Butterflyfish forming mated pairs? Chaetodon Behavior/Sexing    2/28/12
Hi,
<Hello Adam>
As we know many species of butterfly fish form mated pairs. I have two questions which I am not sure you can answer. 1. If one of the pair dies does the other fish find a new mate, or carry on alone, or mourn and die I am particularly interested in Chaetodon Trifasciatus.
<I would tend to believe the remaining fish would pair up with another. 
Why the C. Trifasciatus, an extremely difficult Butterflyfish to keep and an obligatory coral feeder.
 2. How do the fish know what sex the other is as there are no external differences. I imagine it must be a chemical sense?
<There are physical differences in some species and egg bound females tend to be a little plump.  Other than that I will ask Bob to comment here.><<Can't tell in almost all cases I've seen UW. B>>
 Regards,
<Ditto.  James (Salty Dog)>
Adam.
Re Butterflyfish forming mated pairs? Chaetodon Behavior/Sexing 2/28/12
Hi,
<Hello Adam>
When I swim in my local beach I see many of these butterfly fish and they always seem in pairs.
<Many species do pair up.  Have you been able to identify the species you have seen?>
As we know it is unlikely that they will both live to the same age. Is there an area where they would pair up as the local fish are fully grown at about 5 to 6 inches.
<Likely so.  Bob is an amphibian and dives their waters/observes their behavior often and may input here.>
Therefore once in my area how would they find a new mate, or do they swim over large distances or the currents take them into "communal" zones.
<Butterflyfish generally spawn at early dusk and usually only pairs are involved and this is what you may be seeing depending on the species.
During the reef courtship the fish chase each other around, doing headstands and lateral displays.  A pair will circle each other until one swims off while the other follows.  Juveniles are generally found in shallow grassy areas where they feed.  Some species will form large aggregations and swim over large areas in search of food.  There are many species of Butterflyfish and their personalities/behavior can vary.>
 Thanks,
<You're welcome.  James (Salty Dog)>
Re Butterflyfish forming mated pairs? Chaetodon Behavior/Sexing 2/28/12
Hi James,
<Hello Adam>
As in my earlier email the species is Chaetodon Trifasciatus.
<Depending on your location, it may also be a Chaetodon lunulatus which is said
(Randall) to be a subspecies of Chaetodon Trifasciatus.  Either one is a poor choice
for hobbyists as both are obligatory coral feeders with preference to Pocillopora corals.
A gorgeous fish for sure.  Seems like all the gorgeous fish are the ones most difficult to keep.
And this species does indeed form pairs when breeding but I do not believe they, or any Chaetodon species are paired for
their entire life.....more like a hot date. :-)
Bob may/will comment if I am incorrect.>
Regards,
<Ditto.  James (Salty Dog)>
Adam.

Butterflyfishes for Marine Aquariums Diversity, Selection & Care New eBook on Amazon: Available here New Print Book on Create Space: Available here by Robert (Bob) Fenner