Fish Intelligence – Research and Products for Home Experiments – Part 2

Hello, Frank Indiviglio here. Fishes outnumber all other vertebrates combined, with nearly 30,000 species identified so far.  No doubt, the ability to learn has assisted in this success.  In Part I of this article, I recounted my experiences with “educated” electric eels in Venezuela, and gave some other examples of fish intelligence.  I’ll continue here, with an emphasis on memory.

If you would like to experiment with your own pets’ learning abilities, please check out our extensive line of fish feeding products , and don’t forget to write in with your observations.

Memorizing Terrain

Frillfin gobies inhabit tidal pools.  When disturbed, a goby will leap from its pool to several others in succession.  It always lands in another pool, despite the fact that it is jumping without seeing the next pool, and changing directions.

Ichthyologists (fish biologists) have discovered that gobies memorize the location of tide pools at high tide, when they swim about over the area in which they live.  Imagine, if you will, trying to do this yourself while snorkeling…and then consider the how much more complicated the task would be to a tiny fish!

Gobies transplanted to unfamiliar tide pools refuse to jump…if forced to do so, they invariably miss the next pool and hit dry land.  Yet after just one high tide, the gobies learn the new habitat and can then jump accurately from pool to pool.  In what must surely be the most amazing fish memory feat known, the tiny fishes retain their internal map of the new tide pools even if removed from the area for 40 days!

Learning by Observing Others

Sea bass were allowed to watch other bass undergo a training program in which the participants were rewarded with food if they pushed at a certain lever.  Some bass were better than others at learning this skill.  “Student bass” that had watched “smart” bass immediately pressed the levers themselves when given the opportunity.  Those that had observed bass that did poorly in the training took much longer to master the lever trick themselves.

French grunts travel from sleeping to foraging grounds each day. Grunts re-located to new habitats follow resident fishes to and from foraging grounds.  After 2 days, the transplanted fishes are able to find their way to the foraging grounds – an average distance of .5 miles – even if the resident grunts are removed.

Goldfishes that watch others foraging in an aquarium with hidden food are able to learn and remember what areas of the tank held food. When released into the tank, they ignore areas where the original fishes found no food, and head immediately for the most productive feeding sites.

Remembering Bad and Good Experiences

An American eel that was my mother’s (somewhat unusual!) pet for 17 years quickly learned to associate people with food.  It was kept in a high-traffic area, but did not rise to the surface for food unless someone stopped in front of its aquarium…people passing by were ignored.

Perch separated from minnows by a glass partition stop trying to catch the minnows after crashing into the glass.  When the glass is removed, the perch refuse to chase the minnows, apparently associating them with a bad experience.  Note: this lesson wears off when predators get very hungry, so don’t try it in hopes of keeping Oscars with sword tails!

Further Reading

You can read more about fish learning abilities in an interesting Fish and Fisheries article and in The Everything Aquarium Book.

Please write in with your questions and comments.  Thanks, until next time, Frank Indiviglio.

American Eel image referenced from Wikipedia and originally posted by Freida.

Fish Intelligence – Research and Products for Home Experiments – Part 1

Hello, Frank Indiviglio here. The natural behavior of fishes is so interesting that one can easily overlook the fact that they are capable learning in the true sense of the word (i.e. changing behavior in response to experience). Fishes are, after all, the most successful vertebrates in terms of species diversity (nearly 30,000 species have been described so far) – it would be odd indeed if they did not possess some capacity to profit from their experiences.

What All Fishes Learn

Some evidence of fish learning ability is so common that we usually do not appreciate it as such. From guppies to giant pacus, aquarium fishes of all types gather in anticipation of a meal when they see their owner approaching, or if the aquarium light is turned on. Seems simple to us, but these “simple” creatures are associating a large being (us) with food, something that instinct would never cause them to do.

Observations in the Field: Electric Eels

As you can imagine, forming associations similar to “people=food” can increase hunting success in the wild.

Years ago I was involved in fish and anaconda research on a cattle ranch in Venezuela. The ranch owners periodically replenished their stock’s water supply by lifting a gate that separated the cattle-watering channels from a river. As soon as river water began splashing into the channels, the huge electric eels (which are actually knife fishes) that lived in the channels would appear at the gate. They had obviously learned that fishes and other prey were carried from the river to the channel with the flowing water.

Some of these brutes approached 6 feet in length. When grabbed in the mistaken believe that it was an anaconda, one large eel knocked a co-worker off his feet with its electrical discharge.

Home Training Devices

If you would like to experiment with training your own fishes, please check out our extensive line of fish-feeding products. Automatic fish feeders, feeding stations, feeding clips and feeding tongs can be used to duplicate some of experiments described below. Of course, the possibilities are limitless, so please write in with your own ideas.

Following are some other brief examples of piscivorous learning abilities:

Feeding Associations

Archerfish (which feed by shooting jets of water at terrestrial insects) that were fed immediately after a light bulb went on soon squirted water at the bulb, in anticipation of a meal. I find this particularly interesting because archers generally shoot only at moving objects.

I recall that archers under my care shot water at their exhibit door as I opened it – I thought they were reacting to the movement, but perhaps they associated the opening door with food.

Territorial Defense

Male bettas that were shown a rival male directly after a light bulb came on soon began displaying to the bulb, without seeing another fish.

Male sticklebacks perform an elaborate display when confronted by competitors. Researchers hid the competitors from view each time the males exhibited the “head down” portion of the display…in effect convincing the displaying fish that the interloper had fled. Realizing that this part of the display was very effective, the males soon began performing it earlier and more often than usual.

Next time I’ll relate more experiments that give evidence of fishes’ surprising learning abilities. Until then, please write in with your questions and comments. Thanks, Frank Indiviglio.

Further Reading

You can learn more about fish intelligence in the following article: and in The Everything Aquarium Book, which I wrote several years ago.

Electric eel image referenced from Wikipedia and originally posted by StevenJ.

Archerfish image referenced from Wikipedia and originally posted by Okapi.

Crabs That Carry Food and Weapons

Hello, Frank Indiviglio here. The ways of crabs never cease to amaze me, and we are fortunate indeed that so many interesting species do exceedingly well in marine aquariums. Today I’d like to draw your attention to certain hermit, arrow, boxing and spider crabs that increase their survival odds by carrying food or weapons wherever they go.

A Mutually Beneficial Partnership

After locating a suitably-sized sea anemone, the anemone hermit crab (Pagurus prideauxi) places the stinging invertebrate on its shell as a deterrent to predators. The anemone attaches itself to the new home, and may benefit by gaining access to leftovers from the crab’s meals. Anemone hermits that I’ve kept have invariably relocating their protectors when switching their own living quarters from one shell to another.

Anemones as Boxing Gloves

Boxing CrabAnother weapon-bearing crustacean, the boxing crab (Lybia tessellata), goes through even greater lengths to arm itself with stinging anemones. This tiny (2.1 inch) fellow has the fascinating habit of carrying small anemones about in its claws. When threatened, it will rear up on its hind legs and wave the “weapons” at the interloper!

Lunch to Go

The ever-popular arrow crab (Sterorhynchus seticornis) is more concerned about food supply than weaponry, and impales food upon its pointy carapace. This is quite comical to see, because, due to the location of the spine, the crab seems to be carrying food about on the tip of its nose! The stored food is consumed when the crab is safe within a retreat, or in times of need.

A Surprising Discovery

The Atlantic spider crab (Libinia emarginata), a temperate relative of the arrow crab, can easily be collected along the eastern coast of the USA. They are inoffensive towards one another and most tank mates, and make interesting aquarium pets.

I still remember my shock upon seeing one wedging kale (provided as food) into the many crevices of its carapace. Eventually, the small crab looked like a walking patch of marine algae…when it ceased moving, its camouflage was perfect (well, nearly perfect…kale doesn’t grow in the sea!).

I later observed spider crabs to pick at and consume bits of their movable garden, so the vegetation serves two purposes (I have not determined if spider crabs will use inedible materials for camouflage, as do decorator crabs and some others). The spider crabs that I have kept gave up this habit upon reaching a carapace size of approximately 4 inches.

The aforementioned crabs are readily available and adjust quite well to aquarium life. I’ll cover their care in detail in future articles. Until then, please write in with your questions and comments. Thanks, Frank Indiviglio.

Further Reading

The diversity of crab species and lifestyles is astounding. To learn about current research in East Asia (where over 1,000 species have been described) and view some remarkable photos, please visit

I have written about the natural history and captive care of crabs and other crustaceans in The Everything Aquarium Book

Aquarium stocking tips – How do I choose my fish?

Eileen here. We’ve all heard some of the general “rules of thumb” for stocking an aquarium: an inch of fish per gallon, one fish per gallon, more fish = less aggression and countless others. So, what works best? There are several approaches that can help make stocking an aquarium easier.

Research, research, research!!

It’s no secret that I for one am a big fan of research. I’m not talking in depth taxonomic studies or long term observational studies, just a little planning and reading up on what you would like to keep. We get a lot of questions like “I set up a tank, now what can I put in it?” or “I bought this pretty yellow fish, what can go with it?”. You may be able to save yourself some time, frustration and possibly disappointment by planning out what you might like in your tank before setting it up or buying any fish for it. It also helps you decide what you would like to put in it down the road. That way, you will be able to decorate and filter your tank in the best way for what you’d like to keep and you’ll avoid getting those impulse-buy tankmates that eliminate any chance of having that special fish you’ve had your eye on for months.

Along these same lines, be sure you are aware of the behavior of each item you choose and its requirements. There is nothing worse than bringing a fish home, only to learn that you have to spend more time preparing its food than your own, or finding out that it is a known predator of that favorite fish you already had. A few basic aquarium fish books can help a lot with the basics and can even help you find fish you never even knew existed. You could try shooting us a blog question, email ( or giving the store a call at 717-299-5691 if you’re looking for help in this regard too.

An inch per gallon? Only sometimes

This is one of the most common stocking “rules” we hear but is often misunderstood and misused. Let’s compare a few common groups of freshwater fish: tetras, goldfish and cichlids. First, the “inch per gallon” or “fish per gallon” rules have to take the adult size of the fish into consideration. Sure a fish might be one inch when you buy it, but if that fish grows into a footlong adult? That changes things. Then, what about the body mass of the fish and the waste it produces? Six little one-inch-long tetras will certainly affect an aquarium different than a six-inch-long goldfish. Goldfish just produce a whole lot more waste, eat a lot more and have a lot more mass and size behind that six-inch-length than all of those little tetras put together. Next, what about the behavior? That same six-inch goldfish is going to have a far different temperament than, say, a six-inch Green Terror cichlid. Two six-inch goldfish could be perfectly happen in a well-filtered 45 gallon aquarium; the two Green Terrors may well try to kill each other. So do we throw out the “inch per gallon” rule altogether? Not necessarily. It is fine to use as a very general and basic guideline for small fish like tetras, danios or livebearers if you take their adult size into consideration, but don’t take it as gospel if you are keeping anything larger.

Zone Defense: It works in sports, it works in aquariums

When stocking your tank, keep in mind that all of the fish won’t be spending all of their time in the same area of the tank. Looking at your aquarium from the front, you can divide it horizontally into 4 zones. The middle two zones are where a lot of fish hang out. In freshwater, this is where you’d find tetras, angelfish, barbs, and some cichlids. In saltwater, this would be your tangs, damsels, clownfish, and some groups of wrasses. The top quarter section is more of the top dwelling fish. In freshwater, these are fish like hatchets, killifish, rasboras, and mollies and in saltwater, this would be some cardinals and dartfish gobies – fish whose mouths are more on the tops of their heads and point upward for those prey items on the surface. The last section, the one on the bottom of the tank, is home to the bottom feeders – catfish, blennies, loaches, and gobies. Some saltwater fish and freshwater cichlids that spend a great deal of their time in and around rockwork also would count towards this group.

Stocking an aquarium with all three sections – top, middle, and bottom – in mind will help you make the most of the space you have. Instead of having a lot of fish that hang out in one of these zones, choosing fish from all three can give you a more complete look to your tank and can help spread out the activity and aggression throughout the whole tank.

The more, the merrier? Or one big spotlight?

There are two big ways to plan an aquarium – having an active tank with lots of activity and schools of little fish, or have a showcase item like a big saltwater angelfish or showing cichlid and build the rest of the tank around it. Both can be stunning in their own way but take some planning. Having a tank full of different schools of little fish can be interesting and active. You can have a higher number of fish this way but there isn’t one big thing to focus on. Or, you can have one big fish like a Discus and complement it with just a few other little fish for some subtle activity and to help with clean-up. Some of the most interesting tanks can be species-only tanks. These tanks have just one kind of fish like an aggressive cichlid or a saltwater oddity like a frogfish, and nothing else. These are the true “pet fish”. They can be fascinating but aren’t as diverse as community aquariums.

Biotopes – a little piece of nature.

Some of my favorite aquarium have been biotopes. These are tiny pieces of a specific environment where everything in the aquarium is designed and chosen around that location or habitat. For examples, a Caribbean biotopes would have only fish and invertebrates chosen from the Caribbean, so a fish from the Indo-Pacific would not be chosen for this tank. This is how most public aquariums are designed and is one of the more collector’s approaches to aquariums. Instead of relying on the impulse buy, this method is all about planning and choosing the perfect addition for your little slice of nature. Much better than a postcard for remembering that trip to Hawaii!

These are just a few approaches to choosing the next addition to your favorite aquarium. Feel free to share your own and never be afraid to ask if you aren’t sure if what you want is right for you!

Feeding Canned and Live Insects to Marine and Freshwater Fishes – Part 2

Hello, Frank Indiviglio here.  I have always included insects in the diets of a great many fishes, including shallow-water marine species (in the accompanying photo, you can see a number of typical community aquarium species swarming around a cricket).  Please see Part I of this article for further information on the role of insects in marine and fresh water fish diets.

My Experience with Wild Fishes

Whenever I have the opportunity, I toss insects into natural water bodies.  Time after time, be I near a quiet pond in Ohio or a salt marsh on Long Island, the insects never last more than a few minutes before being consumed by resident fishes.  Eventually, I came to believe that terrestrial invertebrates play a great role in the diets of numerous freshwater, brackish and even marine fishes (many insects fly far out over the ocean, especially on migrations, or are carried there by the wind).

Current Research

Just this month (August, 2009), biologists at the National Oceanographic and Atmospheric Administration published the results of a research effort which addressed this very topic.  Surveys of fishes in lakes across North America have revealed that terrestrial insects comprise up to 100% of the diets of some fish species (my own casual observations are rarely validated in such a timely fashion!). 

However, in lakes where development has eliminated shoreline vegetation, insects typically make up only 2% of the diets of the same fish species.  The importance of insects is evidenced by the fact that fishes living in lakes with developed shorelines take in 50% less energy on a daily basis and grow slower than those in undeveloped lakes.  Fortunately, re-planting even a narrow fringe of bushes and grasses along a lake can dramatically increase terrestrial insect populations.

Canned Insects for Aquarium Fishes

Both live and canned insects are eagerly accepted by many typical (and untypical!) aquarium fishes. Canned invertebrates may be better suited for most aquarists, who, unlike their reptile-keeping colleagues, are not often in the habit of maintaining live insect colonies. What’s more, they retain all the nutrition of live insects but are far more convenient to store and use.

Canned grasshoppers and adult crickets are ideal for large carnivorous fishes such as Oscars and other Cichlids, arowanas and many catfishes, and can also be crushed or chopped for smaller species.  Other varieties of canned invertebrates include snails, mealworms, silkworms and young crickets . Freshwater shrimp , also available in cans, are eagerly taken by a wide array of fishes. 

Further Reading

In some habitats, the availability of “junk food” is reducing the role of insects in fish diets.  To learn more, please see

Please write in with your questions and comments.  Thanks, until next time, Frank Indiviglio.