Saw the recent posts about massive die-offs of exotics in Florida due to the cold. Then, this past week, I experienced it a little more directly when my girlfriend's two bettas died while we were on vacation. We had turned the heat down to 53F, and I assume that's what got them as nothing else changed. Lesson learned, I guess.

I'm curious what the explanation is for relatively mild water temperatures (50s and 60s F) being lethal to many fish, while others are perfectly healthy and active all the way from water temps in the 80sF right down to a few degrees above freezing?

Is there a single shared characteristic that delineates these groups (presence or absence of a certain protein?), or are mechanisms of cold tolerance different in different groups? Has cold tolerance appeared and then been lost in certain groups, or simply never evolved for certain tropical fish groups?

I dunno but its a fascinating question. Some tropical critters can survive way lower temp than you'd guess from their native range. Cherry shrimp native to Hong Kong do just fine near freezing in a 20 gal plastic tub in NC all winter. Ive seen Badis and Dario from tropical India & Burma still active and feeding in mid-50s; maybe they could could go even lower but I havent risked it.

There's loads of books and journals on fish physiology. It would be great if you could find out some answers and write us an article for AC. Could be very useful in evaluating potential invasiveness.

It's usually a function of the kinetics of enzyme systems in the various species. The same enzyme in different species will have slightly different properties such as how well it might work at different temperatures. The crucial enzymes would be in the mitochondria, where aerobic respiration takes place, and most of the central nervous system. So there is an optimal temperature range for a species, with too hot or tool cold being equally bad after point.

So do species with broad temperature ranges have more flexible enzymes, or do they have multiple varieties of each enzyme for operation in different temperature regimes?

How much plasticity is involved; i.e., are fish raised in a stable environment unable to tolerate the same range of temperatures as their wild conspecifics?

Is there a cost associated with broad temperature ranges that would push species from stable habitats to develop a more focused temperature range?

So do species with broad temperature ranges have more flexible enzymes, or do they have multiple varieties of each enzyme for operation in different temperature regimes?

How much plasticity is involved; i.e., are fish raised in a stable environment unable to tolerate the same range of temperatures as their wild conspecifics?

Is there a cost associated with broad temperature ranges that would push species from stable habitats to develop a more focused temperature range?

To the first question, basically yes, with maybe a broader optimal range or a combination of enzymes for different functions that may complement each others' functions. For the second and third questions, there is acclimation of enzyme systems to certain temperatures by ectothermic animals like most fishes which can be measured as "Q", the ratio of metabolic rate at a higher temperature to that at a lower temperature. Most famously, frogs have been used as study animals for acclimation. Most fish species can't go from one temperature range to another quickly without some cost, but they can certainly go through seasonal adaptations that may involve the expression of different genes for somewhat different versions of metabolic enzymes. It's a very old-fashioned kind of biology that I've always been fascinated by but never had a chance to really pursue. For NA natives, most of them live in environments at the top of their thermal tolerance range, certainly in the summer.