Saturday, January 19, 2013

Sea Pens (Phylum Cnidaria: Order Pennatulacea) of Singapore

Sea pens (phylum Cnidaria, class Anthozoa, subclass Octocorallia, order Pennatulacea) are radially symmetrical animals living in colonies exhibiting quill-like appearances. They usually live on soft bottoms, such as sandy or muddy substrates, and are able to burrow into the substrates partially or completely.

Sea Pen (Pteroeides sp.)
Like other cnidarians, they possess explosive, harpoon-like cells called cnidocytes. Each cnidocyte contains a secretory organelle (cnidae), which can be a nematocyst that discharges a harpoon-like stinger carrying toxins, a ptychocyst that discharges sticky substances, or a spirocyst that discharges lasso-like threads. Hence while cnidocytes are often called "stinging cells", they do perform other functions apart from stinging.

Veretillid Sea Pen (Family Veretillidae)
The sea pen animal, or polyp, has a simple body comprising a stomach (coelenteron) and a mouth surrounded by tentacles (where most of the cnidocytes are located), appearing like a little flower. The tentacles occur in multiples of eight, and hence they are in the subclass Octocorallia. It does not have an anus, and thus the mouth performs both functions of ingesting food and removing waste.

Sea Pen (Pteroeides sp.)
Unlike most other cnidarians, sea pen colonies are formed from a single large polyp called the oozooid. Secondary zooids then bud off from the sides of the oozooid to form the colony. The oozooid is usually supported by an internal, rod-like, calcareous axis. The fleshy lower part of the oozooid, called the peduncle, anchors the colony in the substrate by burrowing into it. The upper portion where the secondary zooids are found is called the rachis. There are usually two types of secondary zooids - the large autozooids which are feeding polyps with eight well-developed tentacles, and the smaller siphonozooids (with reduced or no tentacles) for pumping water. In some cases, there are a third type - the mesozooids, which are intermediate in form between the autozooids and siphonozooids.

Sea pens are largely filter feeders (feed on plankton). Some species also harbour the unicellular algae, zooxanthellae, which produce food through photosynthesis and share with the host sea pens, in return for shelter and nutrients (waste products of the sea pens).

Sea pens can reproduce sexually or asexually. In sexual reproduction, they usually broadcast their eggs and sperm into the sea. The fertilised egg develops into a free-swimming larva, which will eventually settle on suitable substrate to develop into an oozooid. Asexual reproduction is mainly for forming the colony as discussed in the previous paragraph.

Porcelain crab & brittle star
Sometimes, commensals such as porcelain crabs and brittle stars can be found seeking shelter among the secondary polyps of the sea pens.

The sea pens in Singapore are very poorly studied, and I only managed to tentatively identify them either to the family or genera using Williams' 1995 paper which has an illustrated key and synopses of all the living sea pen genera (thanks to Ivan Kwan for getting the paper for me!). If you spot any mistakes or are able to identify any of the below species, do let me know!

Family Pteroeididae

Often synonomised with the family Pennatulidae, sea pens of this family are bilaterally symmetrical with the autozooids disposed along the margins of large, opaque, leaf-like structures that are in turn arranged laterally around the rachis. The peduncle is thick and the rachis is feather-shaped. The siphonozooids are restricted to the polyp leaves.

The sea pens I have seen in Singapore of this family are probably all from the genus, Pteroeides. According to Williams' paper, Pteroeides spp. usually form bilaterally symmetrical colonies that are stout and feather-like. An internal axis is present throughout the length of colony. The secondary polyps sit on polyp leaves, which are very well-developed and rigid due to the presence of one to many supporting rays comprising long needle-like sclerites. Smaller leaves may be present between the main leaves.

Sea Pen (Pteroeides sp.)
This sea pen with broad, fan-like polyp leaves and obvious rays of sharp, needle-like structures (or sclerites) is likely a Pteroeides sp. It is commonly seen on our northern shores, and often, porcelain crabs and brittle stars can be found among the polyp leaves.

Sea Pen (Pteroeides sp.)
It generally has a fat and fleshy oozooid, and the whole colony is usually white or slightly yellowish.

Sea Pen (Pteroeides sp.)
This is yet another commonly seen sea pen, which should also be a Pteroeides sp. Like the previous species, it has fan-like polyp leaves and obvious rays of sharp, needle-like sclerites.

Sea Pen (Pteroeides sp.)
The peduncle is usually orange in colour, and often, "uprooted" ones can be found on our northern shores.

Sea Pen (Pteroeides sp.)
This purple sea pen is likely to be a Pteroeides sp. as well, with its fan-like polyp leaves and obvious rays of sharp, needle-like sclerites.

Family Virgulariidae

Sea pens of this family are bilaterally symmetrical with the autozooids disposed along the margins of thin, often translucent, leaf-like structures that are in turn arranged laterally around the rachis. The peduncle is usually slender while the rachis is rod-shaped. The siphonozooids can usually be found on the rachis, and hence sometimes giving it a furry appearance.

The sea pens that I have seen from this family fit the description in Williams' paper for Virgularia spp. The bilaterally symmetrical colonies are long, slender, and rigid. The axis is usually rounded in cross-section, and extends throughout most of the length of the colony, and sometimes even extending beyond the tip of the rachis. The polyp leaves are usually short and appear congested, or sometimes with intervals of bare rachis between adjacent leaves. The autozooids are fused for most of their lengths to form relatively thin polyp leaves. The tentacles of the polyps can retract into bulb-like, fleshy tips of the polyps. The siphonozooids are commonly on the rachis between polyp leave, but sparsely distributed on the polyp leaves.

Sea Pen (Virgularia sp.)
This sea pen, probably a Virgularia sp. with the narrow protruding axis, polyps with fleshy tips, siphonozooids on the rachis, certainly appears to fit the description above.

Sea Pen (Virgularia sp.)
This should also be a Virgularia sp. The polyp leaves come in two colours - white and brown.

Sea Pen (Virgularia sp.)
This reddish sea pen with the narrow protruding axis and polyps with fleshy tips should be a Virgularia sp. as well.

Sea Pen (Virgularia sp.)
This completely white with a very furry appearance, narrow protruding axis and polyps with fleshy tips is also likely to be a Virgularia sp.

Family Veretillidae

Members of this family can be distinguished by the cylindrical or club-shaped colonies, which are radially symmetrical throughout the length of the rachis. The axis may be present, reduced or absent. There are no polyp leaves, and the secondary polyps extend directly from the oozooid. The autozooids are distributed evenly over the entire surface of the rachis, with numerous siphonozooids in between. The autozooids lack calyces, unlike members of the similar-looking umbellulidae family, and can retract completely into the oozooid.

The various genera of this family are very similar superficially, and hence they can only be differentiated by their sclerites. As such, I could not identify them to the genera in the field or from photos.

Veretillid Sea Pen (Family Veretillidae)
This thin and rod-like veretillid sea pen is commonly seen on sandy substrates. The axis extends throughout the length of the entire oozooid, keeping the colony erect even when it is exposed during low tide.

Veretillid Sea Pen (Family Veretillidae)
The axis of this orange veretillid sea pen appears to very somewhat reduced, and the rachis is not as firm and rigid as the previous specimen.

Veretillid Sea Pen (Family Veretillidae)
Many veretillid sea pens are similar in appearance to the previous specimen, but have different colours, can be seen on our shores. Until someone examine them more closely, it will be hard to tell whether they are all of the same species with different colours, or they are different species altogether.

Veretillid Sea Pen (Family Veretillidae)
This is another veretillid sea pen which is maroon in colour, but otherwise appears very similar to the previous specimen.

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