Don’t Underestimate the Individuality of Marine Fish

On numerous prior posts here at Saltwater Smarts, I’ve pointed out that a certain degree of behavioral variability is often seen among individual specimens of the same fish species. But what exactly do I mean by this and why does it matter to marine aquarium hobbyists?

The type of “behavioral variability” I’m referring to here is not, for example, an obligate corallivore butterflyfish (e.g. Chaetodon larvatus) suddenly deciding it prefers to eat algae, or a clownfish observing a parrotfish cloaked in its nighttime mucus cocoon and thinking, “Hmm, very clever. I might give that a try myself. To heck with hiding in this anemone!” (Of course, I’m being ridiculous here. Everyone knows clownfishes don’t speak English. They speak Tagalog.)

Rather, I’m referring to different individuals within a given species exhibiting varying degrees of a particular behavior or characteristic, such as the level of inter- or intraspecific aggression shown to tankmates or the tendency to nip at corals or other sessile invertebrates. I’m talking about the sorts of differences that inspire aquarium authors to write things like “The odd specimen might [fill in somewhat uncharacteristic behavior here]” in their species accounts.

For example, drawing on my own experience, I’ve kept at least three different coral beauties (Centropyge bispinosa) in reef systems over the years, and each was/is markedly different in its behavior toward corals. One took no interest in them whatsoever and was a model reef-tank inhabitant; another would occasionally pick at fleshier corals, such as a Trachyphyllia geoffroyi I had at the time, but was otherwise fairly well-behaved; while the third—which I still own and referenced in last week’s post—ignored my corals and gorgonians for several months but then “went rogue” and began nipping at almost everything. I had to move it to a separate FOWLR tank where there are no sessile invertebrates to torment.

I’ve also seen firsthand markedly divergent levels of interspecific aggression exhibited by different individuals of triggerfish species that are generally considered peaceful (as triggers go, anyway), such as Xanthichthys auromarginatus and Odonus niger, as well as among different specimens of Pseudocheilinus hexataenia, the popular sixline wrasse. Many hobbyists who have experience with this oft-feisty species might be surprised to hear that the last one I kept was an absolute teddy bear, never bothering any of its tankmates at all, not even passive ones.

Another tendency that can vary significantly among different individuals of the same species is willingness to accept various foods—or any foods for that matter. One specimen may immediately accept virtually everything offered, another might need considerable coaxing to try anything, and still another might fall somewhere in between.

Beyond the natural intraspecific variations that occur in any animal population, a whole host of factors can contribute to these behavioral differences. The relative size/age/maturity of the individuals, the types of tankmates and the order of introduction to the aquarium, whether the specimens are wild-caught or captive-bred, and whether the specimens were collected at different geographical locations within the species’ natural range are a mere sampling of these factors.

Obviously, these variations in behavior can be a significant issue when you’re trying to create a peaceful, compatible community of fishes and invertebrates. While there’s no way to anticipate exactly how any given specimen will behave once introduced to a system, you can avoid a lot of hassles by researching the “typical” behavior of a given species, factoring in reports and anecdotes about the species from other hobbyists, and erring on the side of caution.

So, fellow salties, have you had any interesting experiences related to intraspecific behavioral variation in fish? Let us know in the comment section below.


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About Jeff Kurtz

Jeff Kurtz is the Co-founder/Editor of Saltwater Smarts, former Senior Consulting Editor for Tropical Fish Hobbyist Magazine, and the aquarist formerly known as “The Salt Creep.” He has been an aquarium hobbyist for over 30 years and is an avid scuba diver.


  1. Lisa Foster says

    Green spotted puffers are supposed to be pretty aggressive, but I had one who was very much a “live-and-let-live” sort of fellow. Did fine in a community – no nipping or chasing at all.

    Inverts were still very much on the menu, though XDD

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