Thursday , 13 June 2024

‘A little bit of mucus’ is new technique to determine sex of hermaphrodite grouper

By RD Dianala

Giant grouper
One of the giant grouper (Epinephelus lanceolatus) in the broodstock cages at the Igang Marine Station. A non-invasive technique developed at SEAFDEC/AQD recently allows for easier determination of the sex of the giant grouper. (photo by RD Dianala)


BENEATH the turquoise waters of Igang Bay in Guimaras Island dwell one of the most feared species of fish in the Philippines. Indeed, urban legend has it that this brown behemoth fish sprinkled with dark spots, which can grow up to 2.3 meters in length and 400 kilograms in weight, is responsible for several human disappearances.

Fortunately, the monster fishes of Igang Bay are comfortably enclosed in sturdy net cages of the Igang Marine Station of the Southeast Asian Fisheries Development Center (SEAFDEC). The breeding of the giant grouper, Epinephelus lanceolatus, the largest species of the delectable fish locally known as lapu-lapu, pugaro, or kugtong, is being closely studied at the station.

A successful breeding program for the giant grouper is hoped to pave the way for the sustainable farming of the sought-after fish commonly eaten steamed, fried, grilled, or stewed in restaurants and many homes. Indeed the importance of breeding the fish is underlined by the “vulnerable” status of its wild population as listed by the International Union for Conservation of Nature (IUCN).

Hermaphrodite grouper invites “invasive pokings”

However, a major bottleneck in their breeding is finding out which fish is male, which is female, and which ones are ready to breed. Sexing the grouper periodically is of prime importance because they are hermaphrodites – that is, they change sex from male to female, at some point in their life.

Previously, the way to determine the sex of the fish was through invasive pokings – technically, cannulation biopsy – into the genital openings of sedated fish to check for eggs using a plastic cannula.

Poking the grouper with a cannula requires skill, causes undue stress to the fish and is difficult to perform considering the size of the giant grouper. Recently, a SEAFDEC study devised a new method to determine the sex and maturity of grouper without subjecting them to the painful pokings.

Biopsy cannulation of giant grouper
Researcher Peter Palma does a biopsy of giant grouper by inserting a plastic cannula into the genital opening to check for the presence of eggs. (Photo by SEAFDEC/AQD)

Researcher finds that “a little bit of mucus” determines grouper’s sex

Apparently, the mucus naturally coating the body of the giant grouper contains the protein called vitellogenin whose presence indicates the fish is a female. The new method simply detects the presence, or absence, of the protein to faithfully determine the sex of the fish. Thus, mucus with vitellogenin, female. None, male.

The method is detailed in the paper, “Reproductive development of the threatened giant grouper Epinephelus lanceolatus” by SEAFDEC researcher Peter Palma and co-authors published in volume 509 of the Aquaculture journal. Their study was funded by the Australian Centre for International Agricultural Research (ACIAR).

Fortunately, the technique only requires scraping off a bit of grouper mucus to a glass slide. The study has revealed that the mucus collection did not cause any skin lesion on the fish during and after the study.

This new technique is a great relief to the grouper and researchers. Imagine the number of painful pokings the fish would have needed to endure as breeders need to periodically confirm that they have not changed sex. Now, “a little bit of mucus” is all that is needed.

This is another milestone in giant grouper research so that there can be more steamed lapu-lapu in oyster sauce without harvesting what remains in the reefs. It turns out the feared giant grouper may have man to thank for saving it from further threat of overfishing, and many painful pokings.

A research assistant holds a sedated mature female giant grouper, about 40 kg in weight. (Photo by PA Palma)

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