Divers chase a sea turtle at the wall break, 12 m deep, San Salvador, Bahamas.
Night Life on Coral Reefs
People retire from their offices or workplaces around the same time Monday through Friday, heading home for the evening. Because the majority of the workforce does it at roughly the same time, this routine transition that occurs creates what we call “Rush Hour.” Such is also the case on coral reefs. Like Osha Gray Davidson repeatedly compares in his book, The Enchanted Braid, coral reefs are very similar to cities. And like cities, this marine metropolis also has its own “Rush Hour” per se. Rather than at about 5 pm and only on weekdays, the rush hour on coral reefs occurs between dusk and dawn and every single day of the week—and year for that matter. It is a significant transition period, where the activity level arouses excitement. Daytime feeders are heading to their shelter for the night, while nighttime creatures are awaking and coming out—hungry and ready to hunt.
The descending sun may still seem relatively bright, but the angle in which it sends its rays on the water reflect most of that light off the surface, rendering little light below. This, however, only sharpens silhouettes for the predators’ keen eyes. In addition, it has been discovered that plankton become more available during darkness. Consequently, feeding is at its climax at this time. Researchers have also found that nocturnal predators display higher predation rates on coral reef fish than do diurnal, or daytime, predators. If one were to only observe a reef during the day, much would be overlooked. That is only half of the life and activity; there is a plethora of fish and sea urchins that are unique to solely the night. Furthermore mating and spawning occurs most frequently during dusk and dawn. These nocturnal creatures comprise advantageous characteristics that allow them to have optimal vision and other senses for hunting during the dark hours.
Because nocturnal species emerge at night with food in mind, diurnal fish are out of sight within ten minutes after sunset. These fleeing fish not only must find a secure refuge for the night, but some also have defense mechanisms or strategies to minimize their chance of risk and lower their vulnerability. Triggerfish (family Balistidae) for example, constrict themselves and their condensed bodies into crevices or other safe havens, where they can lock themselves in. They do this by rising their triggers (hence the name), which are their strong, hinged dorsal spine, so that they cannot be pulled out. Parrotfish (family Scaridae) have one of the most unique defenses; it allows them to sleep anywhere they wish—floating about in open water. They secrete a transparent mucus envelope that starts from their mouth, and encloses entirely around their body. This is thought to block olfactory cues from predators, thus providing one of the soundest sleeps among all fish. Some wrasses (family Labridae) can do this too. Though, most commonly for wrasses is to use their snouts to bury themselves in the sand or gravel. Often they will sleep with one eye exposed above the sand to ensure a secure morning to wake up and come out to.
There is a group of fish, called crepuscular fish, which fall neither in the diurnal nor nocturnal category of fish. Rather they are more active during the transition period of dusk and dawn. These fish are piscivorous as well, thriving in the time when the sun casts its light at the angle causing the distinct silhouettes of their susceptible prey—this is called crepuscular light. Barracudas (family Sphyraenidae) are perhaps the most commonly know fish of this group. Jacks (family Scorpaenidae) are also included in this group of fish. They hunt often together and attack groups of fish sending one or two of them in the middle of the group to force them apart, then they pursue individual fishes. Like the jacks, lizardfish (family Synodontidae) will cut off schools of grunts and then pursue the younger ones. Groupers (family Serranidae) have their own technique because of their lack in ability to hunt down their prey. Conversely they will take over a hole or stealthily sneak up on prey behind sea fans.
Nocturnal fish, in contrast to the diurnal fish, which are predominantly herbivorous, are more commonly carnivorous. This results mostly from the difference in food availability at night. Shrimp and other invertebrates that are found on the sea bottom during the day, move up towards the surface at night to feed on microplankton, making them much more disposed. There are also the crustaceans, mollusks, polychaete worms, starfish and other invertebrates, which are normally not found out during the day, but come out to comb the sea floor for food.
Given the conditions, these nocturnal fish have highly developed and specialized sensory systems and adapted traits as a result of the little to no light. One example of this non-visual sensory system is the mechanosensory lateral line employed by the dwarf scorpionfish (family Scorpaenidae). This gives the fish the ability to detect relative water movement surrounding it. Utilizing this, the predator can detect even the slightest current when a prey may swim past. Not only does this enable to distinguish movement, but it can also determine the size of the passerby as well as the direction it is heading. This is only one quality that aids in nocturnal feeding.
Both their pinkish-red color as well as their huge eyes provides the squirrelfish (family Holocentridae) as well as cardinalfish (family Apogonidae) with an advantage at night. They are close to impossible to spot given their color, and their eyes allow them exceptional night vision. They differ, however, in their choice of feeding; cardinalfish feed mostly in the water column, whereas squirrelfish are benthic feeders, meaning they hunt their food on the sea floor. Drumfish (family Sciaenidae) also feed mostly on small crabs and shrimps.
Some nocturnal fish simply swim and hunt fish the traditional way. Such is the case for snappers (family Lutjanidae); they eat crustaceans and other smaller nocturnal fish. Soapfish (family Serranidae) do the same but with more of a strategic effort involved. They simply sit somewhere acting as if they are sick, when in fact they are waiting for fish that become a little too comfortable around it, and then with a sudden dart, the soapfish attacks its prey.
Grunts (family Pomadasyidae) are active day and night, but their appearance might lead one to think otherwise. During the changeover from day to night, the grunt loses its bright stripes that it comprises during the day, and within minutes after sunset, changes to faded stripes or blotchy spots to be more concealed for its nighttime endeavors. The sweeper’s (family Pempheridae) practices are somewhat akin to that of the grunt, though they are a bit smaller. They travel in schools of two to four hundred. To signal a migration one may use its iridescent scales to reflect light to the other fish from the crepuscular light as the sun descends.
Typified by their snake-like bodies and swimming techniques, eels (family Muraenidae) are at ease during the nocturnal hours. They tend to stay in their crevices in the reef during the day, perhaps only showing their heads. Conversely, at night they swim about scavenging mostly on small fish, crustaceans, shrimp, and octopus. Occasionally they are known to even eat a smaller eel. Their poisonous bodies supply them with the peace of mind that they are rarely in any danger. Sharks will only eat them as a last resort if they are in dire need of food.
On clear nights and when the moon is bright, it may provide enough light to impinge on fish behavior. Triggerfish, which as stated earlier are normally found locked in their crevice at night, will be out and about looking for food, granted the moon be bright enough. It is predominantly daytime fish whose habits are altered by the moonlight, and usually not nocturnal fish. Groupers, surgeonfish, and butterfly fish have all been documented to be out foraging for snacks.
Depending on the type, some octopi are found out only at night. The Caribbean Reef Octopus only comes out at night. This cephalopod, like other octopi, has some 200 suckers with which it has the incredible aptitude to differentiate among objects and shapes, if it were it blinded. Compared to all other invertebrates, they have the highest developed brains; they can learn by experience attributable to their both long- and short-term memories. They also have complex eyes, which compare to ours. They have the ability to change colors is a manifestation of the mood they are in—white being afraid, red being angry, and brown is their regular color. In attempting escape when threatened, they may secrete ink in the water to disorient the enemy.
Crawling the sea floor at night are most common the shrimp, crabs and lobsters. These arthropods taste good to not only humans; they also make up the diet of many fish and other sea creatures. All of these, but especially lobsters, are primarily nocturnal just because they avoid the most predators that way—simply being out of daylight. They hide most of the day in their homes, and come out at night, just because it reduces their chance of being seen while they search for food.
Since my first and, so far, only night dive in Grand Cayman freshman spring break, I have been intrigued by the difference in what one encounters compared to the day. I have kept a dive log and recorded dive information and brief summaries about my dive. The following was my recording after that night dive:
Dive No. 7 Date: 3/13/02 Bottom Time to Date: 142 minutes
Depth: 50 ft. Bottom Time: 50 min. Cumulative Time to Date: 192 minutes
15 ft. stop: 3 min. Visibility: 100ft
Comments: It was my first night dive. It was just John and me this time; his dad stayed home tonight. It was incredible how different it is at night—a whole new experience. We saw lobsters, crabs, a much different variety of fish, eels two octopi, and a huge sea turtle, that we accidentally woke up. It was great, we dove through caves with our own flashlights. What an experience.
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