Digitate hydroids

Identification

Single, unbranched finger-like polyps: a thin translucent to white stalk, from a few millimeters to several centimeters when fully extended, tipped with a small bulb or knob of stinging tentacles. They extend mainly at night to feed and retract completely into rock crevices, frag plugs and even sand when lights come on. Bases look like tiny root-like plates on the surface they anchor to.

Often confused with

Colonial hydroids — colonial forms grow as linked mats or feathery branching colonies from a shared creeping base, while digitate hydroids are separate single fingers, each anchored alone. Also confused with vermetid snail tubes (hard, fixed, with a mucous web rather than a soft retracting finger) and with ctenophores or sponge tentacles on coral; the nightly extension and fast full retraction point to a hydroid.

What it targets

No single host. They are filter feeders, but their sting irritates any coral they touch — zoanthids held closed are the most common complaint — and heavy infestations can sting small fish and seahorses.

How it spreads

Hitchhikes on live rock, frag plugs and macroalgae, and some hydroids pass through filtration in a free-swimming medusa stage. Blooms often follow the nutrient-rich early months of a tank and spread from crevices outward.

Control (safest first)

Watchful waiting is a legitimate first move for a few hydroids: many early blooms are self-limiting and vanish within weeks as the tank matures. Escalate when numbers climb into the hundreds or corals stay closed, because at pest density they rarely fade on their own. Manual removal: pinch or tweeze the stalk off at the base repeatedly — a Reef2Reef keeper found that pinching a few times makes the animal abandon the spot and move, leaving its foot exposed to tweezers or a hermit. Seal stubborn bases and any escape holes with reef epoxy. Nutrient and feeding reduction: hydroids farm planktonic food, so cutting broadcast feeding, improving skimming and siphoning detritus slows them. UV sterilization is suggested by some keepers against the swimming medusa stage; reports are anecdotal. Predators: Halloween hermit crabs are the most-cited helper and one keeper credited them with real control over about a year, though they can only take hydroids whose foot is exposed in the open. Hydroid-eating nudibranchs exist but are rarely available in the trade, and Berghia and peppermint shrimp do not eat hydroids. Fenbendazole (fish bendazole / Panacur-type dewormer) is the heavy option keepers report clearing digitate hydroids, and it is NOT reef-safe as a casual dose: it lingers in rock, can kill snails (turbos may need months before reintroduction per the Reef2Reef fenbendazole thread), some corals and other worm fauna, and one keeper who accidentally dosed at ten times the intended rate described months of recovery. Use only with livestock moved out and a full protocol in hand.

Natural predators

• Halloween hermit crab→
• Lynx / fringeback nudibranchs

Field notes

A Manhattan Reefs keeper's digitate hydroids — clear stem, bulb tip, out only at night — disappeared on their own and never returned, while the colonial hydroids in the same tank took over a rock within a month and were finally torched off outside the tank; the two outcomes in one system explain most of the conflicting advice online. A Reef2Reef success-stories thread shows the middle path working slowly: about a year of pinching stalks so the animals moved into the open, a Halloween hermit picking off exposed ones (more effective on the colonial form), and epoxy for the holdouts — control achieved, total elimination not claimed. The opposite outcome is also on record: a keeper told to wait it out ended up with hundreds of digitate hydroids across a 20-gallon tank, growing from the sand and rocks and keeping zoanthids shut, with no single base to glue over.