Ulua aukea and `ōmilu: these species sometimes cross

A groundbreaking genetic study of two Hawaiian jacks, ulua aukea and `ōmilu, has reached a couple of interesting conclusions.

One is that while they often behave territorially, their genetic makeup suggests that either occasional long-distance voyaging or spawning characteristics establish each as a single genetic group across the main Hawaiian Islands.

Another is that there are occasional crosses between the species. In the neighborhood of 6 percent of the fish sampled were hybrids.

(Image: An ulua (Caranx ignobilis) from the Northwestern Hawaiian Islands. Credit: NOAA's Coral Kingdom Collection, Dwayne Meadows, NOAA/NMFS/OPR)

The groundbreaking study, published in the January-February 2011 Journal of Heredity, was done by researchers Scott Santos and Yu Xiang of Auburn University in Alabama, and Annette Tagawa of the Hawai`i state Division of Aquatic Resources.

Santos has previously done extensive genetic work on the tiny anchialine pond shrimp of Hawai`i that are known as ‘ōpae `ula. His lab website is here.

First, some nomenclature. These fish are in the family Carangidae, whose members are variously called jacks, trevally, crevalle, cavalli, pompano and a few other names. In Hawai`i, big ones go by the general term ulua and little ones papio.

There are more than two dozen members of the family in the Hawaiian Islands, but this study looked at just two very popular sportfish: Caranx ignobilis, aka ulua aukea, aka white ulua; and Caranx melampygus aka `ōmilu aka bluefin trevally.

The white ulua (which turns black when it gets mad) is generally silvery with black spots, and can get huge—more than five feet long. The smaller `ōmilu is much more colorful, a grayish silver with a hint of gold, with blue spots and blue fins.

The researchers write that “catch data imply that (`ōmilu) is more common than any other jack species on Hawaiian coral reefs.” They had anglers collect genetic samples from fish caught on Kaua`i, O`ahu, Moloka`i. Maui and Hawai`ii Island. The samples were from 33 whites and 58 `ōmilu.

“Based on mitochondrial sequence data, we found no evidence of genetic structure in C. ignobilis and C. melampygus of the high Hawaiian Islands,” they wrote.

“Taken together, we conclude that the absence of genetic structure…is due to the active movement of adult individuals and/or the passive dispersal of eggs and juveniles at frequencies sufficient to homogenize populations in the high Hawaiian Islands.”

The crossing of the species has previously been reported. Santos and his partners, interestingly, found that in all the cases they studied, the hybrids are the result of a female `ōmilu crossing with a male ulua aukea. It’s not clear why.

Maybe male `ōmilu-female aukea crosses can’t survive. Maybe male aukea manage to intrude on `ōmilu spawning events, which occur more frequently simply because there are more`ōmilu .

“In either case, the potential importance of these hybrids to the evolution of the genus Caranx deserves further attention,” the writers conclude.

Source: Journal of Heredity J Hered (2011) 102 (1): 47-54. doi: 10.1093/jhered/esq101

© Jan TenBruggencate 2010