Sessile Animal

Basic Information

A Sessile Animal is an animal that exhibits Sessility (zoology) - that is to say, an animal naturally immobile / rooted to the spot on a substrate. On earth, sessility is far more common amongst plants and fungus. While this is not the norm within the animal kingdom on Earth, it's not unheard of. Sessile animals on earth generally live in (or at the edge of) water, which they use to distribute their offspring - most reproduce by budding, but the water is equally capable of carrying eggs or spores great distances.

Sessile animals tend to cluster together in tight groups, a phenomenon known as Clumping.

Earthly Examples:

• Barnacle
• Coral - actually a colony of small, sessile animals with a shared exoskeleton
• Hydra (genus)
• Jellyfish - though just their larval stage, and not all forms of jellyfish have polyp larva
• Sea Anemone - actually electively sessile and capable of relocating itself, but generally doesn't.
• Sponge - exhibiting the opposite pattern from jellyfish, the sponge larval stage is motile and the adult form is sessile

Beyond Earth

Fantasy and Science Fiction allow for rather broader exploration of the concept of sessile animals. Some stretch the definition of "animal", and probably belong in a Kingdom of their own. They do not produce their own food via photosynthesis, and must somehow feed on something external.

• Carnivorous Plant - Technically, it's a plant, but it exhibits behavior more commonly associated with animals here on mundane earth. Plus, there's something cool about a giant venus flytrap. "Feed me, Seymour!"
• Genius Loci - The spirit of a place. May be able to manifest at specific points within a place, but can't leave it's confines.
• Sessile Sentience - Intelligent and self-aware physical creatures who are rooted to the spot. But they could grow from a vehicular substrate.
• Slime or Ooze or The Blob - Semi-liquid gelatinous creatures that sort of cling to one spot and wait for prey to come to them. May go through stages similar to a terrestrial slime mold.
• Symbiotic (or at least commensual) sessile organisms might attach themselves to another creature and ride about on it.

In addition, some Eldritch Abominations and Starfish Aliens may be depicted as sessile just to make them less human. See Alien Biochemistry for additional ideas.

Making the sessile form one stage in a metamorphic life (as it is for barnacles) - or one generation in a cycle might make for an interesting idea. Perhaps one instar is motile but sub-sapient and another sapient but sessile? Perhaps those wierd alien trees are actual the same species as the natives who object to you cutting them down to build your base?

The sea anemone would also be a decent model for a terrestrial ambush predator given a little more in the way of tentacle strength … just the thing to find in a cave, swamp or other restricted environment.

Sources

Bibliography

1. Wikipedia

Game and Story Use

• Sometimes you just need a monster or alien to be different in some major way that makes it memorable or unique. This could fit the bill.
• On earth, most animals are motile, and most plants are sessile. On another planet, however, that could be inverted.
  • An entire planet evolved from tumbleweed!

Building This Character

• A sessile animal needs some means to lure or extract food in it's environment. Consider pheromones or camouflage.
• It also needs some way to defend itself since it can't run away.
  • Poison is a common option. Possibly delivered by thorns or a Cnidocyte.
    • Clumping may also be beneficial in this regards, as it concentrates the amount of toxin that can be delivered. The sessile organisms in the middle of the clump are safest.
  • The very high reproduction rate of r-specialists would serve this function as well, especially if some mechanism exists to disperse the offspring over large areas. See r/K Selection Theory.