Tiny Mobile Snacks: 'Pods and Mysis (March '07)

[zanzimog]

Copepods, Isopods, Amphipods, Mysis
This month's article deals with one of the middle steps in the marine aquarium food chain. In terms of phylogenetic relationships, these guys are all over the map, but they are critically important to the health of your tank, turning microalgae, detritus and debris into healthy snacks for your fish and other inhabitants.

Phylogeny/Biology
All four groups are arthopods (phylum Arthropoda), having crunchy segmented bodies and jointed limbs. Amphipods, isopods and mysis all belong to the class Malacostraca, along with shrimp and true crabs, but each has their own order (Amphipoda, Isopoda and Mysidacea, respectively). Copepods have their own class, the Copepoda. Each group is large and diverse, ranging from about 800 species of mysids to 8500 species of copepods. This means that this article only covers a small sliver of the massive diversity of these groups.

With the possible exception of the mysids, all groups have parasitic species. That often puts people into a panic when they see little bugs, but the vast, vast majority of species we see are either herbivores or detritivores. Suspicious isopods, like this one found by joephys, occasionally crop up with new live rock or live sand, and an occasional newly imported fish will crop up with parasitic copepods ("fish lice"), but these are by far the exceptions.

Identification
Step one is to notice that they are there at all. Step two is to find a magnifying glass, so that you can see they are not just spots on the glass, but actually have legs and stuff. The sizes range from several tenths of a millimeter to a few mm in length, with the copepods being at the small end and the amphipods being larger. Behavior is also different for the groups, with some sitting and grazing, others scuttling among debris, and others swimming free.



The amphipods we normally see are the flea-like species of the family Gammaridae (pic). These are usually some of the largest of the 'pods that live and thrive in marine aquaria. They are often seen scuttling to and from sheltered spots.


Copepods tend to be very small, to the point that they look like tiny spots on the glass if you notice them at all. With a magnifying glass, or a good closeup shot with a camera, they have a large head/thorax, with a single eye and long unbranched antennae at the front, and a narrow tail. They usually have a pair of egg sacs hanging off the sides of the tail, if you can see them. There are benthic (bottom-dwelling) and planktonic species, but the plaktonic copepods are extremely difficult to see. In the ocean, they are the principal link between phytoplankton and higher animals and are major dietary component of many animals (Photo credit, reiverix).


Marine isopods often look like their land-dwelling relatives, the pillbugs. They have a pair of tiny eyes at the front end, tend to be flattened, segmented and without a lot of difference from one end to the other. Most species that we can see are benthic, and are most noticeable on the glass. Most are scavengers and omnivores. The photo below shows a small group of munnid isopods.


Mysis are usually big enough to identify with the naked eye, looking essentially like shrimp, but with brood pouches (if they sit still long enough for you to see them). They rarely perch, instead spending their time swimming in apparently random whirligig patterns.


How Do I Get Them?
The two most common sources of 'pods and mysis are live rock and live sand, but they can also ride along in macroalgae from someone else's refugium. There is an increasing number of companies that will sell amphipods, mysis or copepods, or small collections of unsorted pods. If people are interested, I can post links, with the caveat that I have dealt with most of these companies, have found them reputable, but AB does not endorse the products.

How Do I Keep Them Happy?
Unless you do something extreme, like using Interceptor on your tank, you will probably have some 'pods. The main things limiting their population are food and predation. Most tanks are fed heavily enough to provide enough detritus and microalgae to keep the little guys happy, but it may help to add small amounts of phytoplankton. The really big thing is shelter. If you give them a snug place to reproduce and not get eaten the population can take off.

This is probably the best reason to set up a refugium on the system. As the name suggests, refugia are places where small animals can be safe from predation, and can be anything from a separate tank with a fancy light setup to a HOB filter with some rubble. Refugia are usually set up with live rock rubble and some kind of macroalgae, which provides a perfect environment for pods to reproduce. Most species really like having lots of nooks and crannies, so macroalgae species like Chaetomorpha or Gracilaria are excellent, but I have seen incredible proliferation in substrates as simple as filter pads or sponges.

If you don't have space or resources for a separate refugium, a "pod pile" can be made in the aquarium using rocks and rubble to provide a shelter for small crustaceans to live and reproduce. Growing macroalgae in the main tank, assuming your fish and snails will let you, also helps to give food and shelter for a healthy 'pod population.

Reproduction
Since all species can proliferate in aquaria under the right circumstances, it's pretty obvious that they are reproducing. Interestingly, amphipods, copepods, isopods and mysis all brood their young. The green brood pouch can be seen on the belly of the mysis, above, and you can sometimes make out the egg sacs on copepods. The brood pouches, or marsupia, of the amphipods and isopods are more difficult to make out. Presumably, parental brooding gets the young past some of the critical stages and gives them a fighting chance in the rough and tumble world of marine aquaria. I have had trouble finding out about the stages at which the larvae or juveniles are released, although I did find that at least some isopods release their young as postlarvae. Copepods release nauplii, like brine shrimp, and this stage can last from one week to a year, depending on the species.

So that's the quick tour of the tiny arthopods we call 'pods. They are cute, useful and generally to be encouraged.

For more information check out:

Ruppert, EE and RD Barnes (1994) Invertebrate Zoology: A Functional Evolutionary Approach. 6th Ed. Harcourt Brace College Publishers.

Some articles by Ron Shimek
Copepods
Mysis
Isopods
"Pod" Review