What is a Sea Pen?
The sea pen that is scientifically known as Pennantula aculeata is a colonial marine of the order Pnnantulacea that comprises more than 300 species that are categorized among 14 families. They belong to the phylum, cnidarians that comprise 11000 species of aquatic animals belonging to freshwater and seawater but the species from the latter is of a larger number. Other than this there are 35 extant genera among which 450 species are mentioned but only 200 of them are valid to date. Because of its genus, it is also referred to as pennantula sea pens.
Although the entire group was named after their close resemblance to the feathered structure that is there in a quill pen. But only the pennantula sea pens belong to the suborder subselliflorae are the only groups that resemble a quill pen structure but the other larger suborder Sessiliforae do not possess any feathered structure and either grow in a radiator form or a cube-like structure. Thus the non-feathered sea pen species is commonly identified as sea pansies. They are usually widespread from the polar to the Topical sea and are found both in shallow and deep water.
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Description of Sea Pens
The sea pens attain a maximum height of 46 cm in height and 102 mm in width. They grow a stalk consisting of 20 pairs of branches that are wide and found on the sides of the stalk with a row of polyps on both ends. Polyps are hollow stems with a mouth and tentacles that are seen at the rear end. It generates from the central rigid and erect stalk of the sea pen. The lower part of the centre stalk known as the peduncle, that actually anchors itself into the mud or the sand bed of the sea to balance the sea pens to stand on a location and the upper part of the sea pen known as the rachis is actually responsible for holding the polyps or branches with several polyps. The erect and rigid central stalk is often referred to as the primary polyp or the axis polyp. The entire colony is protected and defended by the calcium carbonate in the form of the central axis rod or spicules. Families of some genera bear very few polyps, for instance, Chunella. But some of the other species may bear as many as 35000 polyps in one sea pen. The axial or the central polyp is strengthened by a central horny rod and it is made up of a little more than just a fleshy stem that is usually regenerated.
Sea pens are octocorals by appearance and belong to the colonial animals that possess multiple polyps resembling miniature sea anemones and comprising eight tentacles. Unlike the other octocorals, the polyps of the pennantula sea pen had special characteristics where it loses its feather-like structure to form a rigid and erect central stem (like rachis). They usually lose all their tentacles to form around and bulging the root or peduncle at its base. The other polyps that are present on the sea pen that is branching out from the central stem function as a water intake structure, a feeding structure with an explosive cell that contains a giant secretary cell also known as cnidocyst and Lynda in the plural. It also acts as its reproductive organ. The Paduncles (root-like structure of sea pen) when anchors itself into the muddy or sandy bed the upper portion that is exposed to the environment stands as tall as 2m in some species, for example, in the tall sea pens.
Many pennatulacens do not resemble with quill pen anymore. For example, a sea pansy, that is found along the Pacific and the Atlantic coasts of North America, biologically referred to as Renitlla, is structured like a flat kidney-shaped plate that consists of two halves that are fused together. All the polyps of all sea pens are attached to the upper part of the body that is not dug into the mud. The members or species of sea pen that can grow up to 2 meters consists of popular genera Anthoptilum, Funciculina, Balticina, Stylatula and Virgularia. But few of the species belong to the shooter group that do not grow more than 20 cm but comprise a few relatively large leaves that are extended from the central stalk, for example, Leioptilus.
Biology of Sea Pen
There are several colours of sea pens that can be seen ranging from yellow, red, brown etc. Some sea pens are bright coloured like orange sea pen which has been one of the famous colours sea pen that can be found in every habitat of sea pen. While some of the sea pens are as tall as 2 meters, that is just the measure of the exposed upper portion of the body that is not anchored with the mud and soil. The sea pens prefer to dwell in the deep water where there is not much turbulence of water where they uproot and the depth can be as deep as 2000 meters (6600 feet) or even more. But they do not float above 10 meters (33 feet) of water depth.
Like many other sessile animals, even the sea pens can relocate and anchor themselves in different locations if needed. They swiftly and comfortably place themselves in the flow of current that ensures the steady flow of the planktons, which are their main source of diet. They usually capture with the help of their tentacles at the end of the polyps. The primary predators of the planktons apart from the sea stars are nudibranches. The sea pens’ predation ability is hindered as they have the ability to clump together and spatially obstruct the predation. They also have an ability to completely deflate themselves by forcing the water out of their body and then retreating into their peduncle. They can deflate or expand either by removing or rushing water into their body through the hollows that are interconnected within the polyps. Some sea pens’ radiate the bio-luminance green light once they are touched.
The polyps of the sea pen coral are of two kinds, namely, siphonozooid and autozooid. In order to inlet the water current through the canal system of the colony, siphonozooid draws the water current. While the autozooid is used for feeding and reproducing either eggs or sperms in female and male sea pens respectively.
Types of Sea Pens
Though there are many types of sea pen species few of the most popular one that is often searched and famous have been described below:
Orange Sea Pen: The scientific name of the orange sea pen is Pitlosarcus gurneyi. The orange sea pen resembles more like a fluffy ostrich plume that can grow up to an average size of half a meter tall and ranging from orange to yellow to white in its appearances. The orange sea pen has a central sturdy stalk or stem that is basically a modified polyp on which several branches are attached with and it is not being a single animal but a colony of polyps that are attached on a thick fleshy emerging branch from the central stem. It has lost its tentacles and is supported by a single rod-like structure of calcium carbonate and a bulging round and thick bulb-like base that is emerged into the mud or sand base to anchor the entire structure of this sea pen.
If they are sufficiently threatened then the entire colony of the feather-like structure can have the ability to submerge into the sedimentation and hence would go completely unrecognized. It extends from the Gulf of Alaska to Southern California.
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Obese Sea Pen: Obese sea pen is scientifically known as Cavernularia obesa which is also commonly known as fat sea pen, sea cactus, sea quill, sea feather, sea finger and octocoral. Obese sea pens have a length of 10 cms and are found at a depth ranging from 2 to 20 meters of Indo Pacific ocean where they are hugely widespread. They usually possess a thick and sturdy axis polyp or central polyp that is internally supported by the calcareous rod-like structure. Half of the body of this sea pen is buried under the sea sedimentation and the rest half exposed to the environment prey on planktons.
They are mostly found on the lagoons and the over harbours where they can find soft mud and sandy bottoms. They usually withdraw into the substrates when they feel extremely threatened. They are also found along the coast of New South Wales in Australia as they mostly prefer the warm and temperate water of the ocean.
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Tall Sea Pen: The tall sea pen is also biologically known as Funiculina quadrangularis and is the tallest species of all the sea pens. This can reach about a height of 2 meters or above in some cases. But the most striking feature of this species is its white central axis stalk that has quadratic cross-sections. This is the reason for which its scientific name is derived as above. In contrast to the slender pen where the polyps are sitting nicely on the branch side, the polyps of this species are settled irregularly and possess a white or pale pink colour.
The range of the species is distributed worldwide. It is mostly found in many locations of the North Atlantic ocean in addition to locations like Japan, Madagascar and New Zealand. It prefers a depth of ocean that is ranging from 20 to 2300 meters and mostly found in muddy substrates.
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Common Sea Pen: Pennatula phosphorea is the scientific name of the common sea pen that can grow up to an average height of 40 cm. It has a deep red colour of its own and can instantly be distinguished from the other species of sea pen. Due to the highly developed muscles and the neural system within the axis stem, it enables the colony of tentacles to move around the seabed to find a better location.
They are the most frequent deeper that is more than 20 meters but they also prefer to dwell at an average depth ranging from 10 to 100 meters. Their common habitat is sandy and muddy substrates. They are widespread in the region of the northeast Atlantic ocean as well as the Mediterranean sea.
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Slender Sea Pen: The scientific name of the slender pen is Virgularia mirabilis and can grow up to a length of 60 cm. They are from the other sea pens by the fact that their polyps are grown on the side of the branches rather than attached directly to the stem branch that is also known as axis-stalk. But it is often confused with the V. tuberculata that also have their polyps attached to the branches that are grown out from the main stem but the only difference is that the latter has very few polyps as compared to the slender sea pen. Sometimes the slender sea pens are seen with 16 individual polyps attached to the single branch whereas the main stem is just a few millimetres thick. The colour of the colony of polyps is either white, beige or orange in appearance.
They are commonly found in the cost of European and British isles. It also stretches its range from the Mediterranean sea in the south to Norway in the north. They are also widespread throughout the North Atlantic ocean. They commonly dwell on sedimentation, shallow sheltered locations or in the depth of water that is around 400m.
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Reproduction
The usual method of reproduction of the sea pen by coordinating the release of sperms and eggs together in the water current and this can occur occasionally or even throughout the year. Both the male and the female sea pens release the gametes through their mouth into the water current where they fertilize into small larva. Their breeding season usually stretches from March to April. The sea pens usually reproduce by spawning. The average size of the eggs of sea pens is 500 to 600 micrometres. The fertilized are developed into planula that is non-feeding by nature and can swim freely into the water. They usually settle quickly into the sediments of the sea beds. Once it is settled, in some time the planula larva metamorphoses into polyps that ultimately becomes the central or the axis stem. The other polyps eventually grow laterally from the main stem. The juvenile sea pen can grow very rapidly and can survive without any feeding for weeks.
Sea Pen Habitat and Geographical Range
The geographical range of the sea pen is quite widespread from British Columbia to Central California. They mostly like to dwell on substrates that are soft like mud or sand where they can anchor easily. They are fond of temperate water but can survive even in the cold deeper ranges of the ocean. They are the inhabitants of the depth of water ranging from the low-tide lines to a minimum of 30 meters of depth and even more for most of the species.
Behaviour and Food Habit
Sea pens are colonial by nature and though they are considered sessile, still they can slowly move up to 40 cms by creeping motion while they are still attached to base under the substrates. One of the main characteristics that are common for all the sea pens is that they can quickly bury themselves. If they feel extremely threatened they withdraw themselves into the substrate in a way that is difficult to even notice bare eyed. While they bury themselves they extract out a lot of water from their system through their canal.
They mostly feed on planktons. Through one of the specialized polyp branches known as autozooid, the prey is inserted into the main stem. The food is then digested by some special digestive enzymes that are secreted by the filaments of the sea pen. The digested particles are then passed to the mesogloeal cells by phagocytizing the particles and then the digestion process is completed.
Conservation Status
In the parts of Pudge sounds, there was plenty of sea pen that habitat the area. But today their numbers have declined to alarming levels. The population of their primary predators such as nudibranches and sea stars have also declined, leaving many sandy substrates vacant. This has immensely affected the species that are on the top of the food chain and the entire marine ecological system. Though the scientists couldn’t able to find any valid reasons for their disappearance the ecosystem is in trouble because of the same.
Fun Facts of Sea Pens
Some of the Fun Facts About the Sea Pen Species are Mentioned Below:-
The three types of nudibranches, the red star and the leather star are the primary predator of the sea pen.
Sea pens are called octocorals as they have each polyp with eight tentacles
They can squeeze out all the water from their body through the colony of polyps so that they can bury themselves into the substrates where the bulbous root is present.
If the sea pens are stimulated, they are capable of bio-illuminating green fluorescent light of their own.
In conclusion, there are 300 species divided into 14 families. They are widespread from polar to tropical regions from temperate to cold water. Their average size is 40 cm long and 102 mm in width. Few members grow up to 2m. They dwell in both shallow and deep water. They are of various colours ranging from red, orange, yellow, brown etc. by the shape they are distinguished as obese, common, slender etc.
FAQs on Sea Pen
1. How Do Sea Pens Survive?
Ans: Sea pens have a structure built where the main propyl is known as axis-stem that is erect, fleshy and sturdy from where the branches of other propyl colonies have emerged and each propyl has eight tentacles on them. Now each propyl has their can functions where one of the specialized propyls catches the planktons and move it into the main stem where it is digested by excretion of digestive enzymes.
2. What is the Locomotion Way of Sea Pens?
Ans: The sea pens usually look for a softer substrate like the muddy substrate or soil bed where they can anchor their base that gives them the stability to stay in that location as they align themselves with the water current to feed on planktons. And then can free their bulbous base and can move around freely to search another location.
