TCI is home to the spineless ancestor of all creatures with a spine.

By Dr. Eric S. Cole

Sometimes scientists get it so right it’s almost magical. In 1999 Neil Shubin and a small team of paleontologists began flying to the furthest north stretch of land above the Arctic Circle, Ellesmere Island, to excavate an exposed bit of rocky hillside during the very brief openings in mid-summer. Five years later, they uncovered what they had only (hopefully) predicted they would find . . . an ancient link between the swimming fish and four-legged land animals “Tiktaalik.”

Shubin’s findings beautifully documented in his subsequent book Your Inner Fish and in a PBS video series (https://www.pbs.org/video/your-inner-fish-program-your-inner-fish-2/) was a triumph of careful study, cautious prediction, and persistence. It revealed a luminous ornament on the evolutionary tree. Shubin proceeded to explore and explain how many of the most peculiar anatomical features of the human body (and indeed all land mammals) make sublime sense when viewed as modifications on the body plan or anatomical design of an ancient fish.

There is an older, even less probable story involving creatures whose descendants now populate the seagrass beds and reefs surrounding the Turks & Caicos and have even crept into some of the saltwater ponds on the Islands. These are “Tunicates” or sea squirts.

Anchored to the mangrove prop roots in a saltwater pond on Providenciales are a colony of yellow Tunicates.

This is a pale variety of Tunicates, simple “filter feeders.”

Alexander Kovalevsky, (a Russian embryologist whose family was Ukranian nobility), first noticed that a genuinely weird group of squishy invertebrates (the Tunicates) were neither mollusks nor worms, but “chordates,” creatures with ancestral features that would one day produce our spinal cord. Though his discovery was nearly 160 years prior to Shubin’s, it is little known or celebrated outside of academic circles, yet it is every bit as profound.

It is difficult (and humbling) to recognize kinship with a gelatinous blob that spends its entire adult life anchored to a substrate and filtering plankton like the simplest animals on Earth do. It took Kovalevsky’s unique fascination not only with the adult body form of marine invertebrates but their juvenile, or larval forms, as well, to recognize the link. When fertilized Tunicate eggs develop, they sprout a muscular tail that works against a cartilaginous rod to swim vigorously and disperse. When satisfied with their new surroundings, they “face plant,” adhering to the local substrate with adhesive pads as they digest their tails and their brains and set up shop passing water across their “gills.” A rod-like organ secretes a sheet of mucus that passes over tiny cilia across the gill-perforated pharynx, capturing filtered particles and delivering them to the stomach. This organ, the endostyle, is remarkable in that it concentrates iodine just as the thyroid gland does in a vertebrate. Coincidence?

This image depicts the interior of an adult Tunicate or “sea squirt.”

In humans (and fish), the thyroid gland also concentrates iodine used to make a variety of hormones that regulate growth and development. Kovalevsky drew attention to the fact that, here in the larva of a seemingly simple form of life, were the rudiments of “chordate” features: a brain and spinal cord running down its back, a mouth, pharyngeal gill-slits, an iodine-capturing organ located below the pharynx, a series of segmentally repeated muscle groups (our modern-day abdominal muscles), and even the cartilaginous disks that reside between our vertebrae—occasionally slipping or herniating to remind us that the body we wear was not originally designed to be vertical or to bear weight.

This tale is poorly known, probably because it is “thrice cursed.” It involves invertebrates, an incredibly diverse group of difficult organisms that have troubled scientists for centuries. It involves Tunicates (a group peculiar even amongst the invertebrates). And, the tale demands attention not so much on the adult body form of this obscure group of animals, but on the animal’s larvae—for it is within this tiny, transient, “dispersal form” that we find the blueprint of all “higher” life*.

It is one of evolution’s more twisted turns that the vertebrate body plan evolved by arrested development. Some ancient Tunicate never grew up, but became sexually mature as a tailed-larvae, forsaking its adult form as a sedentary filter feeder. This evolutionary shortcut is called paedomorphism or neoteny:  retention of juvenile features in the adult body form.

The solitary Bluebell Tunicate is common on islands in the Caribbean and east Atlantic.

Once you develop a search image, you see Tunicates everywhere:  brilliant yellow colonies attached to scaly Halimeda macro-algae in the back-reef environment; beautiful “bluebell” Tunicates anchored to the harder reef materials on a night dive; or grape-like clusters in the anchialine ponds and mangrove communities in the Turks & Caicos interior. They display a riot of colors and appear both singly and within complex colonial structures. When not out snorkeling the inland ponds, and attached to a comfortable chair sifting information through the Internet, I feel a definite kinship with these ancient, invertebrate ancestors.