Clade Profile: Pectinauts

Eryobis was once a world with an ocean ruled exclusively by fish. Many types of coelacanths and holosteans dominated every imaginable marine ecosystem and seemingly nothing could ever dethrone them

That was…, until Eryobis was hit by an object from outer space that carried such incredible energy that the entire first 200-500 meters of ocean around the globe were evaporated and so killing every fish in it.

A few fish did survive in deep water and isolated freshwater habitats furthest away from the impact, but they would never recover from the extinction.
So, now there was an ocean devoid of larger animals in the photic zone, niches free for the taking.
Several groups took to the occasion:
* The conodonts like Nienktvissen and Squidn’ts whose ancestors had survived in the deep sea.
* The Llamplelganae polychaetes which took it upon themselves to fill the most of the niches we typically associate with cephalopods.
* And then the subjects of todays post: the Pectinauts

Pectinauts are bivalves in the order Pectinida, commonly known as scallops. On Earth, scallops are capable of swimming by taking in water and then pushing it out of openings next to the hinge of the shell, giving them a weak jet propulsion system.
On Eryobis, they took this a bit further following the extinction of nearly all fish. In the beginning they were clumsy swimmers as scallops are on Earth, but pretty soon they figured out a way to do it more efficiently. They oriented themselves upright. A large improvement compared to the ancestral laying on the side condition. Accompanying this new orientation, they also modified their foot into a rudder to help them steer, though it would come to be used as a tail in many future forms.
Scallops on Earth can have up to 200 eyes. These tiny eyes are typically located on the edge of the mantle close to the shell and can be retracted for protection. Early Pectinauts also had such eyes, but these eyes were adapted for a life of lying sideways on the seafloor, not for swimming upright in open water. So the Pectinauts had a problem: they could not see what was to their sides or behind them.
For this issue, they came up with a most curious solution. Early in their lifecycle, a dent forms on each valve. As they mature, these dents form slits and within these slits are a number of eyes, which are also typically the largest. This results in Pectinauts having cloven shell valves, a unique appearance and an easy way to tell them apart from other bivalves.

Most Pectinauts can be classified in one of two main groups: frontswimmers or backswimmers. As the names suggest, the main distinction between the groups is whether the mantle is facing to the front or the back when swimming.
Frontswimmers (1-6) are usually considered to be the more basal of the two groups. Some are thought to still closely resemble the ancestral Pectinauts and most of them live in shallower waters and reefs where they feed mostly on algae and plankton. Many of them have modified their tentacles into large appendages for a variety of purposes such as catching food, luring prey and in some cases even as fins.

Backswimmers (10-15) differ from frontswimmers in that they swim with the hinge of their shell moving forwards. Their mantle and tentacles are thus facing backwards while they swim, sort of like cephalopods. In general, backswimmers are quite derived. They occur in many shapes and sizes and occupy a greater variety of niches than frontswimmers do. Their tentacles tend to be thicker, their digestive systems more extensive and their propulsion system more advanced. While smaller and more basal forms often feed on plankton, the larger and more advanced forms tend to feed on algae, corals, crustaceans and in case of the large free swimming forms even other Pectinauts, Llamplelgans and even fish. Some backswimmers have taken on forms that resemble fish quite closely. They turned their feet into large tails and swim by combining this foot motion and jet propulsion.

Then there are a number of outliers among Pectinauts (7-9). Such forms cannot be classified as frontswimmers or backswimmers for their mantles and valve hinges are not facing the direction they’re swimming. Many of these forms love in the deep sea suspended and largely without much motion, patiently waiting to for food to wander into their tentacles. Other forms of these outliers live in shallower waters where they sift through the sediment for smaller molluscs, crustaceans and annelids which they then kill with modified tentacles that sport spikes. It’s generally believed that these outliers are an unnatural grouping that split off from both frontswimmers and backswimmers at various points in their evolution history, but much research is still needed to understand the full picture of the Pectinauts.