Gracile alien fauna - sea squirts

Some of the most intriguing animals related to chordates are sea squirts. Some are sessile, some are free-swimming, some of the latter use the crasiest procreation system found on this planet. They don’t have a notochord as adults, but their larvae are pretty much like little swimming Vetulicolia.

Before tackling the design for gracile alien’s direct ancestor, I decide to play with the alien analog of “sea squirts”, hoping it will make my set of ideas broader.

The top one is the most basic vanilla sessile sea squirt. It retains its full 4-fold symmetry. It’s also quite like an Earth sea squirt on the outside, except it has four exhaust siphons. Within, however, it is quite different.

This design was actually quite important for nailing the “proto-fish”, because it establishes several key points. One, the use of gills for breathing and filtering out food, with associated hearts. (Which is, incidentally, the same in Earth’s sea squirts, but their structures are different. This design is more like the gills of an Earth’s bivalve.) Two, its four gastric channels begin in a common atrium. Three, it uses direct pulsed pumping to ventilate its gastric/gill channels, with sphincters in the intake and the exhaust - this is the feature that got carried over to the “chordate” and justified retaining the separate breathing channels in it.

The thing in the middle is an asymmetrical derivative. Its skewed onto one side like a flounder - essentially, only half of its structures remain.

The bottom design is a beauty. It’s a free-swimming sea squirt like an Earth’s salp, which repurposes the top pair of its gastric channels to propulsion. Its symmetry is broken; the top channels are greatly increased and have mechanical valves rather than sphincters. It still has the breathing gill. The bottom pair of channels is purely digestive, the gill repurposed for that end. The wide, shallow atrium is circled with back-facing whiskers; whatever shrimp that this thing bumps into is not going back. It’s much faster and more active than a salp; more an analog of the alien squid in its hunting style.
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