Twilight Echofin

Splitting from its ancestor, isolated populations of Red Echofin found a new Binucleozoan food source, the Twilight Crystal, and descended deep into the Twilight Zone in order to feed on them. Here, they developed an unusual reproductive adaptation in order to survive and propagate in the deep sea.

Twilight Echofin begin their lives as larvae germinated from spores. The larvae are meroplankton, swimming in the water column of the twilight zone and travelling along deep sea currents. Larval Twilight Echofins feed upon Shimmering Marephasmatises, which are plentiful within the twilight zone, piercing their prey with a sharp four pronged jaw and devouring their insides. Larvae will often take on Marephasmatises larger than themselves, being careful to avoid their stinging thread. In order to survive off of a gelatinous food source, larvae must eat a large amount of Marephasmatises to survive.

When Twilight Echofin larvae grow large enough, they will be able to swim against the current using the many pairs of fins which appear on each of their segments. They usually swim in an undulating pattern to slowly move forward, but will move all of their fins in unison to achieve quick bursts of speed to escape predators.

Adult male and female Twilight Echofins will often settle down around twilight seafloors in order to find their favored food, Twilight Crystals. As their name suggests, Echofins use the cone around their heads to find their food using echolocation, and to avoid predators. After locating a Crystal, they will pierce through the red, fungi-like area and avoid the poisonous outer layer by boring holes into the Crystal. Between Crystals, the elongated forms of these Echofins allow them to easily drift in the current, expending little energy between food sources.

Hormone Carrier Lifestyle
The deep sea is a vast void where even finding a meal is a rare occurrence, let alone finding a mate. Echofins are particularly disadvantaged by this, as they require three separate sexes (male, female, and hormone carrier) in order to successfully reproduce. As such a meeting would be extremely rare in the deep sea, the hormone carriers have adapted in an extreme and unusual way to propagate in these isolated conditions.

Hormone carriers are dimorphic from the males and females, with adults being less than a third of the length of the two other sexes. The cone around their head is narrow, giving them tunnel vision but allowing them to fit into their lifestyle. Their jaws also differ from the others, being more stubby and diminutive, as hormone carriers feed by spitting out digestive enzymes instead of by piercing.

After reaching sexual maturity, males and females will create a constant clicking sound with their jaws, producing echolocation which the hormone carriers seek out. After locating their partner, the hormone carrier will insert themselves into their gills, aided by their small size and slim cone. Here, the carriers latch onto the inside of the gills, and will remain there for a majority of their lives.

The carrier will feed on the blood of their larger partner, only taking what it needs to survive by spitting a digestive enzyme around its mouth and slowly rasping at the flesh. Since these carriers are so inactive, barely ever moving, they need to take little to sustain themselves. The carriers will also occasionally dislodge themselves while their partner feeds on Crystals, supplementing their diet by feeding within the bore holes made by their larger partner. The partners will remain close to the carriers if they detach, waiting for them to finish feeding and reattach.

All of this is done so that when a sexually mature male and female Twilight Echofin meet, it is very likely that one will already have the carrier necessary for them to breed. The carrier will release hormones into the surrounding water as the male releases sperm and the female releases spores. With all its complexities, this reproductive adaptation makes it far simpler for Twilight Echofins to reproduce in a lonely void.

Other Information
Because of their food source, Twilight Echofins will often fall prey to Twilight Trapinouts due to their mimicry of Twilight Crystals. The Echofins may avoid this fate if they spot the Trapinouts which are less well hidden with their echolocation.

Though male and female Twilight Echofins mainly feed on Twilight Crystals, they will still occasionally eat Marephasmatises. This is a purely supplemental diet, and adults cannot survive on this prey exclusively. However, this allows the adults to travel along currents in the open water of the twilight zone, surviving off of Marephasmatises until they reach another twilight floor.

Most Twilight Echofin larvae are destined to become hormone carriers. A majority of these carriers will die before they can find a partner, either through predation due to their small size or the difficulty they have gaining nourishment without blood.

Though a single hormone carrier is not enough to harm their partner, if multiple attach to a single Echofin the increased blood drain will cause health problems. As such, male and female Echofins will often attack their smaller partners if more than one is present, persisting until one detaches.