Dive Dry with Dr. Bill

#332: The Sex Life of Gobies

I thought I'd give you a break this week from talk about marine reserves and focus instead on some of the most common but often overlooked fish species in our dive park, the gobies. With the exception of the black-eyed goby which can reach a length of six inches, these small fish rarely exceed two inches. In some years they are incredibly abundant, in other years scarce. I researched their "romantic life" for my recently completed DVD on the "Greenlings and Gobies of Southern California," so I thought I'd spend some time on the "mating" side of "munching and mating" in the Macrocystis. After all, I know most of my regular readers look for such titillating tidbits in these columns. My irregular readers take laxatives or eat All Bran for breakfast.

There are three gobies I see frequently in the dive park: the blue-banded (or Catalina) goby, the zebra goby and the black-eyed goby. A fourth species, the bay goby, is usually in deeper water outside the park. Each of these species has evolved its own methods for reproducing their species. Their range of strategies is pretty interesting stuff for biologists and those with... ahem, prurient interests.

The blue-banded or Catalina goby is a beautiful but short-lived species, rarely living longer than 18 months. It sits out in the open on a rock perch overlooking its territory and darts out to capture food or defend its space against an intruder. The males may have harems of 1-7 females. Sounds good to me, as long as they all have good jobs. However, neither the males nor the females are always what they seem to be... they merely function in those sex roles. These gobies are simultaneous hermaphrodites,. meaning they usually have both male (testicular) and female (ovarian) reproductive tissue present at the same time. If the male tissue dominates, they are said to be male-biased and female-biased if the ovarian tissue is larger and active.

If this isn't confusing enough, blue-banded gobies can change their gender. If the male is munched, the dominant female can quickly transform into a male and take over the territory... and the harem. Having both tissues present at the same time makes this transition a fairly rapid one. If a female of low position in the dominance hierarchy has trouble mating, she can revert to being male. Males mate with more than one female, increasing their chances of passing on their genes. However, in exchange for that, the males have to perform the child rearing duties by guarding the eggs until they hatch.

The zebra goby is more secretive, due in part to its more cryptic coloration and behavior. Rather than sitting out rather prominently on rocks, it stays in protective crevices and under the spines of sea urchins. They are also simultaneous hermaphrodites. Female-biased, male-biased and pure females have been identified. However, pure males may not exist. Most of the individuals die soon after spawning. That seems to be a good argument for just staying celibate if you are a zebra goby! Sex just isn't worth dying for... or is it?

The black-eyed goby is a member of a different scientific genus. All of them begin life as females. The females are sexually mature at about two years and 1 1/2 to 2 inches in length. Then when they are 2 1/2 to 3 inches in length, the females turn into males. You may remember from the sheephead that this makes them sequential hermaphrodites. A species which begins life female and later turns into males is known as a protogynous ("first female") hermaphrodite. The larger, dominant males prepare nest sites either in depressions under rocks or the underside of ledges, and woo the ladies. Smaller males do not spawn or guard nests. The number of males that breed successfully depends on the number of available nesting sites in the habitat. After all, what woman wants to raise kids without a proper home? The availability of nest sites may also affect the timing of the transition from female to male.

Black-eyed gobies court the ladies in an interesting manner. During mating season, the male's pelvic fins turn very dark in color. He displays them for the girls by rising up into the water column head first with the dark fins spread out, then falls back to Earth... er, the bottom. They may also use open mouth displays to entice the babes. Although dominance is often linked to size, females do not choose their mate based on size alone. Instead they look for the males who display the most vigor in their courtship dances. Fortunately Simon Cowell doesn't vote... he's neither a goby nor a female. Scientists assume the girls feel that the guy who has the most aggressive dance steps may also be better at defending the nest with their eggs. I guess I'd better take dancing lessons!

The bay goby is a deeper water species that resembles the black-eyed goby, but lacks its bulging black eyes. I spent some time researching the sex life of this species without success. Now I am assuming it does reproduce, but it must do so very secretively so scientists don't have a clue about its strategy. Either that or the scientists have no desire to spend that much time in the colder, deeper water to watch it do "its thing." I can't imagine there is much grant money available to conduct such research so I guess we'll just have to accept it as one of the mysteries of life below the ocean.

© 2009 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 49, 10:00 AM and 5:00 PM weekdays and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. Please help me climb out of self-imposed poverty... buy my DVD's (see this link). Yes, take Dr. Bill home with you... we'll both be glad you did!

The gobies of Catalina: blue-banded (courtesy of Xiaoyan Li), zebra (courtesy of Erik Erikson)
and the black-eyed and bay gobies. .

This document maintained by Dr. Bill Bushing.
Material and images © 2008 Star Thrower Educational Multimedia