This article is the second in a four-part series that deals with different ecosystems in the world’s oceans. In this segment, we will take a look at life in the sand flats. Subsequent articles will discuss coral reefs and the open sea. Gaining fundamental insights into these habitats will provide any diver with a much greater understanding of the richness of the marine kingdom.
Sometimes when people swim with wild dolphins, the dolphins appear to show a lot of interest in their human counterparts. The dolphins rush in and veer off at the last possible instant, or swim loops and circles around a swimmer, snorkeler or diver. In one way or another, on those days it is blatantly obvious that they are interested in us.
During other encounters, dolphins won’t give us the time of day. Whatever they are doing, they are doing it with other members of their pod, and they just don’t show any interest in humans.
This was one of those “other encounters.” I was snorkeling around a pod of roughly a dozen Atlantic spotted dolphins in the shallow waters of the Little Bahama Bank. I had been with the pod for approximately 30 minutes, when they suddenly stopped moving and turned head down in a vertical position facing the sandy bottom 25 feet/8 m below. As is often the case, the water over the bank was very clear, and it was easy for me to see the sand below the pod. There was nothing of interest as far as I could tell — in fact, nothing at all. To me, the almost sugar-white sea floor looked completely barren and devoid of life.
Slowly, one of the dolphins began to descend. The animal, still head down, stopped about 3 feet/1 m off the bottom. I was bewildered. Suddenly, a small razorfish exploded out of the sand, and an instant later I saw the fish wiggle its last wiggle, as the dolphin swooped in and pinned it with its teeth and jaws before swallowing.
A few seconds later, several more members of the pod descended to within a few feet of the sea floor, and the scene I had just witnessed was repeated time and time again. I watched in total fascination for a few minutes, then decided to dive down to the bottom and see if I could locate a razorfish. To my dismay, I couldn’t find anything but sand.
At first, I thought the dolphins must have gotten all the fish, but as I surfaced for air I was able to see two dolphins capturing their prey. I tried again, with the same result. Then I tried again and again, until finally I saw a razorfish dart across the sand and disappear. No doubt about it, with my sensory capabilities I would have starved to death trying to survive on the sand flats. I would have sworn that the sand was a vast, empty desert. But the dolphins, with their natural sonar used to locate prey hidden in the sand, proved me wrong. There were a lot of living creatures, enough to feed a pod of very active dolphins.
Roughly two weeks later I was back home in Southern California, and once again I found myself swimming over a sand flat on my way to explore a reef community. I was casually surveying what I thought was a lifeless sea floor, when all of a sudden the sand bottom exploded. A 5-foot-/2-m-long angel shark magically appeared right in front of me. An instant later, another startled angel shark arose out of the sand, then another and yet another. The first animal had created a domino effect, and within a matter of seconds I realized I was not alone.
Once again, the sand bottom was filled with life. The creatures around me had not been obvious to me, but that is often the case in the sand biome. Since those dives, I have spent a lot of time exploring and learning about the creatures that inhabit the sand and rubble zones. I have long since quit hurrying over the sand in order to get to the “good diving.” Instead, I’ve learned to slow down and look around in the sand and rubble. I hope this article will help you enjoy an aspect of diving that has been ignored and underappreciated by too many of us for too many years.
Underwater Deserts
Throughout the oases of reefs in both tropical and temperate seas are expanses of sand that visually resemble terrestrial deserts.
Some of these sandy plains extend for hundreds of square miles, while many others cover only a few square yards. In some cases, fragments of dead coral, chunks of rocks and other debris are mixed in with the sand. The zones where the fragments and debris tend to dominate are often referred to as rubble zones. Given time, keen observers will probably notice that some species occur in both the sand and rubble zone, while others show distinct preferences for one zone or the other.
Like the deserts of the topside world, the sandy plains of the marine kingdom are home to many more species than one probably suspects upon first glance, especially if that glance is taken during the day. Primarily due to the fact that there are few places to hide in the sand, during the day a sandy plain often appears to be a void. But dive that same area at night, and the joint is jumping, alive with creatures that seem to have come out of nowhere.
The animals that inhabit the sand biome are considerably different than those that dwell in nearby reef communities, despite the close proximity. Most divers will never see the majority of creatures that inhabit these areas, because so many creatures live in, as opposed to on, the sand and rubble.
For example, in Southern California waters many divers are completely unaware of the amazing variety of polychaete worms that live within the sand. Polychaete worms have the largest total biomass of any sandy plains animals in Southern California waters.
We tend to be aware of a variety of fishes and other animals that are drawn to the sand to feed on the worms, but as a rule the variety and number of worms go unnoticed. The list of creatures we do encounter includes crabs, shrimps, octopi, squids, clams, snails, conchs, flat worms, nudibranchs, sea stars, brittle stars, sea cucumbers, sea urchins, sand dollars, sea pansies, sea pens and cusk eels, as well as a variety of sharks, rays and skates.
Of course, there are not always lots of animals to be seen, but in many locales patience and keen eyes will pay off. In many places that you look, you will discover a pair of eyes looking back at you. During those times of personal discovery, you are likely to encounter a variety of animals and behaviors you have not seen before. At other times when exploring the sand, it seems as if there is “no one home.”
The most vibrant sandy communities are typically found in areas where decomposing debris accumulates and continuous supplies of phytoplankton are available, providing food and some form of protective shelter. There is a distinct lack of plant life in the sand, so most food webs are based primarily upon plankton and decaying debris that drifts into the area. In temperate waters, if a sand patch lies down current from a kelp forest, or in tropical seas if the sand is down current from a mangrove swamp or sea grass bed, a greater variety of species can generally be expected. The variety of sand and rubble inhabitants tends to increase with distance from the beach, as the water gets deeper and the effect of surge and crashing waves lessens.
The Art of Living in the Sand and Rubble
A major difference between a crevice-filled reef and the sand is that while a reef is a latticework of hiding places, the sand is anything but. With few rocks, coral fragments or debris of any significant size in the sandy plains, there are few places for animals to hide other than in or on the sand, or under and amongst the debris. As a result, to survive most creatures that inhabit the sand must be 1) excellent burrowers, 2) able to rebury themselves rapidly if they are exposed, 3) able to stabilize the substrate around them in some manner or 4) superbly adept at the art of camouflage.
Numerous species of crabs and clams are excellent diggers. Many clams spend a considerable amount of time buried in the sandy sea floor with only their paired, snorkel-like siphons exposed. Like the clams that live in mud flats (a habitat discussed in last month’s feature titled “Neptune’s Nurseries: Where Sea Meets Land”), these mollusks use their siphons to take in oxygen-rich water and food while eliminating wastes, without exposing the rest of their bodies to potential predators. If you are a new diver, ask an old salt to point out a clam siphon sometime. Odds are you have seen the siphons before, but never realized they were of vital importance to a living animal.
Many shrimps, such as mantis shrimps, live in self-made burrows. While they sometimes leave their homes, they can often be seen with their eyes peering from the burrow opening as they watch and wait for potential prey to wander past.
A variety of fishes, such as sand tilefish, pikeblennies and jawfishes, also live in self-excavated burrows in the rubble zones of tropical and subtropical seas. Many divers who have explored the tropical waters of the South Pacific have spent entire dives being entertained by the antics of blind shrimp and gobies. In this partnership, the goby maintains a sharp vigil, while a tireless blind shrimp performs housecleaning duties in the burrow. The shrimp usually keeps at least one antenna in contact with the goby. When the goby senses danger, it retreats into its burrow, and the shrimp follows quickly behind.
The relationship between a goby and a blind shrimp is typical of many phenomena that occur in the sand and rubble. The animals are fairly small, and if you are not paying close attention, it is very easy to swim past them without seeing them. But once you do see these partners at work, you can’t help but become fascinated with what you have discovered.
Difficult to get close to, garden eels are often-seen members of many tropical and subtropical sand communities. As divers, we often see eels in the distance, swaying in the current as they stretch out from their burrows in search of planktonic prey. Approach the eels, and they will withdraw. Take your eyes off them for just a moment, and their sudden disappearance into their burrows can be enough to make you wonder if they were merely a product of nitrogen narcosis.
Sea pens and sea pansies often spend their days buried in the sand, but at night they emerge to feed. If threatened or uncovered during the day, they can quickly rebury themselves. The same is true for heart urchins, brittle stars, numerous species of sand-dwelling shrimps and myriad other sand residents.
Many sea pens and sea pansies appear to glow if disturbed by water movement or touched. The glow is known as bioluminescence, or cold biological light. Many animals that live in the darkness of deep water are believed to use bioluminescence in an effort to attract mates and prey, as well as to repel and confuse potential predators. Scientists are uncertain about the advantages of bioluminescence in animals like sea pens and sea pansies that inhabit comparatively shallow areas.
A variety of drably colored tube anemones inhabit the sand and rubble biomes in many regions. These anemones often appear uninteresting when seen during the day, because their tentacles are retracted and their bodies are often buried or partially buried. If uncovered, tube anemones quickly rebury themselves in an effort to gain protection. However, at night they emerge and unfurl their long, flowing tentacles, armed with potent nematocysts used to paralyze and capture.
Even larger creatures like some stingrays, electric rays, and angel sharks commonly bury themselves in the sand. In the case of angel sharks and electric rays, these lie-in-wait predators bury themselves to avoid being detected by potential prey. Those stingrays that bury themselves are believed to do so to go undetected and simply be left alone. When feeding, these rays actively grub around on the bottom in plain view. In contrast, electric rays sometimes appear to “jump” out of the sand to surprise prey. On other occasions, they use their ability to shock their prey while in full view.
These species, as well as many other sharks, rays and skates, are well-adapted to the demands of living in the sand and rubble. These animals take in oxygenated water through an opening known as a spiracle, located near the top of the head. After passing through the spiracle, the oxygen-rich water is passed over sensitive gill tissues. The high positioning of the spiracle enables these animals to draw in water while almost completely buried, without also taking in potentially abrasive grit and debris.
Observant divers commonly discover these hidden creatures by noticing their outlines in the sand. But these animals are so skilled at burying themselves that it is not uncommon to miss them altogether, or to get so close without seeing them that you startle them into swimming away before you ever knew they were near.
In some instances, groups of sand dollars, brittle stars and sand-dwelling worms are so thick that the association serves to stabilize the sand around and within the group, giving these creatures a more secure home and a place where they are less likely to become exposed. This ability enables them to flourish in a habitat that would prove too dangerous to many other species.
Flatfish such as peacock flounder, halibut, turbot, sole, scorpionfishes, stonefishes, snake eels and cusk eels, as well as a number of other fishes, are masters of the art of camouflage. These animals use this ability to blend in and avoid being too obvious to both potential predators and prey. The same is true for a variety of octopi.
Flatfishes are nothing short of natural marvels. They undergo exceptional morphological changes as they mature through their larval stages. When hatched, flatfishes, like most other fishes, have one eye on each side of their head, and they usually swim in what we might call an upright orientation. Like the larval forms of many reef fishes, larval flatfishes live in the water column out in the open sea. As the young flatfishes mature, the animal secretes an enzyme that causes one eye to migrate to the opposite of the head, and by the time these fishes settle into a sand community, both eyes are positioned high on top of the head.
This general body plan enables flatfishes to keep a low profile, while maintaining a sharp lookout for both potential prey and potential trouble. Used like a periscope, their eyes can move independently.
Tests have demonstrated that the ability of flatfishes to match their surroundings is related to their sense of vision. When moved onto checkerboards, these fishes create near-perfect replicas on their own bodies. They have also shown a remarkable ability to make part of their body one color and pattern, and another part a completely different hue and pattern. When blindfolded, they completely lose this ability.
Even given their amazing skills, flatfishes are no match for octopi when it comes to the art of camouflage. While many octopi reside in protected dens and in reef communities, it is not uncommon to encounter some species moving across the sand or rubble, especially at night when they are on the hunt. Octopi display astonishing control over specialized cells in the skin called chromatophores. These cells enable octopi to flush and fade, blanch and darken, and send rippling waves of colors over their bodies. They can also contort their bodies to make themselves look like other marine organisms, from crinoids to lionfish. Normally, the coloration and patterning of an octopus closely resembles its surroundings, and when seen on or near sand it often looks so much like part of the sea floor that divers have a difficult time seeing it at all. But when disturbed it can alter the color, texture, and shape of its skin and body in dramatic fashion.
Not all sand residents bury themselves, nor are they well-camouflaged. For example, whitetip reef sharks are often observed to rest on sand.
Of course, the ability to bury, burrow, stabilize the substrate or use camouflage does not guarantee an animal a carefree existence. A variety of rays and other fishes can often be seen grubbing through the sand and rubble as they search for food. In tropical waters, it is a common sight to see a hogfish blowing “jets” of water into the sand in an effort to uncover prey. Schools of goatfish display a similar behavior, and they can often be seen with their sensory barbels extended while swimming just above the sea floor in an effort to find a meal. Stingrays and eagle rays grub through the sand, often accompanied by barjacks and other fishes that await the opportunity to grab an uncovered but passed-over morsel.
While it can be useful to consider the sand and rubble as biomes that are separate from other marine habitats such as reefs, mangroves and the open sea, these areas are certainly interconnected. It is not uncommon to see creatures like lobsters that one normally finds hiding in crevices, under ledges and in caves in reef communities walking around out in the open on the sand. At times hordes of squid or tuna crabs, animals that normally occur in the open sea, suddenly appear in the sand and rubble. Of course, you shouldn’t overlook the fact that in tropical and subtropical regions, sand originates from corals that are eaten and later defecated by fishes like parrotfishes and puffers. Clearly, the borders to these habitats are frequently crossed by many animals.
Learning about the communities of animals that inhabit the sand and rubble can add a lot of insight and pleasure to your diving. Some sand dives are disappointing, but over time you are likely to have a wide variety of wonderful experiences, if you are patient enough to wait and see what these vital habitats reveal.
Of course, there are not always lots of animals to be seen, but in many locales patience and keen eyes will pay off.
In many places that you look, you will discover a pair of eyes looking back at you, like this flatfish hiding in the sand.
Here Today, Gone Tomorrow
Often, the best way to describe life in the sand is “here today, gone tomorrow.” In a kelp forest, coral reef or rocky reef community, life seems to be much more constant. Once you get a feel for a given area, you have a good idea about what you might expect to encounter on a dive there. By contrast, in the sand community, change is often the name of the game.
On the sandy plains in the waters of Southern California, it is not uncommon to encounter dozens if not hundreds of bat rays when diving the sand during certain parts of the year. Come back to the exact same spot two weeks later, and there is not a bat ray to be found. The sand might appear barren, or it could have been invaded by uncountable numbers of tuna crabs, a horde of halibut, Liticinus sea urchins, brittle stars or common squid. But they too are likely to stay for a limited time only.
The transient nature of the sand is part of its allure. You just never know who or what you might see.
Angel sharks bury themselves to avoid being detected by potential prey.
Octopi display astonishing control over specialized cells in the skin called chromatophores. These cells enable octopi to flush and fade, blanch and darken, and send rippling waves of colors over their bodies.