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Tunicates are chordates. They possess a notochord and spinal cord along the back but no backbone. This alone demonstrates that they are advanced organisms. The name comes from a thick coat many of the adults possess, especially those living on the seashore where they are commonly referred to as sea squirts. Unique among animals, they have the ability to make cellulose used in this tunic. Thought to probably have come from a gene transfer from bacteria.  Some tunicates have a larval stage living in the zooplankton that settle out on to a shore as a sessile creature.

By contrast there are two groups, including Appendicularia, that have permanent dwellers in the plankton. Within the holoplankton they can be very abundant, second only to the copepods. One species, Oikopleura dioica is very common in Milford Haven, particularly in the summer months, and is a species found worldwide. See data below.

Oikopleura dioica has separate sexes and retains larval characteristics with a distinctive long tail containing the notochord and nerve cord. The photographs here show how the creatures are found in the collected samples but when in the sea they live within a house they create by secretions. This house is a fairly elaborate structure which helps them feed by trapping nanoplankton, brought in by the beating tail. When clogged with material a new house is made and the old one discarded. This can happen up to 10 times a day. Life happens rapidly for them as even their development from eggs is quick, maturing to adults in 4-6 days with females able to produce up to 400 eggs. They can bloom very rapidly to reach high densities (53,000/cubic metre has been recorded.) This makes them vital fish food particularly larval flatfish. Overall, an important species as a food link from nanoplankton to secondary consumer. They are important in the removal of carbon as the house, when discarded, sinks to the bottom sediment taking with it organic carbon that it has filtered. The gelatinous house is also “sticky” and so as it sinks (which is slow) so it picks up more material.

Various stages in development are shown below.

Oikopleura dioica tunicate from plankton

Oikopleura dioica. The long tail contains the notochord. What looks like a head is the body or trunk. On the top is a gonad.

Oikopleura larva Tunicate Appendicularia
Oikopleura Tunicate Appendicularia juveniles
Oikopleura larva Tunicate Appendicularia
Tunicate Appendicularia - mature Oikopleura trunk with labels
Oikopleura adult with developed gonad

Adult with developed gonad

Oikopleura larva Tunicate Appendicularia

A statocyst is a sensory receptor for balance. In aquatic environments that is especially important so they know which way up they are.

Data for Oikopleura in Dale Samples

Relative Frequency

From even basic data like this it can be seen how abundant Oikopleura can be through the year. Typically, there is a spring peak followed by a long period of summer and autumn before the decline to winter. For Oikopleura to grow and reproduce it needs a lifetime of access to food rather than food concentration. Its actual food requirement threshold is low. This is a good adaptation for survival in times of food scarcity. Ultimately the population density is determined by competition with other species and the maturation of the gonads. The timing of the latter is down to the temperature of the water. For example full maturity from egg to egg release is 6 days at 15 degrees C; at 20 degs C it is just 4 days. Around Pembrokeshire the summer of 2023 had the warmest sea temperatures on record. Maybe this is why the density was so much higher and sustained compared to 2022.

For more about the data collected please go to the Data section. It will open in a new window
Ascidian, larva

Ascidian Larvae found in the plankton

Two different tunicate, Ascidian, larvae. Below is a closeup of the head. What looks like an eye is a statocyst with a simple ocellus to the side

Head of Ascidian larva with statocyst
Ascidian larva with a dark statocyst
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