The Evolution and Biogeography of Xanthichthys

Triggerfishes have a deserved reputation as pugnacious and problematical fishes within aquariums, but the half-dozen species of Xanthichthys offer a peaceful exception to this rule. Because of their smaller size and zooplanktivorous diet, they are a surprisingly amenable choice for larger reef tanks. Their natural diet is reported to be almost entirely comprised of tiny calanoid copepods, meaning they have zero interest in dining on your corals, though smaller crustaceans ought be wary. Xanthichthys are primarily associated with deeper waters (40-140+ meters), though juveniles can be found as shallow as 5m. Adults are most common along the forereefs of oceanic islands, while juveniles are found in more diverse habitats. These fishes often occur beneath a sharply demarcated line separating waters of different temperature—the thermocline. As an example, X. greenei is found beneath a thermocline of 68-72℉, above which it is 77-82℉. As their preferred temperature is far below that of the typical reef aquarium, Xanthichthys can require a great amount of food to maintain their thermally-increased metabolism.
 

Black=auromarginatus, Blue=mento, Red=lineopunctatus, Yellow=greenei, White=ringens

 The precise biogeography of many Xanthichthys species is imperfectly known, with many having large gaps separating known populations. Whether this is an artifact of their deeper, colder-water preferences or is indicative of a true absence is not fully understood. For instance, is X. lineopunctatus to be found in the regions between the many scattered locations it’s known from? Is their a member of the ringens Complex present throughout the Central Pacific? Do the Indian Ocean and Pacific Ocean populations of X. caeruleolineatus maintain any genetic connectivity across their apparent absence in the Coral Triangle?

Credit: Brian Gratwicke

• Range: Western Atlantic (North Carolina & Bermuda to Brazil)
• Size: 25cm
• Etymology: Latin “gaping”
• ID: Body w/ lines of spots; dorsal/anal fins unmargined
• Aquarium Rarity: Common
• Notes: Juveniles in shallow water, associated with Sargassum

Credit: John Randall

• Range: Antiequatorial, Indo-West Pacific (S. Africa+Reunion+Mauritius, SW India, NW Australia, Taiwan, Ryukyu+S. Japan,+Izu+Bonin)
• Size: 30cm
• Etymology: Latin “lined-dots”
• ID: Dorsal rows of spots conjoined into short dashes
• Aquarium Rarity: Rare
• Note: X. lima is a synonym, both fishbase and Catalog of Fish mistakenly list it as valid
  

Credit: Pacific Island Aqua Pics

• Range: Kiritimati, Line Islands, possibly at nearby Phoenix Islands
• Size: 15cm
• Etymology: After mesophotic reef biologist Dr Brian Greene
• ID: Two rows of spots limited to beneath dorsal fin base
• Aquarium Rarity: Rare
• Note: This appears to be a Polynesian sister species to lineopunctatus, though the extent of its range is very poorly understood.

… More:

The post The Evolution and Biogeography of Xanthichthys appeared first on reefs.com.