Massive numbers of juvenile Blacklip Butterflyfish (Chaetodon kleinii) settle at Molokini and in south Maui1 Comment »
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:
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 twice as deep – down to 100 feet in places, potentially doubling 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.
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.
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.
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.
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. ******************************************************************************
*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.
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!
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 2014.
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.
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?
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?
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!
Looking 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.
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.
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.
Almost every month brings a new underwater discovery that could be titled “Who Knew?” This summer a Giant Moray (Gymnothorax javanicus) has been sighted numerous times at the islet of Molokini. Although common farther west in the Pacific in places such as Indonesia, the Philippines and Australia, this eel is rare in Hawaii. So rare, that in over 26 years of diving in Hawaii this was my first sighting of this species of eel!
I should clarify that it was my first sighting in the wild. My first sighting was actually in the Bishop Museum’s fish collections. There, tens of thousands of little glass jars contain one of the world’s most extensive collections of fishes, thanks to the prodigious efforts of world famous ichthyologist John E. (Jack) Randall. Arnold Suzumoto, Ichthyology Collections Manager who has worked at the Bishop Museum for 38 years, guided us through the aisles and shelves containing over 100,000 fish specimens. Many, such as gobies, clingfish or wrasses, were tiny and easily fit into equally tiny glass vials. Most of the specimens, such as butterflyfish, goatfish, damselfish, etc. were small-to-medium and bottled in various sizes of glass jars. But then Arnold led us to a white plastic 5-gallon bucket sitting on the floor. This, he explained as he removed the lid, contained a specimen of a Giant Moray. But not the whole eel. This species can reportedly reach 8 feet long and weigh up to 77 pounds, rather large for the limited space in such a massive collection. So, in this bucket was just the head of a Giant Moray. And, it filled the bucket!! (*see note at end of article)
But on to the “Who Knew” part of the story. Beginning in June of this year, Justin Haghbin, a divemaster at Ed Robinsons Diving Adventures, spotted a Giant Moray on the back wall of Molokini.
Over the next few months Giant Moray sightings continued to be made by Justin and others. But not within just a 10- or 20- or even 50-foot radius like other species of morays we have known. These sightings were sometimes a half-mile apart! So, we wondered, is it a different Giant Moray at all of these places? Or the same individual? Odds were that it was the same individual, since the species is so rare in Hawaii. But we had to know for sure.
Fortunately, like most animals, there are identifying characteristics if you look closely enough. The Giant Moray has tiny black spots all over its body. So, we compared the spot pattern on the right side of the face in photos taken in the three locations – on the back wall, on the outer western slope and at Enenue. As you can see, the spot pattern is identical.
Molokini is about 0.4 miles in diameter and about 1.3 miles in circumference. Just moving from the outer western slope clock-wise around to Enenue one time is a trip of about half a mile, and it has done this several times that we know of. Then there have been several visits to the back wall, about 1/4 of a mile from the outer western slope. Added up, this eel has swum miles around the perimeter of Molokini in just the few months that we have been aware of its presence.
We are not aware of reports of a moray eel traveling such distances, so we asked eminent fish scientist Dr. Jack Randall, and Dr. John McCosker, Senior Scientist at the California Academy of Sciences and moray behavior expert. Neither had heard of such long-distance movement by a moray.
Morays that we have known have remained in roughly the same location for years, possibly a decade or more, and some divers have reported that they have visited the same moray eel in the same place for at least 20 years. What would cause this Giant Moray to move around so much? Being such a rare eel in Hawaii, perhaps this individual is expending all this energy in search of a mate, something it would not have to do in areas where it is a common species.
Such discoveries are exactly the kinds of things that dive guides have a unique opportunity to document. All it takes is paying attention (and usually a camera) to document marine life behavior that has never been observed before, because while dive guides spend only a tiny fraction of their lives underwater, it is still more than most professional scientists are lucky enough to spend.
* the specimen at the Bishop Museum had been caught on hook and line (sharkline) off Milolii, Kona Coast of Hawaii, in Oct. 1977. It was a male 6.7 feet long and weighed 69 pounds. Information courtesy of Arnold Suzumoto, Ichthyology Collections Manager at Bishop Museum.
Any frequent Maui diver or snorkeler has witnessed innumerable green sea turtles with tumors, some as large as a baseball, and sometimes completely covering a turtle’s eyes. It’s a painful sight, even more so because these tumors are more prevalent in areas where we humans reside, and because we are most likely responsible for their suffering. Indications are that our activities on land change the nearshore ecosystem and make it more likely that turtles will contract this disease. Turtles with tumors the size of a baseball have not been known to recover. Some of these end up stranding themselves and are euthanized, or they disappear from our reefs and we just stop seeing them.
Fortunately, not all turtles are equally afflicted. Many have tumors that do not grow to such large size. And in these turtles, some individuals actually experience tumor regression or disappearance altogether.
We have been lucky to see a few of these fortunate turtles over the years, one as recently as this summer. In 2001 Andy Schwanke began a photo catalog of turtles that we see at the places we dive. This was possible because each turtle’s face has a unique pattern of scales.
In 2002 Juerg Grieder photographed the above turtle at Ulua Beach with small tumors in the back corners of both eyes.
In 2005, off Kamaole, tumors were seen in the armpit and almost covering both eyes.
But In July of 2013, off Wailea Point, the right eye showed no evidence of tumors. Her eyes are closed because she is resting, and we did not want to bother her just to get a photo of her eye open, but it appears as though no tumor is present.
This is not unusual. A 2006 master’s thesis by Joseph Spring showed that 88% of nesting females at French Frigate Shoals had tumors that were regressing. In a 1999 paper by Peter and Ursula Bennett they report a 33% regression in the turtles they monitored.
So, while not unusual, she is the first turtle we know personally (and have photographed) with a good outcome from this disease. Now we will keep an eye out for her and see if she remains disease-free. She is a huge female with an easy-to-remember scale pattern on her face so she will be easy to spot, especially because in the 11 years we have known of her, she appears to have stayed within a small area between the St. Anthony wreck and Wailea Point. If we see her again we’ll keep you posted!