• Why are my Jellyfish Shrinking?

    Why are my jellyfish shrinking? Help!

    This is the most popular question I have received in the last year as folks start to venture into keeping jellyfish as pets. As there are many reasons why captive jellies tend to shrink, let’s start with some basics. First, the one thing I feel most don’t realize is that jellies occur in all oceans of the world and in all temperature zones. There are tropical jellies, temperate jellies and arctic jellies. Each species requires specific water quality parameters, food and care. I will zone in on warm water jellies (77º-80ºF) and cold water jellies (63º-65ºF) for this article as those are the most common being kept as pets these days. And more specifically, moon jellyfish, or moon jellies as they are more appropriately being referred to as the term “jellyfish” is outdated due to the fact that they are not fish at all, but belong to the invertebrate phylum Cnidaria, as do corals and anemones.

    Shrinkage in any living plant or animal generally indicates malnutrition and/or unhealthy living conditions. The unfortunate thing about jellies is that the captive system in which they are being kept (some faction of a plankton kreisel system) maintains a false current, laminar flow characteristic, or circular dynamic to the water flow. So, when a jelly is malnourished and unhealthy, it will still circumnavigate the tank and appear to be “alive” simply because it is going “around” the tank. Unlike fish, that will float or sink, or corals that begin to bleach indicating there is a problem.

    I think the best thing for newbie jelly keepers to do is familiarize themselves with what a healthy moon looks like versus an unhealthy one. Jellies can be resilient if the problem is dealt with quickly and upon immediate observation. Below are a few pictures of unhealthy moons. Note the oral arms hanging down, the curled under bell margin, the lack of visible tentacles and the opaqueness of the bell itself. You may also see holes in the bell or non-uniformity to the shape of it.


    When you have a healthy moon you will visibly see the radial canals through a clear bell, uniformity to that bell, tentacles extending, oral arms held up, not dangling and some coloration from their food source indicating that it is being fully metabolized and utilized throughout the animal. See below for some images of healthy moons.


    Upon noticing some unhealthy issues you can review some of your water quality and feeding habits and fix them. Inadequate acclimation can also have stressed the animal when you received it. Take your time acclimating your new jellies to their new home. Improper foods and feeding habits are a major concern. Most foods on the market are fantastic for fish, but not for jellies. Feeding acidic foods with low pH, target feeding that food, or target feeding in general, can stress the animal. These guys are 96% water and they utilize everything they eat. So, if it is of poor quality or acidic in anyway, they will suffer and eventually show signs of it by not eating and therefore, shrinking. If you are making up your own saltwater be sure to use RO/DI and aerate it for 24 hours prior to using it. Don’t do more than 10% water changes at a time as that can stress them out—they prefer stability. They don’t have a buffering system---just raw cells, so don’t throw them into another universe of water quality thinking you’re doing them a favor by doing a large water change. Take a few days and make several small water changes if you need to increase the quality of their environment.

    Holes in the bell can be caused by small bubbles introduced into their system that get trapped in their bell or radial canals and then work their way out through the mesoglea, another word for jelly tissue. Holes can also appear from bacterial infections generally caused from the use of acidic foods or foods that haven’t been rinsed properly with system water prior too use. And, naturally, holes can be physically created if there are any obstructions in the tank at all like pebbles, gravel, or sand.

    So, all in all, jellies are here to stay as long as we, as jelly keepers, begin to understand these delicate creatures, their requirements and proper handling. Salinities, pH, and temperatures vary with warm and cold water moons. Know what your jelly requires and take it slow. We all need to be responsible pet owners. If you are having problems, there are more and more sites, blogs, and articles being written about the subject. This is great because you can be more informed than ever before about keeping jellies as pets. Pets rely on us to take care of them, so here’s to animal and jelly welfare! Happy jelly keeping!

    Related topic:
    A Few Things to Know before owning a Jellyfish Tank
    Comments 2 Comments
    1. Zoanthrope's Avatar
      Zoanthrope -
      Wow. Have not seen jellies at the LFS. Nice to know there is another "final frontier" critter out there.
    1. SantaMonicaHelp's Avatar
      SantaMonicaHelp -
      First time seen a Jellyfish picture like this!