Deep-sea mystery solved: astonishing larval transformations and extreme sexual dimorphism unite three fish families.
The oceanic bathypelagic realm (1000-4000 m) is a nutrient-poor habitat. Most fishes living there have pelagic larvae using the rich waters of the upper 200 m. Morphological and behavioural specializations necessary to occupy such contrasting environments have resulted in remarkable developmental changes and life-history strategies. We resolve a long-standing biological and taxonomic conundrum by documenting the most extreme example of ontogenetic metamorphoses and sexual dimorphism in vertebrates. Based on morphology and mitogenomic sequence data, we show that fishes currently assigned to three families with greatly differing morphologies, Mirapinnidae (tapetails), Megalomycteridae (bignose fishes) and Cetomimidae (whalefishes), are larvae, males and females, respectively, of a single family Cetomimidae. Morphological transformations involve dramatic changes in the skeleton, most spectacularly in the head, and are correlated with distinctly different feeding mechanisms. Larvae have small, upturned mouths and gorge on copepods. Females have huge gapes with long, horizontal jaws and specialized gill arches allowing them to capture larger prey. Males cease feeding, lose their stomach and oesophagus, and apparently convert the energy from the bolus of copepods found in all transforming males to a massive liver that supports them throughout adult life.
Johnson
G David
GD
Division of Fishes, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560, USA. johnsond@si.edu
Paxton
John R
JR
Sutton
Tracey T
TT
Satoh
Takashi P
TP
Sado
Tetsuya
T
Nishida
Mutsumi
M
Miya
Masaki
M
D000818:Animals; D005260:Female; D005399:Fishes / Q000033:anatomy & histology / Q000145:classification* / Q000254:growth & development; D007814:Larva; D008297:Male; D008675:Metamorphosis, Biological; D012727:Sex Characteristics