Prickle Gill

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The prickle gill developed from the twilight gill by developing further defense against predation. Several of the filter-feeding bristles have enlarged and become stiffer, with pointy ends. These extend past the rest of the bristles, and the prickle gill can use them to fend off attackers. This works best against soft-skinned creatures, as they can pierce the skin and cause tears and bleeding. Armored creatures are too tough for the prickles to penetrate. They can seek gaps in the armor for twilight nippers, but they have little defense against the twilight neodevorator, which remains their most dangerous predator.

As these prickles are modified filter-feeding bristles, they remain hollow and capable of ingesting food, though they mainly rely on the smaller bristles for filter-feeding. This can lead to ingesting a bit of blood or other fleshy material while defending themselves, but this is incidental and not a significant part of their diet. They never seek it out.

An additional part of their defense array has come with sexual dimorphism, through a quirk of genetics. Like the twilight gill, the prickle gill has a large number of mating types, in which an individual can mate with anyone belonging to a different mating type. These are determined by proteins encoded on their gamete casings. There are two genes encoding these proteins. The first encodes ones labeled A, B, and C (D, present in their ancestor, was lost). The second encodes X, Y, and Z. This leads to seven varieties of the former (A, B, C, AB, AC, BC, none) and seven of the latter (X, Y, Z, XY, XZ, YZ, none), multiplied for a total of 49 mating types.

The prickle gill has developed a venom that is derived from the sex protein X. As a result, only the 21 mating types that include X are able to produce this venom; the other 28 types cannot. This results in what appear to be two distinct sexes, though they have many more in reality. Those with X produce a venom that fills the hollow insides of their prickles, making them even more dangerous for predators, as a prick from them is often fatal. They also have adopted a bright turquoise warning coloration. Those without X, however, have only the poking defense, and retain a more subdued periwinkle color. Neither really has the upper hand because the bright venemous ones, while safer against most predators, are at more risk to twilight neodevorators, so they end up balancing each other out.

Furthermore, the prickle gill's eyes have advanced to a pinhole, allowing them to resolve images more clearly. They are the smartest of the gill line, as they've learned to fight predators more effectively. They also often get together in small groups, especially mixes between venomous and non-venomous ones, allowing others to benefit from the venomous defenses. Their body shape has slightly compressed, with broader paddles. Their brain area is bigger, and they have more protective deposits around their central internal deposits, resulting in a blobbier midsection.

Otherwise, their anatomy is much the same as in the twilight gill. They have a central body with four arms: two on either side, one in front, and one in back. The back one contains an anus, the side ones are paddles, and the front one carries eyes, vibration-sensing hairs, chemical sensors, and the brain at its base. The back and side arms support a panoply of branching gills. The front and side arms support the bristles, which are hollow and lead to digestive tracts in each side arm and then to the central stomach. They have membranes between the arms which can expand to protect vulnerable areas when needed or to provide an additional form of propulsion. The central body also contains three hearts. They have a hydrostatic skeleton and can adjust the shape of their limbs. They reproduce by emitting gametes from a hole near the base of the anal arm, which the mate ingests through the bristles. After mating, the young grow as buds at the base of the front or back arms. These young are radial at first, but they detach after the gills and bristles have developed.