The first time I was asked to go grunion hunting, I was extremely flattered. You see, I was new to the San Diego area, and my girlfriend asked me if I wanted to go down to the beach in the middle of the night to bag some grunion. She looked me right in the eye and described grunion as slender, 5- to 6-inch/13- to 15-cm long, silvery fish that slithered up on the beach by the thousands when they mate. Flattering myself, I was certain my girlfriend was trying to deceive me into going to the beach alone with her in the middle of the night. I pretended that she had convinced me to go grunion hunting. Man, was I ever feeling clever.
It was only a few days after the new moon, and because it was dark and late, I was certain we would have the beach to ourselves. Fishing licenses in hand, we stopped by her house to grab a couple of flashlights and a gunnysack. I really felt silly going along with all of this, but I didn’t want to let on that I really knew that her intentions were to get me alone on that beach.
Twenty minutes later we were at the water’s edge — along with dozens of other people who were armed with flashlights, buckets and bags.
Unbelievable. Grunion exist — and so did my big ego, but it was rapidly being reduced in size. I confessed, and we had a great time gathering a bag full of grunion.
Grunion belong to a family of fishes known as Antherinidae, a group commonly referred to as silversides. They are found only along the shores of Southern California and the northern portion of Mexico’s Baja peninsula. The reason they come ashore is to mate and for females to lay their eggs in the wet sand. Grunion make their beach excursions shortly after high tide only on the three or four nights following the new and full moon during certain, very predictable parts of the year. Fertilization takes place as the females are burying their eggs. Since this time interval produces the highest tides of the month, the eggs remain covered until they hatch two weeks later, and the fry rush out to sea with the waters of the next cycle of highest high tides.
Grunion are not the only marine animals that leave the sea in order to mate and lay eggs. Sea snakes invade many Indo-Pacific islands to lay their eggs. Sea kraits, creatures that are commonly referred to as sea snakes, are distinguished by the fact that they mate and bear young in the water. Of course, seals, sea lions and walruses typically mate and give birth to their young on land, but for the most part, marine creatures mate and have their young in the water. If we keep the discussion to fishes, the fact that grunion leave the water to mate and lay eggs makes them an especially rare exception.
How They Do It
With more than 24,000 species of bony fishes and more than 1,000 kinds of cartilaginous fishes (sharks, rays and skates), there are hundreds, if not thousands, of reproductive strategies in the world of fishes. In this article, we’ll take a peek at the sex lives of fishes, but there is no way to even get close to covering all the reproductive strategies that fishes employ.
In fishes, reproductive behavior is generally divided into phases of courtship, spawning or mating, and parental care. In most cases eggs are fertilized outside the female’s body in an act called spawning, but copulation and internal fertilization is the method employed by sharks, rays, skates and some guppies. With mostly minor variations, members of the same family of fishes tend to use similar strategies.
In damselfishes such as sergeant majors and garibaldi, the male cultivates algae in a nest site that he builds and maintains as part of his effort to woo willing females. Elaborate courtship and mating rituals soon follow. The male vocalizes and performs a courtship dance above his nest to attract females. After a female deposits her eggs in the nest, the male fertilizes the eggs and then chases the female away.
Over time the male will attempt to attract a number of females, often eating a few of the ripening eggs of a previous mate to make room for even more eggs from a newcomer. The male vigorously defends the nest from potential intruders such as sea stars, octopi, urchins and snails. This “single male/numerous female” strategy is known as polygyny, and it is common in habitats where males are able to defend resources such as food or a nesting site.
Male jawfish take their reproductive responsibilities a bit further, protecting fertilized eggs by holding them in their mouth until they hatch. In some species of marine catfish, the fry continue to seek cover in the mouth of the male for an extended period after hatching.
Male sea dragons and sea horses qualify for the title of Mr. Mom. In sea dragons the eggs are attached to the belly of the male, who guards them until they hatch, while in sea horses, males actually keep the fertilized eggs in a belly pouch until they hatch. In essence, it is the male that bears the burden of “pregnancy.” In sea horses, as soon as one brood hatches, the female is quick to deposit another batch of eggs into the male’s pouch, where he fertilizes them.
Changing Cycles, Sexes and Partners
In grunion, the mating cycle is governed by the seasons and is in conjunction with lunar and tidal cycles. But that is not the case with all fishes. Many species, such as rainbow wrasses and convict tang, gather to spawn on a daily basis. Gathering in large groups, males and females simultaneously release sperm and eggs into the water, where fertilization occurs. In this case, the gametes (sex cells), fertilized eggs and hatchlings receive no parental protection and are merely unattended members of the community of plankton.
However, it should be pointed out that mating in broadcast-spawning reef fishes often occurs in low light or at a time and place where currents are most likely to carry eggs out toward the open sea, away from concentrations of potential predators that live around the reef. Mother Nature is a clever lady, indeed.
Other species provide a startling contrast. As examples, wolf eels and some butterflyfishes form monogamous pair-bonds that last for years. Anemonefishes, commonly called clownfishes, live in small schools that are dominated by a single monogamous pair. If one member of this pair dies or is removed from the school, another fish acquires the mating fish’s role, and in some cases this requires that at least one fish undergo a midlife sex change. However, polygamy, with both sexes having multiple mates, is the rule for many species.
Having access to members of the opposite sex when all factors, such as the time of year, water temperature, place, tides, availability of food and absence of predators, are right is of critical importance. Schooling, a behavior found in approximately 20 percent of adult fishes, such as many snappers, grunts and jacks, offers constant access. Another successful strategy for maximizing an encounter with another member of the same species is for each individual to be both male and female at the same time. Amazingly, lots of fishes — many sea basses, groupers and hamlets, to name a few — employ this strategy and are known as simultaneous hermaphrodites, possessing both male and female reproductive organs. Spawning repeatedly over short periods of time (often less than one hour between spawnings), these fishes tend to alternate between their male and female roles.
Many other fishes, like those species described in the genus Anthias and many wrasses, take this amazing role reversal to an even more astonishing level. Many members of these species change sex in midlife. Schools of Anthias are dominated by the largest male, but when he is removed from the population, the dominant female changes sex and becomes the dominant male.
All sheephead, a common wrasse found in Southern California waters, are born females. They function first as breeding females, then later in life, depending on the needs of the population and the availability of territory, many transform into sexually functioning males. In some species, some individuals change sex in midlife, some never will and some might or might not.
Salmon spawn in the streams of their birth, but they travel over great expanses of ocean during their lifetime before gathering at their birthplace to mate and lay eggs in the last acts of their lives. Scientists are still trying to come to grips with all the cues that help salmon determine the place and timing of their massive spawning runs.
Competition for mates is often intense. Male threespine sticklebacks compete for females by trying to build the “best” nests. The winners pass along their genetic code with the cooperation of willing females.
Some competition involves physical displays, as well as violent physical confrontations. In many blennies, males that are competing for breeding territories or mates try to drive away other males by raising fins or opening their vividly colored mouths in order to look as big as possible. Displays are intended to ward off competing same-sex members of the same species, but fighting — with fatal results — does occur in some species. As a rule though, the bigger or brighter male is likely to achieve dominance without violence. The displays help to avoid life-threatening injury that could occur if the males were to fight, and simultaneously the displays establish mating or territorial dominance.
While many of us are familiar with the songs of male humpback whales, it seems somewhat surprising that some male fishes serenade in an effort to convince a potential partner of their worthiness. Some male midshipmen woo potential mates with a song, but other sexually mature and functioning males are incapable of belting out a tune. These “sneaker” males often try to “steal” a quick mating with females that show interest in a singer. In salmon, a similar scenario exists in which smaller “jack” males try to sneak in to spawn with willing females while larger males are fighting amongst themselves for the right to mate.
Other types of displays are very common. Because a male in most species initiates reproductive behavior by attempting to convince willing females that he is Mr. Right, the male often develops bright coloration patterns, as well as accessory structures on its fins. In some cases, the males are considerably larger than the females.
These characteristics are known as secondary sexual traits. While they often help males (and in some cases, females) outcompete their foes and allow them to strut their stuff in front of the ladies, the traits can make bigger, more colorful males more obvious and inviting to predators. For this reason, secondary sexual appeal has its costs as well as its benefits.
No Bones About It
Sharks, rays and skates make up a group of fishes known as cartilaginous fishes. Their skeletons, unlike those found in most fishes, lack bone. All cartilaginous fishes copulate internally and produce well-developed young, especially by comparison to bony fishes, whose offspring typically undergo major changes on the way toward adulthood and sexual maturity.
Sharks are somewhat of an experiment in nature, giving birth in a variety of ways. Some species, such as horn sharks and swell sharks, lay eggs; some, like blue sharks, Caribbean reef sharks and lemon sharks, bear live young; and some eggs hatch inside the mother, but the young continue to develop inside of dear ol’ mom. In these instances, intrauterine cannibalism is generally part of nature’s plan. This means that the first hatched, or the “biggest and baddest” sibling, devours its own brothers and sisters while still inside the mother’s body.
In all cases, litter sizes in sharks and their kin are quite small when compared to the number of offspring produced by spawning bony fishes. In blue sharks, a litter may vary from less than 10 to as many as 200 individuals, while in great white sharks it is believed that only one or two pups is produced each year, or perhaps every second year. On the other end of the spectrum, a female bony fish known as a Mola mola, or ocean sunfish, can produce an astonishing 28 million eggs at a time.
There is no parental care for shark, ray and skate offspring. The energy investment that goes into producing a single, fully developed offspring — a miniature adult at birth — is huge by comparison to the energy investment of bony fishes in fry that will undergo enormous changes on the path to adulthood. Not only are fewer young produced, it usually takes sharks, rays and skates considerably more time — years longer in some species — to reach sexual maturity. As a result, populations of sharks, rays and skates will collapse quickly and dramatically if they are fished with the intensity that many species of bony fishes are pursued.
While many of the already discussed scenarios probably seem bizarre, they are only the tip of a wonderfully diverse and intriguing iceberg. In some fishes, such as silversides, females are actually capable of reproducing without males. All of the necessary genetic material is contained within the egg. She uses the sperm from the males of an associated “sister” species to mechanically begin embryonic development. The poor males are deceived into “believing” that they have mated with a female of their own kind. In some frogfish, the only function in the life of the comparatively tiny male is to produce sperm for his much larger mate. The devoted male begins the relationship attached to his love, and he is soon absorbed into her body, where he continues to produce sperm, thus fulfilling his purpose in life.
Increasing the Odds
No matter how the reproductive game is played in the world of fishes, the end goal is always the same — to pass one’s genes on to future generations. Within a given species, competition for mates is fierce. This competition maximizes the odds of producing the strongest young with the best chance to grow and successfully reproduce, thus helping to ensure the survival of the species.