Thursday, August 09, 2012

Horseshoe Worm (Phylum Phoronida) of Singapore

Horseshoe Worms (phylum Phoronida) are marine worms with a crown of tentacles (lophophore) arranged in a somewhat horseshoe shape if viewed from the top (hence the common name), appearing like two separate crowns but are actually connected as one. They live in tubes made from chitin (like our finger nails) that support and protect their soft bodies. Horseshoe Worms superficially resemble the annelid tube worms, but their bodies are not segmented, unlike the latter. They are filter feeders, and gather tiny plankton or other organic matter with their tentacles.

Black Horseshoe Worm (Phoronis australis)
Horseshoe worms perform respiration through the lophophore, and while they are heartless, the main blood vessels can contract in waves to transport the blood, which interestingly contains hemoglobin (like our blood). They mostly reproduce sexually externally, broadcasting their eggs and sperms. Some are hermaphroditic possessing both male and female reproductive organs, while others are dioecious with separate sexes. One species is known to reproduce asexually by budding or splitting, and generally horseshoe worms are able to regenerate lost body parts readily.

Black Horseshoe Worm (Phoronis australis)
One species, the Black Horseshoe Worm (Phoronis australis), has been recorded in Singapore. This species lives commensally with various species of tube anemones from the family Cerianthidae by boring into the walls of their tubes. While the tube anemone does not appear to benefit or get harmed by this relationship, the horseshoe worms get a good foothold, food (bits and pieces collected and dropped by the tube anemone) and protection (the tube anemone's stinging tentacles, and when there is danger, the tube anemone will retract its tentacles, providing an alarm signal for the horseshoe worms to retract their tentacles as well).

Black Horseshoe Worm (Phoronis australis)
As many as 100 worms may be hosted by a tube anemone, though in Singapore the most I have personally seen is about 50 worms. This species can be found all over the world in the tropics and even temperate regions.

  • Emig, C.C. (2003). Phylum: Phoronida. In B. Grzimek, D.G. Kleiman, M.Hutchins. Grzimek’s Animal Life Encyclopedia. 2: Protostomes (2 ed.). Thompson Gale. Pp 491–495.
  • Stampar, S., C.C. Emig, A.C. Morandini, G. Kodja, A.P. Balboni & F.L.D. Silveira (2010). Is there any danger in a symbiotic species associated with an endangered one? A case of a phoronid worm growing on a Ceriantheomorphe tube. Cahiers de Biologie Marine, Mar. 51: 205–211

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