bivalves text index | photo index
Phylum Mollusca > Class Bivalvia > Family Malleidae
Hammer oyster
Malleus sp.
Family Malleidae
updated Oct 2016
Where seen? The strange T-shaped clam is commonly seen Cyrene Reefs, lying freely among the seagrasses sometimes partially buried in the sand with the T-shaped portion of the shell below, anchoring the clam. The clams are said to be found in colonies. Some are shaped like tongue depressors or spatulas and seen wedged upright crevices among rocks and rubble. According to The Gladys Archerd website, most live in the crevices of coral rocks or on reef flats in tropical regions. Malleus are NOT true oysters which belong to Family Ostreidae.

Features: 8-12cm. The two-part shell is thick and some are obviously T-shaped. 'Malleus' means 'hammer' or 'mallet' in Latin. The hinge is on the 'horizontal' portion of the 'T' and the valves held shut by one large adductor muscle that lies at the cross of the 'T'. It has wavy edges along the 'vertical' portion of the 'T'. The outer shell is often encrusted with calcareous algae and other organisms. The inner shell is partially lined with mother-of-pearl. The 'T' shaped part of the shell anchors the clam. Byssus threads are produced near the hinge.

Those shaped like tongue depressors stuck in crevices are sometimes confused with
Wader tree oysters (Isognomon isognomum) that have a similar shape and also stuck upright in crevices. It is difficult to tell them apart without ripping them out of their hiding place and looking at the inside of the shell. On the inside, Hammer oysters have a small depression at the hinge and a small area of mother-of-pearl, relative to the shell length.

Human uses: In Indonesia and the Philippines, they are collected. The shell may be used for shellcraft or as lime.

Sometimes partially buried.
Changi, May 12

Pulau Semakau, Feb 09

Spatula-shaped clams seen sticking out
from among rubble and rocks.
Pulau Semakau, Feb 09

Sometimes seen lying on the ground.
Cyrene Reef, Jul 09

A look at the inside of a dead clam.
Terumbu Semakau, Dec 11

Hammer oysters on Singapore shores

Photos of Hammer oysters for free download from wildsingapore flickr

Distribution in Singapore on this wildsingapore flickr map

Changi, Jun 10
Photo shared by Loh Kok Sheng on his blog.

St. John's Island, Feb 11
Photo shared by Loh Kok Sheng on his blog.

Terumbu Semakau, Nov 12
Photo shared by Loh Kok Sheng on flickr.

Pulau Sudong, Dec 09

Terumbu Berkas, Jan 10

Terumbu Salu, Jan 10

Pulau Salu, Jun 10

Pulau Senang, Jun 10



  • Tan Siong Kiat and Henrietta P. M. Woo, 2010 Preliminary Checklist of The Molluscs of Singapore (pdf), Raffles Museum of Biodiversity Research, National University of Singapore.
  • Abbott, R. Tucker, 1991. Seashells of South East Asia. Graham Brash, Singapore. 145 pp.
FREE photos from wildsingapore tagged with Malleidae. Make your own badge here.
links | references | about | email Ria
Spot errors? Have a question? Want to share your sightings? email Ria I'll be glad to hear from you!
wildfactsheets website©ria tan 2008