I recently bought a sea pen (I don’t know which kind, but it looks more-or-less like a large grey and white feather) that doesn’t seem to be doing too well in my tank. No matter how I try to plant the foot, it just keeps climbing out of the gravel. I have about 4 inches of crushed coral (roughly 3-5mm in diameter) and even when I cover the whole thing it pops out of the gravel within a day or two. I don’t understand why – does it need really fine sand? Also, what are the lighting requirements of these guys?
This is one of those cases where I’ll start the answer by admonishing you for not checking into the care and requirements of the animal before you bought it. Aside from that, it’s very hard to give advice on the care of an animal when its identity is unknown, and this is why it is so important to research your purchases before you have them in your aquarium. At least if you find out the care and maintenance of one species you can keep an eye out for it to appear in your local shop – if you buy a random animal from the local shop, it’s impossible to even guess from which ocean basin it originated, let alone what the species is likely to be. Frankly, I have no idea which species of sea pen you have from this description (most likely it’s one of the 40 or so tropical species of Virgularia or Pteroeides from the Indo-Pacific, but that’s just a guess). So having said that, I can’t give you any specific answer about your animal, but I can try to give you some general info on sea pens in general and these genera in particular in the hope that it helps.
First off, lets start with explaining what exactly a sea pen is. They are cnidarians and along with the true soft corals and gorgonians, they are members of the Subclass Octocorallia. Pennatulacids (the technical names for sea pens and sea pansies) are actually colonies of many polyps (like a coral head) rather than a single animal (like an anemone), and there is some pretty cool division of labor among the polyps. When a larva first settles, it metamorphoses into “founder polyp” which becomes the stalk from which the other polyps in the colony arise via asexual proliferation. The colony is composed of a series of gastrozoids (the feeding polyps responsible for capturing food and nourishing the colony), and siphonozoids (respiratory polyps responsible for moving water into and out of the colony to allow for gas exchange). The entire colony is typically arranged into a bulb, which is buried in soft sediments (very fine sands and muds on the sea floor) and an exposed portion along which the remainder of the polyps are found. Both the bulb and the upper portion of the animal are strengthened by an axial rod that consists of a mixture of hard organic material (like a gorgonian) and calcium carbonate (like a stony coral), but the animals are highly contractile and can expand or contract greatly depending on conditions. The “feather-like” shape is an adaptation for increasing the spread of gastrozoids into the water column to maximize the prey capture rate of a colony while also minimizing drag in the turbulent water in which these animals are typically found. Each gastrozoid is capable of producing gametes for sexual reproduction but all polyps in a given colony are either male or female. The animals free spawn and produce planktonic larvae that develop in the water column, so reproduction in the aquarium is highly unlikely.
OK, having explained what they are, let’s talk about how they live. You asked what the lighting requirements are for these guys, and the answer is pretty much whatever you have available. Species in these genera lack symbionts and as a result are generally uncaring of lighting conditions – many are nocturnal (active only at night) and even the species that are expanded during the day are unlikely to be bothered by whatever your ambient lighting happens to be. Given that they are nonphotosynthetic, it should be simple to deduce that they need to be fed, and in this case, fed quite a lot. These animals typically prey on an abundance of tiny plankton such as invertebrate larvae, rotifers, ciliates and the like, so you’ll probably need to start culturing rotifers if you hope to keep it alive (even newly hatched Artemia are typically too large for them).
In order for them to feed at all, however, you first need to make them happy enough to extend fully. In addition to the obvious requirements of water conditions, making a sea pen happy generally involves two primary things: 1) proper current (these animals seem to be found most commonly in areas that have moderate flow rates – something on the order of 5 cm/s would be a reasonable guesstimate), and 2) very fine sand (or better yet mud) in which to bury their “foot” (as I explained above, this is really a highly modified polyp that provides the attachment point for other polyps in the colony). Crushed coral (especially stuff in the 3-5mm range) is simply too coarse for the animals to borrow, and if it tries to burrow at all (which I doubt), it will likely cause as much damage as good due to laceration of the bulb by this coarse sediment. Most sea pens seem to go through a period where they extend fully on top of the sand before burying into it completely, so seeing a fully expanded sea pen is generally a good sign, even if it is not buried in the sediments yet (it is still likely to be reasonably healthy and likely to try burrowing if presented with the proper conditions). They also like to be able to retract completely within the sediments for protection, and are rarely happy unless they can do this.
Although some species (such as the unusual Cavernularia which looks more like a bottle brush than a typical sea pen) could thrive in a bed as shallow as 4", many species need something on the order of a full body length (and I’m sure many people would suggest something closer to 1.5-2 body lengths of the fully extended animal) of very fine sand for them to burrow and establish properly. Given that these particular genera average something on the order of 15-60 cm in height, that means you’re going to need a pretty darn deep sand bed to have a reasonable hope that this animal will survive long-term in your aquarium.
Given what you’ve described as your setup I would have recommended that you avoid a sea pen, and if you hope to keep it alive for any length of time, you’re going to have to make some major changes to your setup. I don’t mean to sound harsh, but it seems you were ill-prepared for this purchase and there is probably little chance that the animal will survive in your tank with the current design. I would suggest that you consider asking this question prior to purchasing an animal like this in the future to save yourself and the animal an unpleasant experience...
If you’re really set on getting a sea pen, I’d suggest that you first consider switching to a deep sandbed aquarium design. These animals are found exclusively in soft sediments in the wild and simply never occur in areas with anything approaching crushed coral-sized sediments. These soft sediments are essentially fine sands and muds (granulated sugar-sized particles and smaller), and if you’re interested in pursuing this avenue, there are plenty of articles on deep sandbeds out there for you. Discussing deep sand beds, their design and maintenance is another article (actually an entire series) in itself, and I’ll leave it to you to research more on this option if you decide to go this route. You’ll also want to look into setting up a feeding culture of rotifers, or obtaining some artificial rotifer feed such as A.P.R. (Artificial plankton - rotifer) or Golden Pearls Rotifer to feed your new pet, which should probably be supplemented with a phytoplankton product to stimulate a feeding response, if not feeding the colony directly. This will ensure that you have the proper sized particles and a ready supply of food for your new critter once it establishes itself. Finally, remember that I explained that many sea pens are entirely or primarily nocturnal – well, there is a reasonable chance that your pen will only occasionally be open and visible during the day when (if?) it settles happily into your aquarium. This is a double-whammy for you because you may not see it all that often and you'll need to make a concerted effort to feed it at night.
Despite those drawbacks, I really think that these colonies can be a fascinating addition to a reef aquarium which provides for their needs, and mine is one of my personal favorites. I wish you luck with your new addition, and hope that this information will help you to provide a suitable home for it!