Pennatulacea
Sea pens, also known as the order Pennatulacea, are colonial marine cnidarians that belong within the Octocorallia group. Although some molecular studies have been conducted on deep-sea species within the order, information on shallow water species is still limited. The order consists of 14 families and 35 extant genera, with an estimated 200 valid species out of the 450 described. Sea pens have a widespread distribution, ranging from tropical to temperate waters globally, and can be found from intertidal zones to depths exceeding 6100 m. They are classified as octocorals, along with sea whips (gorgonians).
While the group is named after its resemblance to antique quill pens, this comparison only holds true for sea pen species belonging to the suborder Subselliflorae. Those belonging to the larger suborder Sessiliflorae lack feathery structures and instead grow in club-like or radiating forms. Commonly known as sea pansies, these sea pens lack the feathery appearance.
The earliest accepted fossils of sea pens date back to the Cambrian-aged Burgess Shale, and there may be similar fossils from the Ediacaran period that could represent the origins of sea pens. However, there is still ongoing debate and uncertainty regarding the nature of these early fossils.
Geographic distribution plays a role in the genetic variation among sea pen species, as they are capable of inflating themselves with seawater, allowing them to be carried by currents. Numerous populations of sea pens can be found in 🇮🇳 Indian waters, and genetic differences have been observed in their polyps due to dispersion and adaptation to different environments and islands. The polyps, or tentacles, are instrumental in the protection of sea pens and other species.
Sea pens are colonial animals with multiple polyps, resembling miniature sea anemones, and each polyp possesses eight tentacles. Unlike other octocorals, sea pens have specialized polyps with specific functions. One polyp develops into a rigid stalk known as the rachis and loses its tentacles, forming a bulbous base called the peduncle. Other polyps branch out from the central stalk, forming water intake structures, feeding structures with nematocysts, and reproductive structures. The entire colony is reinforced with calcium carbonate in the form of spicules and a central axial rod.
Sea pens anchor themselves in sandy or muddy substrates using their root-like peduncles. In some species, such as the tall sea pen, they can rise up to 2 meters in height. Sea pens can exhibit bright colors, with the orange sea pen being a notable example. They are typically found at depths below 10 meters and prefer deeper waters where turbulence is less likely to uproot them. Some species can inhabit depths of 2000 meters or more.
Although sea pens are primarily sessile, they have the ability to relocate and re-anchor themselves as needed. They strategically position themselves in the path of currents to ensure a continuous flow of plankton, their primary source of food. Nudibranchs and sea stars are their main predators, with some species feeding exclusively on sea pens. The clumping nature and spatial unpredictability of sea pens can make it challenging for sea stars to prey on them. When touched, some sea pens emit a bright greenish light through bioluminescence. They can also forcefully expel water from their bodies for defense, quickly deflating and retracting into their peduncle.
Like other anthozoans, sea pens reproduce by releasing sperm and eggs into the water, either seasonally or throughout the year. Fertilized eggs develop into larvae called planulae, which freely drift for about a week before settling on a substrate. Mature sea pens provide shelter for other animals, including juvenile fish. The analysis of growth rings on the rachis suggests that sea pens may have lifespans of 100 years or more, assuming the rings are indeed annual in nature.
Some sea pens exhibit a rare type of symmetry called glide reflection symmetry among extant animals.