Playing the Odds
Survival and reproduction on the reef
While diving a coral reef, vibrant and bursting with activity, it’s easy to think of this ecosystem as a ‘magical kingdom’ with a community of strikingly beautiful creatures living in serene harmony. Yet, the appearance of serenity is a facade. Life within this coral metropolis can be very difficult. It’s a realm of “eat or be eaten,” with its own brand of ‘mean streets’ and a harsh set of rules. Let’s look at a few of the survival and procreation strategies used by marine creatures you are likely to see on Wakatobi’s reefs.
Survival of the fittest
Watching a pair of ornately colored adult butterflyfish swimming together, foraging amongst the fingers of branching coral, makes one wonder how such delicate creatures make it so far. Yet they do, rising above the challenges presented in the underwater realm during their journey from juvenile to adult. In fact, each species of fish we see on the reefs at Wakatobi is a result of a unique survival and reproductive strategy that that evolves into unique ways to beat the odds.
The most important underlying rule of nature remains the survival of the fittest. This doesn’t necessarily mean being the biggest or the meanest, but simply that, for a species to flourish, some members of each generation must live long enough to successfully pass on its genetic heritage through propagation. The variations on this strategy are endless, and can involve everything from hiding, armoring and attacking to simply putting out more offspring than competitors in hopes that at least a few will beat the odds and make it to breeding age.
The tasks of seeking and establishing a mate, then brooding and rearing youngsters can be both a perilous as well as delicate matter. Ultimately, when it comes to propagation within a set of species it all comes down to finding a suitable mate.
Making the Connection
Unique markings and colors serve as important identifiers for marine species to differentiate others of their kind from the maelstrom of bodies and colors on the reef. According to Boston University marine biologist Gil Rosenthal, as a reef fish retreats, distance and motion can make it difficult for predators to perceive fine details and distinguish closely spaced outlines of contrasting colors. So at a distance, spots and stripes blur together, helping even stationary fish merge into the background of the reef and the ocean beyond.
Color can also speak the language of love on the reef; when it comes to marketing yourself as a suitable mate, presentation is everything. Sometimes having the right “suit” is not enough, thus requiring additional tactics such as incorporating particular body postures and movements to draw further attention to the opposite sex.
While color can be a key component for finding a mate, for some species like the frogfish, drawing attention visually is not necessarily the best form of flirting. For a long time, scientists have known the color plays a role in sexual selection and defensive tactics, says marine biologist Les Kaufman. Frogfish survive by making themselves as inconspicuous as possible, so when it comes time to broadcasting their intentions, they do so chemically, by releasing distinctive scent or ‘pheromone’ into the water. This is handy as subtlety is important when there are plenty of other species of fish that would also happily make you a meal.
“Scientists have long known that color plays a role in sexual selection and warning of danger.” Les Kaufman
For some fish, reproduction is at the very top of their priority list. It is their most important achievement and they take sexual competition extremely seriously. So in order to counteract competition, some fish delay their sexual maturity and begin life as a functional male or female and later change to the other sex! For example, the squarespot anthia begins its life as a female in a harem with one dominant male. The female clearly distinguishes herself by showing a bright orange color, whereas the male is a beautiful purple and displays a square spot on each side of his body, hence their name. If the male dies or is removed from the harem, the highest ranking female will rapidly change sex, color and size in order to replace him and take over the command of the harem. As a juvenile female in a harem, the squarespot anthia also benefits by the protection of the dominant male.
The squarespot anthia begins its life as a female in a harem with one dominant male. As a juvenile female in a harem, the squarespot anthia also benefits by the protection of the dominant male. Photo by Mark Vanderlinden
So you think you’ve found the right mate, but have you? In the world of fishes, the odds for a species’ continuing successful rate of survival can boil down to a numbers game. In most marine species, the fecundity (egg production capability) of a female is directly related to her size and even her age. The larger and older she is so is her capacity to produce larger batches of bigger and better eggs (roe). Bigger and better eggs produce bigger and stronger larvae, which can translate to a larger number of that brood more likely to survive to adulthood. This is the attraction point that drives male suitors instinctively after the larger, more mature females. Here again, it’s survival of the fittest, or strongest.
Most reef fish are egg layers, with complex methods of distribution. The majority of these fishes are pelagic spawners (jacks, butterflyfishes, wrasses, hawkfishes, angelfish); meaning they rise up and release their eggs and sperm high in the water column to send the eggs seaward. During this activity, some species like grouper and snapper will gather in large aggregations at a specific spot to spawn during a precise phase of the moon. For others like parrotfish, it is common for a moderate sized group of breeding fish to congregate, typically at or near dusk, synchronizing the release of their eggs and sperm en masse with others members of their species to avoid predation. The larger the aggregation of breeding animals, the higher the odds that the cycle of eggs, which can number well into the millions, will make it through the larvae stage, then onward to juvenile after a considerable loss along the way. A lucky few will make it through to adulthood and thus repeat the cycle.
One of the other environmental factors that will play into this cycle is when and where a spawning will take place, such as with lionfish. During times of strong outgoing currents the lionfish will increase the range of distribution for the eggs, thereby increasing the potential success rate and distribution assuring survival of the species.
Crevalle jacks are pelagic spawners (as are butterflyfishes, wrasses, hawkfishes, and angelfishes), meaning they rise up and release their eggs and sperm high in the water column to send the eggs seawards. Photo by Doug Richardson
By contrast, fish like damselfishes, triggerfish, anemonefishes, gobies, blennies and other similar species, will lay their eggs on the bottom (some will build a nest in advance) and attach them to the substrate (or solid bottom) like the cluster of eggs seen above. Afterward, some will zealously protect the eggs up until the day of hatching. On that day, the larvae are left to their own devices as they drift away. Nest builders are rare on the reef but fascinating to watch; they are highly industrious as they clear a site of all algae and debris and fiercely guard it. They are also protective parents after the eggs are deposited, and they will continue to guard the burrow until the eggs hatch.
Growing Up On the Reef
When settling on the reef, some larvae use their sense of smell and hearing to find a place where individuals of the same species are already living. This guarantees they’ll find food corresponding to their diet and that the environment is well adapted to their species. It also means that they are in direct competition with some of other residents on the reef. Welcome to life in the big city.
Some juveniles quickly understand that instead of hiding from predators, they may as well face them and be part of their team!
Because fish are not all born under the same lucky star, some have to be more creative than others when finding their path through life. Some juveniles quickly understand that instead of hiding from predators, they may as well face them and be part of their team! They wear a specific color, an electric blue, which is actually a clear advertisement of the service they can offer to predators. Commonly known as “cleaner blue,” is a blue wave that fish can see very well and therefore identify, as seen here on two cleaner wrasses grooming the pufferfish. By offering cleaning services, this removes the predator-prey relationship that normally rules the juvenile’s life.
Life is tough on the reef, and fish don’t get to rest during any phase of their life. No matter how charmed or successful that life might be, sooner or later they will eventually fall prey and completing the circle of life.
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