A Christmas story of the coral reef variety

It has been a while since I had a chance to write something about the reef tank. Given that it is Christmas, I thought I would pick a topic that related to the season, but before I get into that, I will introduce the latest addition to the tank. As I mentioned in the post An almost-blind killer and its watchman, the shrimp goby-pistol shrimp symbiosis was no longer visible. Unfortunately it was not because the pair had moved, but rather it appears that the goby passed away. The shrimp is still in roughly the same spot. Last week I finally managed to get a potential replacement, which is a yellow watchman goby, or yellow prawn goby, Cryptocentrus cinctus(Herre). This species was described in 1936, and

Yellow watchman goby, Cryptocentrus cinctus, with pistol shrimp, Alpheus djiboutensis. Photo: Nick Hobgood, licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.

comes from the Indo-Pacific. The first week, the specimen in the reef tank was stationed by the far left wall of the tank, but at the time of writing this it has moved to under the rock with the anemones on the right hand side of the tank. It is currently only about 3 cm long, but may grow to 8 cm. I am hoping that it will eventually find the pistol shrimp and pair up, but there is of course no guarantee that this will happen.

The Christmas-associated part of this blog will still link to the general theme of symbiosis that I started a few blogs ago, but in this case it is a somewhat one-sided association between a worm (Phylum Annelida) and a coral (Phylum Cnidaria). While the reef tank had this pair many years ago, they no longer inhabit the tank. One of the reasons is that the coral in question (a fairly non-descript species in the family Poritidae, and specifically the genus Porites) is fairly difficult to maintain in a reef tank. These SPS (small polyp stony) corals are generally beige-brown in colour with very small polyps. They require very strong light and high water movement. Many species are so called finger corals, but some species are encrusting or make more or less round rocks, e.g., the boulder coral, P. solida.

On occasion, these corals are for sale as Christmas tree corals. This is a misnomer, because the Christmas tree reference is to a symbiotic polychaet (=many bristles) worm, the Christmas tree worm, Spirobranchus giganteus (Pallas), which lives embedded in the coral head as a commensal symbiont. These tube worms are obligate symbionts (although see Skinner et al. 2012) of a variety of coral species (Floros et al. 2005), but in the reef tank trade they are usually sold with Porites colonies. It is viewed as a commensal symbiosis because the coral does not normally benefit in any way from the presence of the worm, and it may in fact be damaged by the tubes constructed by the worms. DeVantier et al. (1986), suggested that polyps growing adjacent to the worms may be protected from predation by the crown-of-thorn sea star, Acanthaster planci(L.) (Phylum

Assorted Christmas tree worms, Spirobranchus giganteus on a porite coral. Photo Nick Hobgood, licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.

Echinodermata), thus ensuring that the coral colony survives, which is important as these corals persist for many years and can be dominant in shallow waters. Ben-Tzvi et al. (2006) observed similar protection from predation on corals. Potts et al. (1985) reported on one specimen on the Great Barrier Reef that was at least 677 years old, for example. Pratchett (2001) showed that coral symbionts do affect dietary preference by the crown-of-thorn sea star, although he did not specifically test Christmas tree worms. The worm on the other hand gains a very secure burrow in the calcareous skeleton of the coral, and an excellent perch for feeding.

Why Christmas tree worm? Well if you look at the photograph it becomes rather obvious. Each worm has two specialized palps, which make up a branchial or radiolar crown. Each tentacle has a feathery look to it, which gives it a large surface area. These appendages serve the dual purpose of food filtering and respiration. When the worm is feeding, the crown is extended from the tube, but when disturbed it can be quickly withdrawn and covered by a lid, or operculum. As the Latin generic name implies, the radiolar crowns are spiral, and they look like small trees. In addition, they are extremely colourful, reminiscent of the decorated Christmas trees. What determines the colour morphs is something I haven’t been able to establish, but suffice it to say that these animals are extremely attractive, and a favourite subject of underwater photographers.

A relative of the Christmas tree worm is present in the UNBC reef tank. This is a Hawaiian feather duster, Sabellastarte spectabilis (Grube), which is in the family Sabellidae. The crown differs considerably from those in the family Serpulidae, and in particular, the tube housing the worm is leathery rather than calcareous, and these worms lack an operculum. Nevertheless, they feed in a similar fashion and you can see the reaction to disturbance if a fish gets too close, for example.

Merry Christmas and a Happy New Year to you all.

 References
Ben‐Tzvi, O., S. Einbinder, and E. Brokovich. 2006. A beneficial association between a polychaete worm and a scleractinian coral? Coral Reefs 25:98.

DeVantier, M., R.E. Reichelt, and R.H. Bradbury. 1986. Does Spirobranchus giganteus protect host Porites from predation by Acanthaster planci: predator pressure as a mechanism of coevolution? Marine Ecology Progress Series 32: 307-310

Floros, C.D., M.J. Samways, and B. Armstrong. 2005. Polychaete (Spirobranchus giganteus) loading on South African corals. Aquatic Conservation: Marine and Freshwater Ecosystems 15: 289–298.

Potts, D.C., T.J. Done, P.J. Isdale, and D.A. Fisk. 1985. Dominance of a coral community by the genus Porites (Scleractinia). Marine Ecology Progress Series 23: 79-8.

Pratchett, M.S. 2001. Influence of coral symbionts on feeding preferences of crown-of-thorns starfish Acanthaster planci in the western Pacific. Marine Ecology Progress Series 214:111-119.

Skinner, L.F., A.A. Tenório, F.L. Penha, and D.C. Soares. 2012. First record of Spirobranchus giganteus (Pallas, 1766) (Polychaeta, Serpulidae) on Southeastern Brazillian coast: new biofouler and free to live without corals? Pan-American Journal of Aquatic Sciences 7: 117-124.

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