More Marine Madness

Tuesday, January 21, 2025

More Marine Madness

Today’s special animal friends are the Sea Pens, which are weird. They are marine cnidarians in the order Pennatulacea, which includes about 200 species in 35 genera and 14 families. Cnidarians are marine invertebrates characterized by an uncentralized nervous system and the use of cnidocytes or cnidoblasts, specialized cells with flagella that can catch prey or inject venom. Jellyfish are cnidarians. Cnidarians’ bodies are made of mesoglea, a jelly-like “extracellular matrix” within a membrane that allows the animal to hold its shape against water pressure.

Sea pens are more like coral than they are like jellyfish. They form colonies on the sea floor and are filter-feeders. They are considered “octocorals,” because each polyp is a symmetrical octogon. Unlike most corals, the various polyps in a colony morph to perform specialized roles in support of the whole. The roles include the root or peduncle, which anchors the colony in the sea bottom; water intake structures; reproductive elements; feeding polyps with stinging nematocysts; and more. The structures get added support from calcium carbonate spicules.

There is a great deal of variety among the 200-ish species, so we’ll cover just a few of them. The orange or fleshy sea pen, Ptilosarcus gurneyi, is native to the northeastern Pacific Ocean. It roots itself in a sand or mud substrate at weirdly specific depths from 46 to 738. In addition to orange, they can be yellow or white; they are up to 18 inches high. The colony feeds on plankton, can move from place to place, and reproduces itself by producing both eggs and sperm using specialized polyps known as autozooids. Several predatory sea stars eat orange sea pens.

The radial or purple sea pen, Actinoptilum molle, is found off the coast of South Africa. They usual attach to sandy ocean bottoms, and they have been found over 1,000 feet deep. Instead of looking like a feather, they look more like the duster end of a pole duster, with their polyps growing all around their central stem. They can appear in colors other than purple, but they do not change color, and their population density in desirable habitat can be high. Pierre’s armina, a recently discovered nudibranch (sea slug) is known only because it feeds on purple sea pens.

The tall sea pen, Funiculina quadrangularis, can grow over six feet high. It is found in cold water in both the northern and southern hemispheres. Its structures form habitats for several species of crustaceans, especially in their juvenile phase, and the immature polyps of the brittle sea star. Tall sea pens have not been rated by IUCN, but they are protected by governments in waters around the United Kingdom and in the Mediterranean. They are very vulnerable to damage by lobster and shrimp trawlers, which scrape them from the sea floor.

Marine Life Information Network (MarLIN) article on sea pens

Many sea pens are bioluminescent. Here are pictures of some other species. They’re weird: Real Monstrosities website, living up to its billing.