The Canadian fighting fish apologizes when you show it a mirror. That's mendel's joke, which I think is actually funnier than mine if you know the stereotype of Canadians as a people so mild-mannered that a Canadian will say "Sorry" when someone steps on his foot. (My Canadian friends tell me that this is not so much a stereotype as an accurate description.) The stereotype of mere extreme politeness is more widespread, though, so I used this joke instead.
"Siamese filking fish" is substitute's joke, though he doesn't remember making it or won't admit to remembering. He was also the one who got me thinking of conjoined twins today in the first place, by pointing me to this picture of conjoined terrapins, or conjoined terratwins, or teratopins. Further idle Googling revealed more pictures of conjoined turtles and tortoises than of any other type of conjoined animal. Is this coincidence or some other factor, or are turtles for some reason more prone to conjoined twinning (or more likely to live for a while when born conjoined)? Google won't tell me that!