Monday, October 08, 2012

Murexes, Rock Shells & Drills (Phylum Mollusca: Family Muricidae) of Singapore

The muricids are often called "rock shells" as they are found on rocky shores, or "drills" as they bore holes on barnacles and other shelled molluscs to feed on them; or just "murex" for the ones with spines. Some species feed on clams by prying the shells open; some species secrete an acid to soften the shells of their prey, and use their radula to scrape a hole through the shells, taking hours in the process, so as to access the meat; yet other species may just feed on other small invertebrates.


The shells vary from species to species, and in fact even within a species they may vary. Usually, they have a raised spire and strong sculpture.  The varices of many species are thick and obvious, and come with spines, tubercles or blade-like structures. The aperture has a obvious and usually deep siphonal canal. The shells are generally thick and strong, and the aperture is corneous.


The ones found on our intertidal areas lay masses of yellow egg capsules which turn purple when they hatch. Other species in deeper waters, however, are known to lay egg capsules of other colours and shapes. Since ancient time, a valuable reddish purple dye had been extracted from various species of murex. Some of the bigger species are also collected for food.

Here are some of the muricids that I have seen on our shores (thanks to Siong Kiat who gave tips on how to identify them!). You may want to take a look at my diagram on the parts of a snail's shell if you are not familiar with the names of the parts, so as to better understand the terms used below.

Subfamily Muricinae

Members from this subfamily are usually elaborately spined and they develop fronded varices (usually three varices per whorl). They usually feed on bivalves and snails, and some may even feed on other snails of the same species.

Rare-spined Murex (Murex trapa)
The Rare-spined Murex (Murex trapa) is seldom encountered in the intertidal area, but sometimes seen in deeper waters on sandy-muddy substrates. It has finely beaded spiral threads and long spines on the varices and the long siphonal canal. They feed on other snails and clams by secreting an acid to soften the shells and "drilling" small holes to access the meat with their radula. This species is often collected for the shell trade, and sometimes collected for food. It grows up to more than 10cm.

Ramose Murex (Chicoreus ramosus)
The Ramose Murex (Chicoreus ramosus) is the biggest murex species in the Indo-Pacific, and can grow to more than 30cm long! It has a short spire, and the varices have strong frondose spines, with the ones near the spire curving backwards.

Ramose Murex (Chicoreus ramosus)
The siphonal canal of the Ramose Murex is moderately long and broad. The aperture is white inside with a pinkish margin. It is usually found in sandy areas, sometimes partially buried. It is widely collected in the region for food.

Fireband Murex (Chicoreus torrefactus)
The Fireband Murex (Chicoreus torrefactus) has a somewhat elongated shell and moderately short and broad siphonal canal. The spire is relatively high and acute. The varices are crowded with short, branching spines. The colour of the shell varies, depending on the presence of any encrusting organisms, ranging from bright colours such as pink, red or orange to dull colours such as brown or black.

Fireband Murex (Chicoreus torrefactus)
The aperture is white, often with yellow to orange lips. They are found among rocks or corals, and can grow to about 7cm long. It is often collected for food and shellcraft.

Burnt Murex (Chicoreus brunneus)
The Burnt Murex (Chicoreus brunneus) can appear rather similar to the previous species, but has several big bumps on the shell.

Burnt Murex (Chicoreus brunneus)
A big lump can be found just above the aperture. The varices have thick, short, branching spines. This species is usually found among rocks and corals in shallow waters, and can grow to about 10cm long. it is also collected for food and shellcraft.

Mangrove Murex (Chicocerus capucinus)
The Mangrove Murex (Chicocerus capucinus) lacks the elaborated spines found in the previous species, and is usually found on mudflats or in mangroves. It still has three thick varices per whorl, and the shell is often coated in mud, growing to about 5cm in length.

Mangrove Murex (Chicocerus capucinus)
This murex has been observed to feed on barnacles, bivalves and small snails.

Subfamily Rapaninae

Members of this subfamily have a distinctive ridged, corneous operculum, and have a raised spire with strong sculpture, sometimes with spines and nodules but lack varices. They are commonly called rock shells as they are often found o rocky shores, and may be covered with encrusting animals. The sipphonal canal is short.

Bituberculate Rock Shell (Thais bitubercularis)
The Bituberculate Rock Shell (Thais bitubercularis) is commonly found on sea walls and big boulders. The shells are very variable, and may or may not have spines. It is believed to feed on barnacles, though no studies have been done to confirm this as yet. They can grow to about 5cm long.

Spired Rock Shell (Thaisella gradata)
The Spired Rock Shell (Thaisella gradata) appears as if it is composed of two triangles looking from the top, with the spire about the same length as the body whorl. They are often seen among barnacles on huge rocks or the seawall, and hence it is possible that they feed on barnacles.

Lacerated Rock Shell (Thaisella lacera)
The Lacerated Rock Shell (Thaisella lacera) can be found in rocky areas, or around loose pieces of rocks in seagrass meadows or sandy areas.

Lacerated Rock Shell (Thaisella lacera)
The shell is relatively broader, and the body whorl revolve relatively further away from the spire. The outer lip comes with a few short spines.

Subfamily Ergalataxinae

Members of this subfamily are small to medium sized. They lack varices, but often have small spiral rows of nodules. They usually occur on rocky and sheltered shores. Some species are called "drills" as they drill through the shells of oysters and other sessile shelled-animals to feed on them.

Musical Drupe (Morula musiva)
The Musical Drupe (Morula musiva) is a small snail up to about 2cm long, easily identified by the alternating rows of black nodules and brown nodules. It is commonly found on sea walls and rocky shores with big boulders.

Musical Drupe (Morula musiva)
The aperture is white with brown patches. It feeds on snails and clams, such as false limpets and oysters, by making circular holes through their shells. The body of the animal is greenish.

Rumphius Drupe (Morula rumphiusi)
The Rumphius Drupe (Morula rumphiusi) is smaller, usually about 1cm long. It has spiral rows of nodules of the same colour. The lips are dark brown.

Shouldered Castor Bean (Drupella margariticola)
The Shouldered Castor Bean (Drupella margariticola) is another small snail with spiral rows of nodules. It has a distinctive purple aperture with a few small bumps on the columella near the siphonal canal. The body is greenish. Studies have shown that it eats a variety of animals, such as corals, barnacles, bristleworms, clams and snail eggs. It is also capable of drilling holes through the shells of its prey.



References
  • Abbott, R. T., 1991. Seashells of Southeast Asia. Graham Brash, Singapore. 145 pp.
  • Carpenter, K. E. and V. H. Niem (eds). 1998-2001. FAO species identification guide for fishery purposes. The living marine resources of the Western Central Pacific. Volumes 1 to 6. FAO, Rome. pp. 1-4218.
  • Gladys Archerd Shell Collection at Washington State University Tri-Cities Natural History Museum. 2008. Retrieved September 26, 2012, from http://shells.tricity.wsu.edu/ArcherdShellCollection/ShellCollection.html.
  • Oliver, A. P. H. 2012. Philip's guide to seashells of the world. Philip's, London. 320 pp.
  • Tan, K. S. & L. M. Chou, 2000. A guide to common seashells of Singapore. Singapore Science Centre, Singapore. 168 pp.
  • Tan, S. K. & H. P. M. Woo, 2010. A preliminary checklist of the molluscs of Singapore. Raffles Museum of Biodiversity Research, National University of Singapore, Singapore. 78 pp. Uploaded 02 June 2010.
  • Tan, S. K. & R. K. H. Yeo, 2010. The intertidal molluscs of Pulau Semakau: preliminary results of “Project Semakau”. Nature in Singapore, 3: 287–296.
  • World Register of Marine Species. 2012.  Retrieved Oct 3, 2012, from http://www.marinespecies.org.

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