Cavernularia obesa

Cavernularia obesa, also known as the sea cactus, are colonial marine cnidarians in the class Octocorallia which includes all soft-bodied corals like gorgonians and sea whips^[1][2]. C. obesa belongs in the order Pennatulacea which are also known as sea pens^[1].

Pennatuloids can be distinguished from other octocorals by a basal peduncle anchoring colonies into the substrate^[1]. Their colonies are also made up of different types of polyps, including autozooids, siphonozooids, and acrozooids.^[3]

Cavernularia obesa is different from other Pennatuloids because of several unique characteristics. They exhibit an elongated ellipsoidal body shape as well as eightfold radial symmetry and eight tentacles.^[1][4]

Cavernularia obesa is widely distributed and found in the northeastern Pacific Ocean as well as on the coast of southern Japan.^[5][6] They inhabit sandy shores, mudflats, silty bottoms, and rocky outcrops.^[3] They also help structure benthic communities providing a habitat for other invertebrates and fish from shallow intertidal regions to depths of 6,100 meters.^[1][2]

The species adjusts its behavior based on a light-dark cycle as well as a temperature cycle^[7]. During the day, they contract and bury themselves into the sediment while at night, they expand and rise above the surface becoming more active when the temperature is around 25°C^[8][9].

In C. obesa, both males and females release sperm and eggs into the water which is controlled by light and not each other^[10]. Even if males and females were to be separated, they would still follow the same timing meaning their external fertilization is driven by the environment and not by their communication^[10].

Sea pens have a two-phase life cycle of planktonic larvae and sessile adult forms^[6]. Mature adults are half-buried in the sediments relying on planktonic larvae for the spreading and distribution of their species^[6].