Sunday, January 27, 2013

Hydrozoans (Phylum Cnidaria: Class Hydrozoa) of Singapore

Hydrozoans (phylum Cnidaria, class Hydrozoa) are radially symmetrical animals which 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 can perform other functions apart from stinging.

Hydroids (Lytocarpus sp.)
The body of an individual hydrozoan comprises a stomach (coelenteron) and a mouth surrounded by tentacles. They can be solitary or colonial. The above photo shows a colonial hydrozoan with a chitinous skeleton, exhibiting a plant-like appearance. Other sessile colonial species may build a hard calcareous skeleton instead.

Mature hydrozoans may exist either as a polyp or a medusa (more precisely a hydromedusae), or alternate between the two forms. A polyp is sessile, and has a cylindrical body column housing the stomach.

Hydromedusae (Aequorea pensilis)
A hydromedusa has an umbrella-shaped bell housing the stomach. Unlike most other cnidarians, the medusae of hydrozoans may not always occur as free-living jellyfish. In many colonial species, they just occur as buds on the surface of the colony for sexual reproduction purposes. And while an individual polyp is sessile, some species form motile colonies that drift on the surface of the water, along with the currents, wind and tides.

Hydrozoans may either feed on plankton by gathering them with their tentacles, or sting and paralyse bigger prey. Many hydrozoans can sting very painfully, and some may result in serious allergic reactions in human, though death cases are rare.

Hydrozoans can reproduce asexually or sexually. The polyps can clone themselves to produce more polyps, or produce medusae by budding. The medusae may be free-swimming jellyfish, or as mentioned earlier, remain attached to the colony in some colonial species. The medusae is the sexually-reproductive stage of the hydrozoans' life cycle, and they have separate sexes. The eggs and sperm are broadcast into the sea for external fertilisation. The fertilised egg develops into a free-swimming larva, and depending on the species, will eventually either settle on a suitable substrate to develop into a polyp and or a medusa. For colonial species, the polyp will clone itself to produce more polyps, and in some species, may build either a chitinous or calcareous skeleton.

The hydrozoans that I have seen in Singapore are mostly from the subclass Hydroidolina. Members of this subclass are commonly called hydroids. Unfortunately, I am unable to identify most of them, as I couldn't seem to find resources on the hydroids of Singapore. If you are able to identify any of those which I couldn't identify or spot any mistake, please do drop me an email!

Hydromedusae (Aequorea pensilis)
The hydromedusae, Aequorea spp., are sometimes seen on our shores. They have a glassy transparent appearance, and a shallow furrow is present between the stomach and the margin of the bell. Radial canals, appearing like lines to the naked eye, mark the furrow. Based on Nicholas' and Joo Yong's paper in 2012, tThe above hydromedusa should be Aequorea pensilis, which has up to 160 or more radial canals. Contact with this hydromedusa may result in sharp itchy rashes with light or intense swelling that can persist for several hours.

Hydroids (Lytocarpus sp.)
Lytocarpus spp. are hydroids with feather-like appearances. They are known to give painful stings.

Hydroids (Class Hydrozoa)
This unknown hydroid species forms pinkish red colonies.

Hydroids (Class Hydrozoa)
This is yet another unknown hydroid with orange-coloured colonies.

Hydroids (Class Hydrozoa)
I also could not identify this yellowish hydroid with long and flimsy-looking colonies.

The above unidentified hydroid colony is quite small - about 5cm tall.

Hydrozoan Polyps
From some of the resources I have found online, the whitish, flower-like organisms are likely to be hydroids as well.

This hydroid with yellowish to brownish polyps can be quite abundant on some of our shores.

  • Chou, L. M. 1998. A Guide to the Coral Reef Life of Singapore. Singapore Science Centre. 128 pp.
  • Erhardt, H. and D. Knop. 2005. Corals: Indo-Pacific Field Guide. IKAN-Unterwasserachiv, Frankfurt. 305 pp. 
  • Ruppert, E.E. and R.D. Barnes. 1991. Invertebrate Zoology (International Edition). Saunders College Publishing. U.S.A. 1056 pp.
  • World Register of Marine Species. 2012. Retrieved Jan 15, 2013, from
  • Yap, W. L. N. & J. Y. Ong. 2012.  A survey of jellyfish (Cnidaria) around St John's Island in the Singapore Straits. Contributions to Marine Science 2012: 57–74

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