Sabellarid Reef
Sabellarid reefs are composed of sabellarid worms that collect suspended sand grains in agitated seawater and secrete a protein-containing mucus to cement them together, forming a tube-shaped burrow. The starting end of the tube is sealed shut by the sabellarid worm. At the other end is the operculum which can open and close to protect the worm inside. The length of the burrow is from several to 30 centimeters. Size and length depend on those of the worm inside.
When a large number of sabellarid worms gather and build tubes, they form a massive biotic reef. In the waters off southeastern Florida, in the West Atlantic Ocean, there is such a biotic reef parallel to the beach that is hundreds of meters wide and stretches more than 300 kilometers. This type of biotic reef is formed from the bonding of a large number of sand grains. Therefore, it is also referred to as a sand reef. Today, sabellarid reefs mostly develop along the coasts in temperate and tropical areas on average from the ocean surface to 10 meters in depth or more. In colder waters, sabellarid reefs mainly develop in the subtidal zone. In the past, sabellarid agglomerates have been collected from the seafloor at a depth of 100 meters. Some scholars speculate that this may be due to strong local seafloor currents.
Sabellarid reefs develop in agitated coastal waters or on rocky coasts with regular high-energy wave action. Colonizing sabellarid worms need sand particles (or shell fragments) brought by water currents to build their burrows and food. In agitated bodies of water are suspended sand particles. These worms continue to collect these and bond them together to create their burrows. At the same time, their metabolic waste is taken away by the strong currents.
On an ecological time scale, a large number of entangled and cemented tube-shaped burrows provide abundant hard substrates and result in complex three-dimensional structures, which can serve as habitats for many benthic organisms to attach, incubate, and grow, similar to the ecological functions of tropical coral reefs. On a longer geological time scale, sabellarid reefs act as barriers for landside beaches behind them. Sabellarid reefs that develop in the intertidal zone can slow the rate of erosion of coastal rocks that serve as epiphytic substrates. Therefore, the continued development of sabellarid reefs can protect coastlines and change coastal topography. They serve as barriers, enabling the gradual accumulation of sand on the landside behind them to promote the seaward accretion of sandy coasts.
In 2009, for the first time, scholars from Taiwan (Chien-Hsun Chen and Chang-Feng Dai) reported large numbers of Idanthyrsus pennatus (Peters, 1855) on seawall tetrapods with sabellarid reef formation along the Nanbin and Huaren coasts south of Hualien Port. This is the first scientific record of a sabellarid reef dominated by the burrows of Idanthyrsus pennatus in the West Pacific. (Before that, there were no living or fossil records.) This reef developed on seawall tetrapods at a depth of 1-7 meters. The surrounding seafloor is of sandy sediment. The average thickness of sabellarid reefs is around 10 centimeters. At depths of 3-5 meters, sabellarid reef thickness can be up to 50 centimeters. Based on the amount of time since the placement of the tetrapods, the average accretion rate of these reefs is 4.2-10.0 centimeters per year. If we factor in the damage to the reefs by typhoons each year, these sabellarid reefs were constructed very quickly. According to survey results, the density of sabellarid worms is 5,400 per square meter, gradually decreasing with increasing water depth.
Exhibited here is sabellarid reef specimen obtained from the subtidal seawall near the Nanbin coast in Hualien County. Figures 1-3 are of the same specimen taken at different magnifications. By observing this sabellarid reef specimen, it is possible to see that Idanthyrsus pennatus is not very selective in terms of the construction of its burrows. It uses different types of sediment particles suspended in the water. The diameter of is burrows is less than 1 centimeter and thickness is 0.2-0.3 millimeters, with smooth internal walls and coarse exterior. On the exterior of the burrows are attached growing benthic foraminifera (small, pink, lump-like protrusions in Figures 2 and 3), encrusting bryozoans, and small polychaete calcareous burrows (indicated by yellow arrows in Figure 3), barnacles, and oysters.
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Scientific name: Idanthyrsus pennatus (Peters, 1855)
Phylum Annelida
Class Polychaeta
Order Sabellida
Family Sabellariidae
Genus Idanthyrsus
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Published studies on Taiwan area sabellarid reefs:
Dai, C-F., et al., 2014 Regional oceanography of Taiwan. National Taiwan University Press, Taipei: pp.292-293 (in Chinese).
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