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Coralline Algae Target Phenomena – What makes those bull’s-eye patterns underwater?


This being National Archery Day (in addition to the much more significant Martin Luther King Day), I decided to finally look into and try to understand something that we see regularly underwater, but that has puzzled me my whole diving career. That is: patterns that look like archery targets which grow on shaded vertical rock walls underwater.

Coralline Target cropped 640 Makena Landing

A Coralline Algae Target with thin rings. Approximately 12 inches in diameter. Makena, Maui. Photo: P. Fiene


I guess I knew early-on that these concentric circles were a type of seaweed called crustose coralline algae. What are coralline algae? They are a group of red algae composed of fused plant filaments that have calcareous deposits within and between the cell walls. The crustose coralline algae look like a thin crispy layer, usually pink or purple, covering the surface of a rock.


In my mind all these years, the striking white concentric circles were the crustose coralline algae, and I paid no attention to the uninteresting background. My eyes saw only the white rings.


But I had it all wrong! The living part is the purple-pink background! The white rings, it turns out, are where the coralline algae has been killed. To finally learn this is poetic justice because I have always been amused when my divers think that bright white coral colonies are extra beautiful, when in reality they have just been killed and the white is the bare coral skeleton.

Coralline algae target - thicker rings 640

Coralline Algae Target with thicker rings. About 12 inches in diameter. Makena, Maui. Photo by P. Fiene


This white ring disease (or complex of diseases?) was named “Coralline Target Phenomena” by Smithsonian coral reef scientists Mark Littler, Diane Littler, and Barrett Brooks in a short paper they wrote for Reef Encounters in 2007*. In the paper they proposed that the white rings are caused by an unknown agent (a bacterium, group of bacteria or other pathogen) that kills the pink coralline algae filaments in discrete concentric strips leaving just the white calcareous part exposed.


They did not have the opportunity to observe a target over time, so they made an educated guess at an explanation hoping that it would elicit information from colleagues. Whatever the agent causing the disease, they hypothesized that the pathogen kills a strip and then “leap frogs” over living tissue to infect ever-larger bands as the circle increases in diameter.


I contacted one of the authors, Barrett Brooks in the Botany Dept. of the Smithsonian, to find out if he knew of any new studies or information on the subject. He replied, “The phenomenon is still entirely open for study. No one, to my knowledge, has focused additional study into the matter.”


He also offered some additional comments to clarify what is known and not known: “Although bacteria were found present in the targets, they could be a result of some lethal activity, and maybe not the cause. How microbes work on reefs is a huge hole in our knowledge. I’m not entirely sure of the cause proposed, nor am I sure that there is only one cause. There is a range of “targets” from small diameter/thin bands, others large with thicker bands. It certainly looks like the bands travel/radiate outward, but who knows… The full answer is still out there.”

Coralline Algae Target - thick rings 640

A Coralline Algae Target with thick rings. Makena, Maui. Photo: P. Fiene


Next in my search for a possible explanation I asked marine biologist Cory Pittman and he offered this tentative hypothesis for how the bands might form:


“Assuming the cause is microbial (not necessarily bacterial – the bacteria could be secondary to an initial infection by something else), I wonder if the pattern might result from an interaction between a microbe and a defensive response by the algae? Assume, for the moment, that the agent colonizes a new patch of its host algae (cyst landing, contact with a vector…) establishing an initial point of infection. A colony spreads out from that point killing the underlying algae. The algae around the colony respond by ramping up production of a defensive chemical until it reaches a high enough concentration to suppress the growth of the microbes. The microbes respond by entering a migratory phase and “crawling” forward until they arrive at algae that hasn’t built up enough of the defensive chemical to resist them. Then, they start to consume tissue, again, until the next band of algae builds up resistance. With repetition this forms the target pattern. Meanwhile, the older, resistant bands of algae may begin to slowly overgrow the dead portions..


That would be analogous to how some terrestrial plants respond to attacks by insects. Neighboring plants sense chemicals released by the attacking insects (or the infected plant) and respond by producing defensive compounds that would be too metabolically expensive to produce all the time, but serve to check the spread of the insects ‘when needed’.”


Coralline algae target - thicker rings both  arrows

Photo taken at Makena, Maui by P. Fiene

The photos taken in Hawaii provide some clues regarding the progression of the disease over time. First, at the center of the target the coralline algae shows some re-growth over the dead white rings (yellow arrows) indicating that those rings have been there longer and the coralline algae has had time to recover.


Both green micro-algae and young starts of foliose brown (or red?) algae that colonize the dead surfaces are more prominent on the inner rings (black arrows) than on the outer rings, mirroring the progression of the disease and also suggesting that it takes some time (perhaps a matter of weeks?) for the “targets” to reach their full size.


Coralline Algae Target close-up-Makena Landing copy

Photo taken at Makena, Maui by P. Fiene.



In our Hawaii photos, the coralline algae surrounding the inner rings is often starting to regrow as indicated by the fine white edges (green arrows). The coralline algae around the outer rings shows no such white edge/active growth.






Coralline Algae Target with pink arrows- CP

Photo taken at Hekili Point, Maui by Cory Pittman


Also, some portions of the dead surface in the outer rings still retain faint pink pigmentation (pink arrows) suggesting that those patches have recently died. Again, this emphasizes the progressive formation of the “targets.”







In addition to determining the cause of the “targets,” there are many other intriguing questions to address, such as exactly how quickly the bands are formed, what keeps the targets from expanding to more than about a foot in diameter (in Hawaii), do the other species of algae that colonize the dead surface have any influence on the process, does anything relevant happen at night (just in case something else but a microbe is involved)…

With our curiosity now piqued, we will be keeping our eye out for “targets” to monitor and photograph over time.


by Pauline Fiene


*Mark M. Littler, Diane S. Littler, & Barrett L. Brooks. 2007. Target phenomena on south Pacific reef: strip harvesting by prudent pathogens? Reef Encounter 34:23-24.

Transparent Moorish Idol recruit – How long does it take to become opaque?


As Hawaii’s divers and fish-watchers are well aware, this summer will go on record as the largest settlement of juvenile fishes (recruits) recorded in historic times. Estimated millions of Blacklip Butterflyfish, thousands of Yellow Tangs, and hundreds or thousands of individuals of numerous other species settled out of the plankton on reefs on Oahu, Maui, Molokini, the Big Island and most likely other islands as well.


The settlements are not over yet, however. As of mid-September, recruits of some species are still settling. As our dive guide Warren Blum puts it, “We’re receiving new products all the time.” One of these “new products” is the Moorish Idol.


As divers, it is incredibly rare for us to see a reef fish in the first few days after it has settled on the reef. Most species, when they arrive, are transparent and/or silver, and hide down in the reef, some emerging only at night. During this time they undergo what is called transformation, where they become opaque, add color, and can change dramatically in body shape and fin length. Normally, by the time we notice new recruits, they have their color and look like juvenile fish.


Because Moorish Idols are comparatively large when they settle out of the plankton (about 3 inches), and because they are more likely to be out during the day before they have fully transformed, we get to see them while they are still partly transparent. From a distance a newly-settled Moorish Idol looks somewhat like an adult Moorish Idol with its pale night colors (think the-world-before-color-TV).

newly settled Moorish Idol fro blog

A recently-settled Moorish Idol (left) and a transformed juvenile (right).


But up close it becomes evident that the fish is actually transparent and we can see its spinal column right through its body!! This is advantageous for avoiding detection in the open ocean, but it is not ideal for life on the reef, where it must be visible in order to claim a territory or attract a mate (and where a transparent body would render it the ultimate wallflower!).

Moorish idol close-up

Recently-settled Moorish Idol, Sept. 6, 2014. Body is mostly transparent and spinal column is visible.


We were lucky to photograph one of these new arrivals over a period of days. Not only was it transparent with some opaque silver inside it, but its body was differently shaped than an adult. And already it had encountered predators which had taken bites out of several of its fins. This was actually a bonus for us because when we went back the next day to find out how much it had changed, we were able to find the exact individual, among the several that had settled on this particular section of reef, by looking for its unique pattern of bites.

idol trifecta 650

Recently-settled Moorish Idol on Sept. 6, 7 and 8 (left to right)




We have often wondered how long it takes for a newly settled transparent fish to transform into its adult colors, and now we have a much better idea. While we don’t know when this individual arrived or how transparent it was the day it arrived, we know that on Sept. 6 we could still see through it. The very next day the spinal column was less visible and the day after that the fish was completely opaque in that area. Along with the color acquisition, the dorsal fin became dramatically shorter and the snout became more developed – an impressive, almost magical transformation, especially between the first two days we saw it.

Two weeks later, although the fins were healing and filling in, this individual fish was still identifiable by bites and tiny color markings. It was still residing over the same small patch of reef. As an adult, it will have a larger range, but for now it is staying right where it settled after spending months in the plankton. One of the very lucky few to do so.

Young and adult Moorish Idols

Left: “Our” young Moorish Idol on Sept. 23, 2014. Right: What a full grown adult looks like

Part Two – Juvenile Blacklip Butterflyfish (Chaetodon kleinii) swarm divers and turtles in competition for food

strength for blog

The arm of Ed Fingers gets picked by juvenile Blacklip Butterflyfish. Puu olai, Maui. Aug. 27, 2014

Strength in numbers is not a phrase that applies to the recent settlement of estimated millions of Blacklip Butterflyfish (Chaetodon kleinii) at Molokini and off the south coast of Maui. This many fish compete mightily for the limited amount of available food. We didn’t realize how serious the competition was until we had the opportunity to observe these masses of fish in two completely different habitats separated by 3 miles.


Molokini is a little volcanic cone that sits 3 miles off the coast of Maui in about 300 feet of water. Because it is so far offshore it is often exposed to currents and sometimes upwellings, both of which bathe the island in food for plankton-eating fish and invertebrates. Thus, the estimated millions of Blacklip Butterflyfish that settled at Molokini in July and August and September of this year (see Sept. 2 blog post) have been well-provided for by virtue of Molokini’s geographical position. They have distributed themselves roughly evenly around the crater, finding habitat all around it, and feed by picking the abundant plankton out of the water a few feet from the protection of cracks and crevices in the rock and reef.


The Blacklip Butterflyfish that settled along the coastline of Maui, however, have not been able to distribute themselves evenly because much of the bottom is flat, open sand. The sand provides no shelter so they have been forced to congregate over the isolated rocks and reefs. In addition, these Maui areas do not receive as much current, and therefore planktonic food items, as does Molokini. Such high concentrations of fish mean that competition for the limited plankton around these rocks is fierce. Yes, they are plucking tiny planktonic animals out of the water, but in addition they are performing a behavior that is very unusual for fish – juvenile or adult. They are swarming over us, picking who-knows-what off any exposed skin, dive gear and camera equipment.


once in our lifetime to send

Victoria Martocci is swarmed by juvenile Blacklip Butterflyfish. Puu olai, Maui. Aug. 27, 2014.


Leslie's hand with butterfliesIf you remember the old (and effective) Johnson’s Wax TV commercial for OFF mosquito repellent you will be able to picture what diving during this time has been like. In the commercial when the man puts his untreated arm into the terrarium of mosquitos they immediately swarm his arm. This has been the scenario at certain rocks off of Maui. As soon as we come close enough to the rock these youngsters swarm us, looking for any part of us that might be edible.


Ann and JD

Ann Quinn and JD Brill hold still while juvenile Blacklip Butterflyfish pick at their masks. Puu olai, Maui. Aug. 14, 2014.


Of course, we are not the only visitors to these rocks. Turtles, which are used to receiving some attention from several species of adult fish who clean their shell of algae and organic matter, are swarmed as well.

whole turtle coverered for blog

blacknoses on turtle face for blogCompetition for food drives them to us. The ones that are bolder and more willing to approach large animals such as turtles and divers might be rewarded with more food. But they could also be exposing themselves to greater danger by moving farther away from their shelter. The ones that stay near the rock – perhaps they miss out on a good food source, but they may also be safer from predators. Interestingly, on days when there was current on these rocks, the fish were up in the water feeding and not at all interested in us. The current was presumably bringing them all the food they needed.


whole turtle being cleaned

It has been six weeks since they began swarming us and as their numbers decreased, so did the frenzy of unusual behavior. They stopped swarming divers and turtles, perhaps because there was enough food in the water for their reduced numbers. Or, perhaps as they get older, like many animals, they lose the fearless behavior and become more cautious.

While it lasted it was a wonderful experience out of the norm – to have fish coming to us instead of fleeing from us! In 35 years of diving in Hawaii we have never seen swarming behavior like this.

And if you’re wondering what it looked like from the turtle’s perspective….

inside the mask







Text and photos by Pauline Fiene


We would be so interested to hear if anyone else has observed behavior like this from a species of reef fish.


Part One – Massive numbers of juvenile Blacklip Butterflyfish (Chaetodon kleinii) settle at Molokini and in south Maui


Have your eyes ever beheld so many individual things that they felt full? That is the way my eyes felt three weeks ago while diving off of Molokini and south Maui. They felt so full that I actually had the sensation they were itching.


This is because on July 8 thousands of juvenile Blacklip Butterflyfish (Chaetodon kleinii) appeared overnight on reefs at Molokini and parts of south Maui. A month later on Aug. 4 another “shipment” arrived bringing the numbers into the estimated tens of thousands. And then a week later on Aug. 11, there was another astounding settlement which seemed to bring the numbers into the hundreds of thousands or millions. And these really were three distinct events. One day we were diving and everything was normal. The next morning, we descended and it looked like this:

IMG_0596 slice-Molokini for blog

Hundreds of thousands of juvenile Blacklip Butterflyfish (Chaetodon kleinii) settled at Molokini in the summer of 2014. Photo: Pauline Fiene


Yes, it is an exceptional summer for numbers of newly settled juvenile fish (called recruits) of many species throughout the Hawaiian islands – various surgeonfish including yellow tangs, goatfish, other butterflyfish, parrotfish, trumpetfish and others. But, massive numbers of Biblical proportions seem to have occurred only with the Blacklip Butterflyfish, and have taken place mainly at Molokini and off the Makena area of Maui (with large, though not this massive, numbers reported in north Kohala on the Big Island and on the north shore of Oahu, and few to none in West Maui, Kihei or Maalaea, the Kanaio Coast of Maui or the island of Lanai).


Not wanting to use the word ‘Biblical’ lightly, I thought I’d try and put a number on this incredible event. So, I took the photo of a section of reef at Molokini (above) and estimated the linear distance in inches contained in the photo. I then counted the number of fish visible in the photo and arrived at 16.8 fish per linear inch of visible depth. And then I multiplied 16.8 by the estimated number of inches around the outside rim of Molokini and along the inside of that same rim. The number…….. are you ready…….. was over 1.7 million Blacklip Butterflyfish.


This estimate is based on the number of fish that could be counted in the depth range visible in the photo (about 20-40 feet). The fish actually extended more than four times as deep – down to 170 feet in places, potentially doubling or tripling this number. The estimate also does not include those distributed throughout the center of the crater. On the other hand, although the fish numbers appeared similar around the entire outer rim and along the inside, the surface area of acceptable habitat might not be as high in every part of the rim as it is in the photo. So obviously, this must be considered a rough approximation. When the tens of thousands of Blacklip Butterflyfish juveniles off of south Maui are included (another rough estimate based on a photo count – photo below), a safe statement to make would be that at the peak (the week of August 11) there were probably in excess of two million Blacklip Butterflyfish juveniles in the Molokini and south Maui areas that we dive (Wailea and Makena).


What on earth could cause such an epic show of nature’s ability to procreate?

Two of the most likely contributors to such numbers are the survival of an unprecedented number of larvae, and currents which concentrated and then brought the larvae near the islands.


final Tess' spawning diagram whiter sharper


Understanding larval survival requires knowing about the life cycle of most reef fish. A pair of adult fish release their eggs and sperm into the water at precisely the same moment and no parental care follows. Within a couple days the fertilized eggs hatch and tiny transparent larval fish only a few millimeters long emerge to develop for weeks or months. This takes place up in the water, not on the reef where the fish will eventually reside. This is also where the vast majority of them die.* This summer, however, for reasons unknown, massive numbers of larvae survived.



larval Chaetodontid 28 days post-hatch, 6mm

A larval butterflyfish 28 days after hatching from the egg. It is only 6 mm long. Used with permission from Frank Baensch (

The larvae that make it through that critical period of 2-3 days post hatching, will, when developmentally ready, detect the proper habitat on the reef by smell or sound or other signal. They will swim down to the reef, often still in a partially transparent form, tuck into a crevice in the reef and begin to take on the colors of the juvenile fish. This process, called metamorphosis, is almost completely under the radar as far as we divers are concerned. We do not see the larvae up in the water and we very rarely see the transition stages once they settle on the reef. By the time we notice them, they are juvenile versions of adults.



one kleinii smaller

A juvenile Blacklip Butterflyfish, about 30 mm long. Photo: Pauline Fiene

The juvenile Blacklip Butterflyfish, however, appeared overnight, as fully formed juveniles. Instead of arriving on the reef as larvae, scientists have learned that a minority of species metamorphose in the plankton and arrive on the reef as juveniles. This strategy might have been key to the massive scale of the Blacklip Butterflyfish settlement here on Maui. Most larvae have an internal clock ticking, so that when they are at the right developmental stage they have a limited window of time to detect suitable habitat and swim to it. If currents do not bring them close enough to a reef habitat that they can swim to, they die, unable to find shelter and the food they require. But, because the Blacklip Butterflyfish apparently has the ability to metamorphose into its juvenile stage in the pelagic environment, and because it is a plankton eater (and not dependent on eating algae or sponge or invertebrates, etc), it can survive longer in the planktonic environment, giving it a few more days for currents to bring it close enough that it can swim to the reef environment that it needs.


In addition to larval survival and “helpful” currents there are other factors that could have played a part in this wondrous settlement of juvenile Blacklip Butterflyfish. Some of these are fertility of the adults that spawned the eggs to begin with, sea conditions such as temperatures that are favorable to the eggs and developing larvae, plentiful food for the larvae, low predation on the larvae and probably other factors which we do not even know about yet. Most likely, this once-in-a-lifetime event was a fortuitous combination of two or more of these factors.


Whatever the reason, these fish are, unfortunately, probably not here to stay. Conditions in the plankton were clearly excellent for the larvae, but their requirements as adults living on a reef are completely different. Just a week after the August 11 settlement, the numbers had decreased slightly. Previous large settlements have shown that numbers continue to drop, most likely through predation and competition for a limited food supply.


blacklip butterflyfish, Maui

Thousands of Blacklip Buttterflyfish settled in the Makena area of Maui. There are over 2,400 fish in this photo. Photo by Pauline Fiene


You could dive your whole life and never be witness to an event like this. In the memory of this generation of Maui divers, obvious settlements of exceptional scale have happened only three times in the past 30 years on Maui: 1984 with Fantail Filefish (Pervagor spilosoma – estimated at tens of thousands); September 2008 with Blacklip Butterflyfish (tens of thousands) and our current July-August 2014 Blacklip Butterflyfish settlement in the probable millions.


Events such as this one are a great opportunity for us to learn about the life cycles of marine animals that determine how our reefs are populated. Life cycles which are happening all around us all the time, but not in a way that we notice – until it makes our eyes itch.

by Pauline Fiene

You can read more in Part Two –

Juvenile Blacklip Butterflyfish (Chaetodon kleinii) swarm divers and turtles in competition for food


*A 2014 paper (V. China, R. Holzman. Hydrodynamic starvation in first-feeding larval fishes. Proceedings of the National Academy of Sciences, 2014; 111 (22): 8083 DOI: 10.1073/pnas.1323205111) reports that 90% of fish larvae die from what the scientists call hydrodynamic starvation, which is the inability to eat due to the viscosity of the water preventing the larva’s ability to propel itself forward. The water drag on such a tiny 3 mm larva keeps it from being able to lunge at and inhale the planktonic food, so it starves.

Dr. Jack Randall – Hawaii’s Renowned Fish Scientist – Scuba Dives on his 90th Birthday


This past week, as you may have heard, former President George H.W. Bush celebrated his 90th birthday by jumping out of a plane, performing his 8th skydive. But did you know that here in Hawaii, a locally well-known septuagenarian marked his 90th by scuba diving? Dr. John E. Randall, former Curator of Fishes of the Bishop Museum and member of the Graduate Faculty in Zoology of the University of Hawai’i, was taken for the dive by former student Dr. Richard Pyle from the dive boat of good friend Dr. Gordon Tribble. He even shared the dive off Waikiki with two of his grandchildren, Sandra and Sean!

Jack's 90thdive3

Dr. John E. Randall after a scuba dive with his grandchildren on his 90th birthday

Dr. Randall, or “Jack” as he is known, has led an incredibly rich and adventurous life, so the fact that he dived on his 90th birthday is actually not that surprising. As a 26-year old, Jack built and then sailed his 37-foot ketch from California to Hawaii where he earned a PhD in Zoology. He married Helen Au, also a graduate assistant in Zoology. They sailed the ketch with daughter Lori (age 2.5) to Tahiti for research on fishes with support of a research fellowship from Bishop Museum and Yale University. While an assistant professor at the University of Miami, he directed a marine biological survey of the Virgin Islands National Park, followed by four years at the University of Puerto Rico as a Professor of Zoology and Director of the Institute of Marine Biology. He returned to Hawaii in 1965 as Director of the Oceanic Institute for a year before becoming the ichthyologist at the Bishop Museum in Honolulu.

Jack’s long career has resulted in the description of 731 valid new species of fish, more than any ichthyologist in history! At 90 he continues to describe and write about fishes, and this year will publish his 14th guidebook on fishes, entitled Coastal Fishes of the Red Sea with a Russian and a German as coauthors. It includes all the fishes of the Red Sea to 200 m (total 1072 species).

More exciting however is his soon to be published memoir, Fish ‘n’ Ships! I have had the opportunity to read much of it, and while I can’t talk about it just yet, I will say that it is a page-turner. Jack has packed so many adventures (and close calls!) into his life, and he has lived through so many interesting periods in US history that I could not put it down. The book should be published before the end of 2015.

by Pauline Fiene

Do Hermit Crabs Kill Snails for Their Shells?

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This is what I have wondered for most of my diving career.

The 6th edition of Ruppert/Barnes Invertebrate Zoology textbook published in 1994 seemed to make it pretty clear: “Hermit crabs always use empty shells and never kill the original occupant.”

So did the 7th edition published in 2004: “These distinctive decapods appropriate discarded snail shells for use as portable domiciles.”

Then a couple months ago I came upon this scene.

Scene 1 Old shell and new

A yellow hairy hermit crab (Aniculus maximus), in a scrappy remnant of a triton’s trumpet shell, was reaching way into the opening of a beautiful intact triton’s trumpet (Charonis tritonis) shell. What was going on in there?? Was the triton snail alive and in its home shell? Or was the shell empty? Was there another hermit crab already occupying the shell?

hermit entering triton 2

I moved the hermit crab aside and, after convincing myself that the shell was not occupied by a hermit crab, reached in and felt the snail completely retracted, with the operculum (trap door) tightly sealing the opening. Since it takes energy to hold the operculum tight, I knew that the triton snail was still alive.

So, what was the hermit crab trying to do? Clearly it was in need of a new shell because it was exposed from the rear by the broken apex of the shell. Was it just inspecting the shell to see if it was occupied? Or did it have a darker plan? Since there was no way to know, we had to continue on our dive with our questions unanswered.

When I got home I Googled “Do hermit crabs kill snails for their shells.” Many aquarists reported hermit crabs killing snails in their tanks as a common occurrence, mostly to eat the snails, but sometimes occupying the shells as well. Is this behavior a result of being held in captivity where food availability, shell availability and snail refuge options are not representative of those found in the wild? Or is this normal behavior that happens in the wild as well?

Scene 3 Dead triton and hermit with flesh in claws

The next day by chance we ended up diving in the same location. I had no expectation of seeing the animals again, but as we approached the ledge, there they were in exactly the same place. As we got closer though, we could see that something was different. The hermit crab still had its claws inside the opening of the shell, but not as far in as yesterday because the triton snail was protruding from the aperture – and was motionless. The hermit crab, with its claws full of snail flesh, had apparently killed the snail!

operculumLooking more closely, the operculum had small chips all around the edge, indicating that the hermit had been attempting to gain access to the snail by picking away at the operculum with its claws.

The following day when we returned to the site, all the participants were gone. Since the triton snail was dead and could no longer crawl away on its own, we theorize that the hermit crab was able to remove the dead snail, occupy the shell and carry it away.

To find out if anyone else had seen anything like this, I asked people who I knew had been diving a really long time and were known for paying particular attention to what they saw. Not surprisingly, Linda Marsh, a divemaster with over 15,000 dives in Hawaiian waters and owner of Kauai dive operation Bubbles Below, had had a similar, but more “interactive,” encounter over a period of five days.

Linda’s yellow hairy hermit crab was in an old partridge tun shell, and when she found it trying to get at a triton’s trumpet snail she actually separated them and swam the hermit crab about 100 yards away. The next day, however, it was right back on the triton snail! She separated them again on two more days, each time swimming the hermit far away, but the following day the hermit would be back on the triton, digging around the operculum. Eventually within a 24-hour period the hermit successfully killed the triton snail and she saw it occupying the triton’s shell.

With this evidence, I contacted Dr. Brian Hazlett, hermit crab expert and Professor Emeritus of Ecology and Evolutionary Biology at the University of Michigan. He was aware of only one report in the literature of a hermit killing a snail and occupying the shell. Another researcher, Edwin Iverson, also was aware of only one example in the literature.

Petrochirus diogenes Keoki

Giant Hermit Crab (Petrochirus digenes)
Photo by Keoki Stender.

The paper they cited was one published by Dr. Jack Randall (yes, world-reknowned fish biologist) FIFTY years ago. In that paper Dr. Randall described a long-term study in St. John, Virgin Islands where he and some colleagues built a fence around an elliptical area of seagrass and sand that housed numerous queen conchs they had tagged. One day they found one of their tagged conchs missing, and in its place an empty eroded queen conch shell that had not been inside the enclosure initially. Later they watched as a hermit crab, Petrochirus diogenes, attacked a queen conch, and subsequently occupied the queen conch’s shell. They determined that this species of hermit had climbed the fence, consumed one of the conchs, moved into its shell and climbed out of the enclosure, leaving its old shell behind. In total six of their tagged conchs were killed and occupied by this species of hermit crab.

Our question had finally been answered. At least two species of hermit crab (Aniculus maximus in Hawaii and Petrochirus diogenes in the Caribbean) kill snails for their shells.


Scene 4 showing damage to snail and chipped operculum

But one question answered invariably begs numerous follow-up questions. For me, one of these is how did the hermit crab kill the triton snail? As you can see in the photo to the left, other than a few small gouges out of the snail, there is no obvious cause of death. One possibility is that the triton snail, being forced to remain fully retracted in it shell, could not effectively circulate enough water across its gills and died from lack of oxygen. Or, the muscular tension required to keep the operculum tightly sealed could have led to anaerobic metabolism and lactic acid build-up, something that might have reached toxic levels. Either of these or the combination of them could have led to the death of the triton snail, and once the snail’s muscles relaxed, the crab could have consumed the original builder and moved into its beautiful new home.


Written by Pauline Fiene. Photos by Pauline Fiene unless otherwise credited. Mahalo to John Hoover, Dr. Brian Hazlett and Cory Pittman for discussion and insights; to Warren Blum for underwater observations, to Keoki Stender for generous use of his Petrochirus diogenes photo; and to Linda Marsh for her recounting of such a fabulous and rare multi-day encounter.

Iverson, Edwin S. 1986. Predation on Queen Conchs. Strombus gigas, in the Bahamas. Bull. Mar. Sci. 39(1): 61-75.

Randall, J. E. 1964. Contributions to the biology of the queen conch, Strombus gigas. Bull. Mar. Sci. Gulf Carib. 14: 246-295.