So many suggestions that I made 3 votes the limit this time.
Negligible Senecense 5+1, Telomerase Regeneration 3+2, Cell Regeneration 1+2, Horizontal Gene Transfer 3+1, Burrowing Tendrils 3, Lure Chemicals 5, Size Increase 5, Gilltube 2+2, Closed Circulatory System 4+1
The murklurker can live for hundreds of years, suffering virtually none of the effects of aging. Generally it does not, however, as accident, predation, starvation or disease catch up with all lurkers eventually. Cancer is the number one killer of murklurkers, as faulty cell regeneration mechanisms are often hijacked by the countless viruses responsible for horizontal gene transfers between individuals and the resulting growths damage or occlude the more specialised tissues of the lurker.
The lurker sports a simple non-valved circulatory system that pumps 'blood' around the body through peristaltic motion; even though lurkers are generally sessile they slowly ripple within their shells to keep the flow of oxygen to all their tissues. Simple folded and feathered tubes on the dorsal side of the lurker absorb oxygen for transport through the circulatory system, and the same muscle motions that move blood around the body keep water flowing into and out of their shells to replace the oxygen supply. Most lurker shells now have small holes on the top to facilitate a gentle flow of water through the shell.
Hunting is still achieved through the extended proboscis, now extending to some thirty centimetres from the shell opening, but attractive chemicals are released from a gland near the mouth of the shell to attract prey. The lurker still lacks any sort of nervous system or form of perception, so the snare response remains purely automatic and pressure-based.
The lurker still resembles the cavecatcher in many aspects; the shell is essentially unchanged apart from the dorsal 'breathing holes', and the distinctive proboscis remains. The lurker is fully a metre long at its adult stage now, dwarfing most other creatures, but spends most of its life concealed within one of its many shells. The lurker retains the migratory nature of its ancestors, except for its reproductive tendencies.
Instead of producing young on its proboscis, the lurker has evolved a pair of tendrils that serve the same purpose (duplicates of the proboscis that have been adapted). When the lurker is ready to breed it extends the tendrils in the hope of receiving male cells from another member of the species. When male cells reach the tendrils on the current they begin to spawn new lurkers as growths on the tendrils. Once these growths have reached a certain stage, the lurker leaves its shell and begins the slow march across the sea bed in search of a new home. On its path, the tendrils dig holes in the muddy silt of the bay floor and in the process bury the infant lurkers. The lurkers develop in the safety of the mud until they have started to form their first shells and are ready to hunt on their own.
This migration is one of the most dangerous times for lurkers, who are hunted by the umbrella slugs that serve as the current apex predator; descendents of the swimslugs who employ a large radial fan to push themselves through the water at speed. With only its proboscis to defend itself, migratory murklurkers are often attacked by one or more umbrella slugs and eaten alive at the parts where the proboscis cannot reach. Damage to the circulatory system often leads to them 'bleeding out' and becoming easy prey for the umbrella slugs or other scavengers.
During the shelled stage of its life the murklurker has one particular predator, the siltworm, another slug descendent that crawls across the silt eating various minor creatures and often competing for prey with the murklurker. Siltworms have thicker skins than the murklurker's usual fare and can avoid being pierced by the proboscis. They instead are attracted to the lure chemicals of the lurker and enter through the mouth-hole, burrowing their way into the lurker and eating it from the inside out. The siltworm secretes a chemical that attracts more of its kind when this happens, so whole swarms can descend upon a murklurker shell when one finds their way in.
GENERATION 4:
Murklurker
An enormous white slug-like creature that secretes a cave-like shell bound to the ocean floor. About a metre long, with a pair of distinctive gill-tubes running along its back (when exposed) and 'breathing holes' along the shell. An extendible proboscis is used for feeding, and two genital tendrils can be extended from either side.
REPRODUCTION: It lets out male cells in the current, and polyps grow on the genital tendrils of other blobs where they land. The polyps are buried in silt during migration and emerge as infant murklurkers.
MOVEMENT: Moves by rippling its body in between shell stages.
SENSES: Detects pressure changes in the water through receptors on its skin.
EATING: Proboscis extends to catch swimmers passing the mouth-hole of the shell. Attracts prey with lure chemicals. Occasionally harvests empty former shells of opportunists.
PREDATION: Adults predated by siltworms. Infants or migratory adults predated by siltworms, umbrella slugs and swimslugs. Niche predator; only targets active swimmers such as swimslugs and umbrella slugs.
COMPETITION: Severe competition for niche by siltworms combined with very passive hunting strategies leave the murklurker at threat in its home environment.
ENVIRONMENT: A shallow, murky sea amongst multiple islands. The ancient coral reefs are restricted only to the deeper waters; millions of years of cavecatcher and murklurker shell buildup have transformed the environment, creating a shallow, murky and silt-ridden environment. Light is limited, and the plants that grow here tend to have thick stems that extend to large, wide leaves at the surface. As a result, even less light gets through to the seabed and the murky bottom is very dark.