Gag grouper and the ovary-infecting nematode Philometra incognito

The gag grouper (Mycteroperca microlepis) is a popular reef fish in Florida, and it is the most widely distributed of all Florida grouper species. Given the popularity and importance of this fishery within the state, the FWC is dedicated to learning more about gag and the factors affecting this important fishery.

FWRI scientists in the Marine Fisheries Biology group who study fish reproductive dynamics are conducting a three-year Marine Fisheries Initiative (MARFIN) study assessing male abundance and the factors affecting the reproductive potential of gag grouper in the eastern Gulf of Mexico.

infected ovary
The ovary of a gag grouper infected by Philometra incognito, a parasitic nematode.

Researchers need to capture these fish in order to study them, which provide a unique opportunity for our fish health team to obtain and examine samples from adult female gag for a specific parasite. This parasitic round worm, called Philometra incognita, was discovered and named only recently (2015) by FWC biologists and their colleagues. It is commonly found in the gonads of gag grouper. It occurs on both the Florida Atlantic Coast and in the Gulf of Mexico and probably reaches sexual maturity at about the same time that the host fish is spawning.

Parasitic nematodes of the family Philometridae can cause significant damage to the gonads of wild and cultured marine fish, and FWC researchers are studying how P. incognita may be affecting the reproductive success of gag grouper in the Gulf of Mexico.

Gag grouper are caught on hook-and-line from a charter boat in the Gulf of Mexico, and the gonads of female gag are brought back to the lab in St. Petersburg for analysis. Philometrids infecting the gonads of gag grouper are first observed without a microscope during standard dissections. Large, red nematode specimens are extracted with forceps and fine paint brushes from the host gonad, and tiny male worms are collected under a microscope from washes of ovaries.

A map showing the Madison-Swanson Marine Protected Area (MPA). Gag grouper infected with Philometra incognito were collected in and around this protected area in the northeast Gulf of Mexico.

To determine the health and reproductive consequences of infection, researchers evaluate the ovarian tissue and cells. The presence of these worms in the ovaries may affect the fish’s fecundity, or their ability to produce abundant, healthy offspring.

Because fish parasites can react in a number of different ways to environmental conditions or changes, scientists are also hoping to learn whether these worms can be used as an indicator of ecosystem health. This can be accomplished, in part, by comparing the prevalence of P. incognita and its effects on host fish both inside and outside of the Madison-Swanson Marine Protected Area in the northeast Gulf of Mexico. If the parasites can help to indicate ecosystem health, then they may be useful in identifying regions in the Gulf of Mexico that could use more attention.

Learning more about the host-parasite relationship and understanding the effects of P. incognita on gag reproduction will be beneficial for the long-term health and productivity of this important fishery.