Pete Giwojna

Dear Michelle:

It’s very difficult to say if the colonies of almost microscopic little palm trees you noticed could be hydrozoans because hydroids appear in our aquariums in so many different forms. I can tell you that it is uncommon for hydroids to appear and grow in a newly established aquarium that is not receiving daily feedings of newly-hatched brine shrimp (Artemia nauplii) or some other planktonic prey (e.g., copepods or rotifers). As a rule, they normally only become problematic in nursery tanks and dwarf seahorse tanks where newly-hatched brine shrimp are abundant, so if you’re new 20-gallon aquarium has not been getting daily feedings of brine shrimp or other planktonic live foods, then I would think it was unlikely for hydroids to become established in such a set up.

But judging from your description, it’s certainly possible that your miniscule palm trees are newly formed polypoid hydroid colonies. I would test them to make sure by providing a heavy feeding of newly-hatched brine shrimp to your 20 gallon aquarium and then watching closely to see if the tiny colonies of palm trees are able to ingest it.

If the palm trees are feeding heavily on brine shrimp, you should know it by the change in their coloration. Hydroid colonies are typically transparent or a translucent milky white in appearance. The hydromedusae stage is clear as glass and many times the polyps appear translucent at first as well. Until they begin eating, that is — once they begin gorging on newly-hatched brine shrimp, the colonies often take on an orange or pinkish hue, or assume various shades of brown or yellow. So watch for a change in the coloration of the palm trees after you add the newly-hatched brine shrimp, and look closely to see if you can detect any of the baby brine shrimp adhering to the palm trees (a magnifying glass will be most helpful for this).

In the colonies of tiny palm trees are preying on the baby brine shrimp, then there’s a very good chance that they could be hydroids or stinging animals of some sort, and they should be eliminated from your dwarf seahorse tank. If not, then they are probably harmless filter feeders that strain bacteria and other microscopic organisms from the water or that feed via photosynthesis.

If you treat your tank with fenbendazole (brand-name Panacur), Michelle, it won’t harm your maiden’s hair but it will certainly destroy the nudibranchs and tubeworms or feather dusters that are living in it. So let’s make doubly sure that your little palm tree colonies are hydroids before you eradicate them. Here’s some additional information on hydroids that may help you identify them:


It can be very challenging to identify hydroids because there are about a zillion different species of hydrozoans and the different types have different characteristics and are often vary remarkably in appearance. There is considerable variation within the species as well, and the same type of hydroids can appear vastly different depending on the size of the colony and its stage of development, conditions in the aquarium, and their predominant diet. And, of course, the different stages of the life cycle of these amazing animals are so entirely different that they were long believed to be different types of cnidarians altogether, and different species names were often assigned to the same hydroid in different phases of its life cycle.

The typical hydroid colony has a stem with a variable number of polyps growing on it, and each of these polyps bears numerous tentacles that are liberally studded with knobby nematocysts (batteries of deadly stinging cells). There are many different kinds of hydroids and they appear in the aquarium in many different guises: many colonies are stalked; some have fingerlike projections, others look like tiny pink fuzzy balls or appear like cobwebs (the webbing kind usually spread along the bottom or grow on the aquarium glass along the substrate). The "snowflake" type of hydroids seem to be particularly common in aquaria, whereas other species look more like crystal chandeliers, and some species form bushy colonies as they grow that serve as microhabitats for Caprellid skeleton shrimp and other tiny crustaceans.

Even a large hydroid colony appears harmless to the naked eye of the untrained observer. It takes a much closer look to reveal the dreaded ‘droid’s lethal nature, as described in The Complete Guide to Dwarf Seahorses in the Aquarium:

"Studying the colony under high magnification, one soon becomes lost in an extraordinarily complex, living world–a microcosm in which a beautiful but deadly ballet is conducted on a microscopic scale (Rudloe, 1971). Hungry polyps, some resembling snapdragons, others looking more like daisies or tulips, expand their knobby, translucent tentacles, slowly flexing and languidly waving them about, lulling the observer with their slow-motion ballet — until they abruptly and quite unexpectedly snap up a bit of planktonic life, stinging it, drawing it in with one violent contraction, digesting it, and then re-expanding like a blossoming flower to hunt again (Rudloe, 1971). There are many such polyps in a colony, hundreds of them, each of which is armed with many tentacles and countless nematocysts, and at any given moment, some of them will be dormant and still, some will be expanded and lazily casting about for prey (Rudloe, 1971), and still others actively feeding (Abbott, 2003)."

The feeding or nutritive zooids are the distinct individual animals in a hydroid colony that are responsible for capturing and digesting prey; as such, they bear the nematocyst-studded tentacles. But you need high magnification in order to appreciate the true beauty of living hydrozoans, or to differentiate between different species of hydroids, or to observe the zooids going about their deadly business.

Hydroids are insidious because they start out so small and insignificant, yet spread so quickly under ideal conditions (e.g., a nursery tank or dwarf seahorse tank receiving daily feedings of Artemia nauplii). Many species can spread asexually by fragmentation as a microscopic speck of the parent colony. All of the troublesome types have a mobile hydromedusae stage, which look like miniscule micro-jellyfish, and can spread sexually in this way as well (Rudloe, 1971). The mobile medusae swim about with a herky-jerky, pulsating motion and are often mistaken for tiny bubbles due to their silvery, transparent, hemispherical bodies (Rudloe, 1977). These tiny jellies often go unrecognized until they begin to settle and are discovered adhering to the tank walls. They will have a large "dot" in the middle of their bodies and smaller ones at the base of their nematocysts (Abbott, 2003). Both the polyp stage and the medusa stage sting (Rudloe, 1977) and are capable of killing or injuring seahorse fry. Multiple stings can kill the babies outright, but they are often only injured by the nematocysts, which damage their integument and leave them vulnerable to secondary infections. Many times it is a secondary bacterial or fungal infection that sets in at the site of the injury which kills the fry.

Hydroids can gain entry into our aquaria in many ways. For example, they are notorious hitchhikers. Both the colonial polyp stage and the free-swimming micro-jellies can thumb a ride on live rock, macroalgae, hitching posts, sand or gravel, specimens of all kinds, or within so much as a single drop of natural seawater (Abbott, 2003). Beware of fuzzy looking seashells! Very often hydrozoans come in on the shells of the hermit crabs or snails we purchase as aquarium janitors (Abbott, 2003). Or they may be introduced with live foods, or even among Artemia cysts, in some cases it seems. They can even be transferred from tank to tank in the aerosol mist arising from an airstone or the bubble stream of a protein skimmer.

Best of luck with your new dwarf seahorses, Michelle! Here’s hoping your micro palm trees are harmless and that you never have to deal with an infestation of hydroids.

Pete Giwojna

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