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Phylum Arthropoda > Subphylum Crustacea > Class Malacostraca > Order Isopoda
Marine isopods
Order Isopoda
updated Mar 2020
Where seen? These tiny segmented animals are sometimes seen our shores, often swarming in numbers at low tide over rotting fruit or dead matter. The most commonly seen isopod on the beach are sea slaters (Ligia sp.) or sometimes called sea cockroaches although they are not insects and look nothing like cockroaches (if you ask me).

What are isopods? They are crustaceans like crabs and prawns. There are about 4,000 described species of isopods that live in the sea. Some are found in freshwater, and some are terrestrial (these are usually called wood lice or pill bugs). Most marine isopods are tiny (5-15mm long). But one deep sea isopod Bathynomus giganteus can grow to 40cm long! Many may be parasites on other marine creatures.

Some unidentified beach isopods
swarming over rotting fruit.
East Coast, Aug 09
Features: 1cm or less. Their segmented body is flattened downwards (instead of sideways for amphipods such as amphipods) with legs that are more or less similar. Isopoda meaning 'same feet' while Amphipoda means 'different feet'. Their eyes are NOT on stalks. Some can give a nasty bite.

What do they eat? Beach isopods are scavengers, nibbling on whatever recently died on the rocky shore. At low tide, they swarm over the beach looking for the recent dead. Some isopods found in the mangroves nibble holes in dead wood and can cause much damage to wooden man-made structures.

Sisters Island, Jan 11

Sisters Island, Jan 11

Cyrene Reef, Jul 11
Blood-sucking isopods: Some isopods are parasites of fishes. These usually have specialised piercing mouthparts for sucking blood and terminal hooks on their limbs for clamping onto their fish host. Some parasitic isopods attach to the gills of shrimps, feeding on the blood.

Fish isopod.
Chek Jawa, Aug 13

Chek Jawa, Aug 13

Fish isopod.

Terumbu Pempang Tengah, Apr 13
'Tongue-biter' isopods: Some isopods replace the tongue of a fish! From the Raffles Museum of Biodiversity Research "a parasitic isopod living within the mouth of the spotted scat. It has eaten away the scat's tongue (!) and has lodged itself in the tongue's place. No one really knows what the isopod/tongue lives off. Some studies suggest that they feed off the animals the fish eats; other studies suggest that are blood drinking mouth leeches within the fish."

Marine isopods on Singapore shores
On wildsingapore flickr

Other sightings on Singapore shores

Cyrene Reef, Aug 10
Photo shared by Marcus Ng on flickr.

Links References
  • Niel L. Bruce, Melissa B. Martin, Kerry A. Hadfield & Barbara F. Nowak, Redescription of the fish parasitic “tongue biter” Cymothoa rhina Schioedte & Meinert, 1884 (Crustacea: Isopoda: Cymothoidae) from Singapore, 29 June 2016, Raffles Bulletin of Zoology Supplement No. 34: 331–342
  • Edward E. Ruppert, Richard S. Fox, Robert D. Barnes. 2004.Invertebrate Zoology Brooks/Cole of Thomson Learning Inc., 7th Edition. pp. 963.
  • Lim, S., P. Ng, L. Tan, & W. Y. Chin, 1994. Rhythm of the Sea: The Life and Times of Labrador Beach. Division of Biology, School of Science, Nanyang Technological University & Department of Zoology, the National University of Singapore. 160 pp.
  • Jones Diana S. and Gary J. Morgan, 2002. A Field Guide to Crustaceans of Australian Waters. Reed New Holland. 224 pp.
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