A MOST UNUSUAL CLAM

This clam, which is commonly referred to as the “”watering pot clam” or the “water spout clam,” is one of the world’s most unusual clams [if not THE MOST unusual clam!]. Its early growth stage is totally different than that of the adult stage. Its shell begins as a normally shaped clam with two tiny embryonic valves, but they soon become covered as new shell is secreted over them. The right valve becomes incorporated into a thin long tube (a.k.a., siphonal tube), and the left valve becomes incorporated into an unusual-looking inflated disc with tiny perforations. The left valve also is vertically partially buried in sediment. For orientation purposes, the buried (bulbous) end is referred to as the anterior-most part of this bivalve, whereas the long tube is the posterior part. 

A

B

C

Three views of the same shell, A-C, (10 cm long = 4 inches) of Verpa  from the Singapore, China area.  A. Side view of entire shell. B. Posterior end of shell. C. Anterior end of shell (disk somewhat broken on its top edge).

As the shell of Verpa grows, it becomes more deeply burrowed, and the long-part of the tube can become bent or curved because of growth disturbances. The tube is not cemented to other shells nor to the substrate, but grains of sand and small pebbles and shell fragments may adhere to the thin calcareous tube. The end of the tube, that is buried in the sediment, ends in a fringed-thin disk, and beyond that, a bulbous disk that bears numerous tiny perforations (holes) open to the environment (like a shower head). This fringed disc with perforations is unique among bivalves. 

The shell tube can be up 12 cm (5 inches) long. This clam is a filter feeder that prefers to live among sea grasses growing on muddy/sandy ocean-floor conditions. This clam, furthermore, is known to occur in shallow-tropical waters from the Indian Ocean to the East Indies (especially the Singapore, China and the Philippines), but, apparently is becoming increasingly uncommon/rare [with some local extinctions having taken place]. Although five species have been reported in the literature over the last many years, it might be that this bivalve is on the verge of going extinct.

Classification: 

Phylum Mollusca

Class Bivalvia

Order indeterminate (not well established) possibly Anmalodesmata

Family Clavagellidae 

Genus Verpa [see WoRMS, 2024 at---http://www.marinespecies.org]

Species: Five named species and many synonyms.

The earliest known clavagellids that might be ancestral to Verpa are of late Mesozoic age (100 to 66 million years ago).

If you want to learn more about tube-dwelling bivalves and their adaptations, read the very interesting paper by Savazzi (1982). [It is a useful and interesting guide to all of the various types of these bivalves]. Note: When Savazzi wrote his paper, he used used the now no-longer-used name "Penicillus" for the tube-dwelling bivalve discussed in this present blog.

Reference: 

Savazzi, E. 1982. Adaptations to tube dwelling in the Bivalvia. Lethaia, v. 15, no. 3, pp. 275–297.

Additional Useful Literature:

DeLany, S. T. 1971, “It’s a Bivalve, You Know.” The Tabulata, April 1, 1971, pp. 20-21.