closely related to humans than to any other component in the gelatinous zooplankton community. There are approximately 70 species of appendicularians. In European waters, however, only two families of appendicularian are known to be present – Oikopluridae and Fritillaridae. There are 17 species from these two families found throughout Europe, with seven being sampled at the Plymouth marine station. Species from both of these families have been sampled in Ireland.

The body of the appendicularians live in a gelatinous house, known as a test. These gelatinous houses are typically very small, however some have been recorded with a diameter of 1m, the size of a baseball bat. Amazingly, these houses can be replaced multiple times a day. Appendicularians have a unique way of feeding, making use of their gelatinous house. They create water currents through the whipping of their tail, trapping microorganisms in the gelatinous house, ready for digestion. These gelatinous houses also often contain bioluminescent properties, unlike many other tunicates. Each appendicularian species has its own specific pattern of fluorescent granules.

Ecology:

Within the marine food chain, appendicularians play an important role. They feed on algae and protazoans. Appendicularia can be incredibly abundant and so are an important food source for other zooplankton and larval fish. They are often a preferred food choice for fishes over copepods. Their gelatinous houses create a diverse microbial ecosystem with up to 1000 times higher levels of flagellates, bacteria and ciliates than the surrounding environment. Discarded houses form marine snow. Marine snow is the name given to the millions of small organic particles that slowly sink to the ocean floor, feeding many small organisms along the way and bring some much needed nutrients to the depths of the ocean. The test of the appendicularianis is also of great ecological importance as it has been found to trap viruses creating a sink effect.

Life cycle:

The appendicularian average lifespan is between 5 and 7 days. Appendicularia are semelparous, meaning they only reproduce once in their lifetime and reproduce sexually. They are commonly referred to as class Larvaceae as the adult appearance is similar to larvae of other tunicate groups like the sea squirts. Prior to maturity the larva undergoes a tail shift where the tail moves from a rear to a ventral position and twists 90 degrees to the newly developed trunk. Once mature the unique gelatinous house or test of the appendicularian is secreted. Appendicularians are holoplanktonic, spending their lifespan floating in the plankton, and are exclusively marine, mainly being found in the top layer of the water column.

By Aisling O’Flynn