Biological adaptations for tunicates to become terrestrial animals

Question

I am currently designing evolution paths for tunicate which are a chordate invertebrate marine animal. In their larvae stage they are mobile and resemble tadpoles but in adult form some attach to hard surfaces and become sessile, whilst other free float and some remain with a similar design as the larval form.

Because of their larvae stage it is easy to see how their notochord could evolve into vertebrae and then have a similar evolution path as fish to amphibians but another interesting aspect about tunicate is that they are the only animal to produce cellulose.

From reading about high strength bioengineered materials from cellulose nanofibers it made me wonder if tunicates outer layer could be a cellulose exoskeleton and if it could possibly take an evolutionary path closer to arthropods?

Or could there be other biological adaptations that I have missed out?

What plausible biological adaptations could a tunicate evolve that will help them become a terrestrial animal, other than a vertebrate path similar to fish to amphibians? And is the cellulose exoskeleton a likely path that could lead them to evolve into forms similar to arthropods?

Answer 1

This is a tough one!

Tunicates have two huge strikes against them as land animals:

• They are sessile, which makes it hard to crawl out of the water
• They are filter feeders, which makes it hard for them to live outside of the water. (@AlexP's noted the exception of Larvacea, but I'm avoiding that route)

Nothing is impossible in biology, but as any mother knows, not everything is easy. I'm tempted to cheat somewhere. I could cheat by doing parallel evolution - a later evolution of chordates by skipping the adult stage (starting from Appendicularia/Larvacea, perhaps) as has been long speculated for the original evolution of vertebrates. But that's not much fun. So I'm going to cheat by stretching the definition of the word plausible.

1. A population of tunicates, feeding on the coral reef, developed a need for rapid dispersion in adult form. Too many bleachings, too many crown-of-thorns, maybe an unsung Megalodon that learned the pleasures of grazing on coral reefs. For whatever reason, the adults would get a warning a few days before the reef they were attached to was doomed. They evolved to secrete an acid to dissolve the coral skeleton substrate, float free in the water, and attempt to attach to a site somewhere else.

2. The tunicates become colonized by symbiotic organisms that produce methane and/or hydrogen, similar to human flatulence. The resulting gas bubbles are stored in the center of the pharynx, and turn out to have some clever engineering way of helping to make the filter feeding easier.

3. With most other corals wiped out by rampaging Megalodon veganus (yes, this one turned out to have been misnamed), tunicates come to dominate the ocean reefs of tattered cellulose. Some specialize to become small, but some become immensely large and develop social organization. Filled with flammable gas, and absorbing atmospheric air, their warrior caste floats on the top of the sea like a mine, waiting to detonate the helpless placid megalodons and drop scraps of their flesh to be gathered to feed the queen below.

4. Plate tectonics has divided the planet into many isolated seas. Successful tunicates form a great colony of modified warriors whose collective hydrogen gas bubbles lift the fertilized queen entirely free of the water. At last, the tunicates have taken to the air!

5. The next hundred million years is an exciting time in the planet's evolution. Tunicates experiment with wings, fins, sensory organs of every description, produced by exaptation of the nervous system and notochord during metamorphosis. These tissues, which formerly were lost, are now free to migrate and rearrange into new structures. Overall these animals, being derived from the ancestral adult stage, show little sign of segmentation. However, they develop advanced routing mechanisms for controlled branching growth in their tentacles, which arise by specialization and duplication of the incurrent siphon. They spread from sea to sea, forming mixed colonial organisms with their distant relatives, becoming obligate symbiotes with a wider range of capabilities.

6. Their war on the vertebrates has pushed that luckless group to the fringes of the ocean, where walking catfish have begun to invade the land for short periods. But the tunicates, whose pharynx now serves as a gizzard to grind the flesh of their victims, have become quite skilled at swooping at them from above. Their lengthy, dextrous tentacles pierce the catfish in a half dozen places before any possibility of escape.

7. The extinction of the vertebrates has created new challenges. Shore tunicates patrol the seaside for crabs and rotting fish, but several species have adapted to consume the native plant life. These tunicates roost in the position of an epiphyte, and bore deep into the plant with cellulase-secreting tentacles. Sessile once again, they have begun to lose some of their ancestral traits that permitted them to take to the air - but their carnivorous relatives now have something else to prey on.