Pyramidellid snails (pyramid snails)

Identification

Tiny snails, about 2-8 mm long, white to pale cream, with a slender, elongated, slightly tapering shell shaped like a miniature rice grain or pyramid. By day they hide at the base of the clam, in shell folds and in the byssal area; after dark they line up along the top of the shell just below the mantle to feed. Look for them with a flashlight a few hours after lights-out, plus clear jelly-like egg blobs on the shell near the base.

Often confused with

Harmless baby cerith snails and Collonista grazers, which wander rock and glass grazing algae and never line up on a clam mantle at night. Rissoid and other small white snails cause similar confusion; behaviour on the host at night and the elongated pyramid shell separate the parasites.

What it targets

Tridacna giant clams first, and also large snails such as Turbo and Trochus, plus occasional reports on other molluscs and feather duster worms. Feeding irritates the mantle, causes poor extension and gaping, and heavy infestations kill clams, especially small ones.

How it spreads

Almost always arrives on an infested clam or large snail. They reproduce readily in aquaria and can build to hundreds on a single host, spreading to other clams in the same system.

Control (safest first)

Manual removal and shell scrubbing: lift the clam out into a container of tank water, pick off every snail and scrub the whole shell, especially the base and byssal area, with a soft toothbrush to remove egg masses. Repeat every few days at first, then weekly, for about a month. This is the method every source rates as the real cure. Night inspections with a flashlight: snails missed by day are visible lined up below the mantle after dark, which is also the best moment to check other clams and large snails in the tank. Quarantine and inspect every new clam before it joins the display, including a scrub of the shell base: prevention is far easier than clearing an established population. Wrasse help (see predators): useful support that knocks numbers down, with the caveat from keepers that the snails feed at night while most wrasses sleep, so fish alone rarely finish the job. Avoid chemical dips on the clam itself: sources warn anything strong enough to kill the snails risks harming the clam, and no dip reaches eggs hidden in shell folds.

Natural predators

• Sixline wrasse→
• Eight-line wrasse
• Green wrasse

Field notes

One keeper described pulling hundreds of snails off several clams on the first night, then only eight across all clams the next cleaning, and planned every-other-day scrubbing for a month before dropping to monthly checks; the most affected clam opened fuller than it had in weeks once cleaned. Another keeper used the grazing Trochus snails as collection points: the parasites rode up onto the Trochus shells at night, so the keeper lifted each Trochus out and rubbed the tiny snails off by thumb, reporting the population fell from dozens per snail to an occasional one or two, while a sixline wrasse in the same tank made little visible progress. Several keepers only connected the dots after losing a small maxima clam that had never settled, realising in hindsight the parasites had likely arrived with one of the clams and gone unchecked because daytime inspections showed nothing.