Hydroids! What the heck are they? Not your standard everyday word that marine aquarists would include in their forum chats or conversation topics. However, you may know more than you think regarding these unique animals. By the Wind Sailor is a species of hydroid, which may not ring a bell to many of you, but if I were to say that the infamous and potentially life threatening Portuguese Man-O-War or the Sea Wasp were also a hydroid, you probably would do a double take. Well, the truth of the matter is they are both free floating hydroids, NOT jellyfish. You may also be surprised to know that many of our tanks contain one or more species of hydroid, visible or not. Hydroids have been around since the Pre-Cambrian period some 540 million years ago and their are roughly 10,000 species within the phylum Cnidaria.
Many species of hydroids may look similar in appearance to jellyfish but their appearance is only skin deep. Hydroids consist of a solitary polyp similar to anemones or a colony of polyps, evident in Stylaster or Millipora(fire coral) that secrete a carbonate skeleton giving it its rigid form. They contain a mouth, inner wall(gastrodermis), outer wall(epidermis) and jelly-like body(mesoglea). Tentacles, ranging in number and length, housing specialized cnidoblasts(nematocysts) are encapsulated for defensive and feeding purposes. The main difference between the solitary hydroid and the hydroid colony is the fact that colonial hydroids have numerous polyps, responsible for the overall health of the animal. Each polyp is interconnected by a tube or stolon which is responsible for food transport. If one or a few polyps becomes injured or dies, it doesn’t necessarilly mean the hydroid colony will eventually die, whereas if a solitary hydroid becomes injured or diseased, it means the whole hydroid dies.
There are two morphological forms of hydroids: the medusa and polyp stage. The medusa stage consists of a small, free swimming, translucent animal. Depicted with a bell-shaped body, the hyrdoid in this morphological stage is equipped with numerous tentacles armed with stinging nematocysts. These nematocysts are responsible for paralyzing food for them to feed on as well as protection. Some hydroids are more toxic than others as far as their toxic chemical makeup. Food collection can also be obtainable by releasing a mucus on the umbrella/bell shaped body. The plankton is then trapped and transferred to their mouth(gastrovascular cavity). Propulsion and movement are at the discretion of the current, however, they do have a velum, which is a thickened part of the hydroid’s bell that provides a series of expansions and contractions giving it some mobilization. Think of this stage as the hydroid being depicted as an upside down sea anemone.
The other stage is called the polyp stage. This stage in evident of the hydroid doing an about face and being tentacle side up as is the case with sea anemones. Attaching itself and becomming upright, sessile while standing on its pedal base, the hydroid will normallly bury itself in the substrate or afix itself to live rock or similar hard surface. For the most part these animals host zooxanthellae, which provides the hydroids with the nutrient and food needs. Since they house zooxanthellae, photosynthesis plays a major role in their having any success and thriving in a reef environment. Intense light is ideal and needed for the health and survival of a hydroid and its symbiotic zooxanthellae. As I stated earlier, it is in this morphological phase that each polyp is in contact with all the others through a stolon and they all work together as a whole. This is beneficial because if one part of the hydroid becomes infected or diseased, it doesn’t necessarilly mean that the whole colony will die. Also, nutrients, waste removal and other variables can be initiated and provided by one or a few polyps that will suffice the whole colony instead of having to make sure that each polyp is fed that is the norm for certain aposymbiotic corals like Tubastrea.
For those reefers out there that are hesitent as to whether to raise their hand or not to the question of…Do you have hydroids in your tank? and should you be nervous and seeking to rid your tank of these animals? The answer is yes and no respectively. Hydroids can enter your aquarium through the hitch hiking process as many living animals do, good or bad. Many times we can’t control this scenario! Often times we can not see the hydroids as their translucent color and small stature AVOID being discovered by the naked eye. As for trying to remove these animals from your live rock or corals, this may be like looking for a needle in a haystack. It is also not worth the efforts either as these represent just another form of filter feeder and acts as a water parameter gauge/monitor. The only trouble I see is if you have a colony of these taking up residence too close to other corals, especially delicate ones, the chance of these hydroids overpowering the coral by injuring or killing it due to strong nematocyst cnidoblasts becomes large. Because of this, it is best to try to reposition either the coral or hydroid before this interaction unfolds and the risk of something bad happens.
Many times we, as conscientious marine aquarists, fail to recognize just how valuable a particular organism can be in our aquarium because we are misinformed or do not take the necessary steps to safe guard or realize the benefits of having the particular organism(s) in our tank. Failure to heed these critical observations could pose more damage than good by trying to remove the animal. Having a stable and thriving reef tank is all too often a mute point because we over dramatize and take steps that work unfavorably in our attempts to create the best case scenario for out livestock in a closed system environment.
The Complete Book of the Marine Aquarium by Vincent B. Hargreaves pp. 196-198