Sand Tiger Shark Attempts to Swallow Tankmate Whole

It is always easy to underestimate the feeding abilities of predatory fish, in this case a sand tiger shark at a marine park in Japan. You see the animal and think, “surely, it can’t eat that” when referring to its tankmates. But the 9-foot sand tiger at Aqua World surprised its keepers when after an extended fasting period, perhaps in preparation of the swimming meal it was eyeballing, the shark tried to eat a 3-foot long whitetip reef shark. The aquarium’s staff had been worried for days because the sand tiger was showing no interest in feeding. That is until, however, the tasty little whitetip morsel came a little too close. The shark attacked the much smaller whitetip and even managed to get most of victim into its mouth. But the 3-foot long shark proved to be a bit too much, as it was seen sticking out of the sand tiger’s mouth and was eventually spit out. Obviously dead at this point, the aquarium keepers removed the whitetip in fear that the sand tiger would return and try to finish the job

The Eyespots Have It

Young Ambon Damselfish on the reef, small and extremely vulnerable to predation. Image: Oona Lonnstadt. The old proverb about a leopard not being able to change its spots now has a new biological footnote after researchers in Australian recently found that fish exposed to predatory danger can, indeed, transform their spots to make them less vulnerable to attack. Working with young Ambon Damselfish, Pomacentrus amboinensis, researchers from Australia’s ARC Centre of Excellence for Coral Reef Studies (CoECRS) have made the remarkable discovery that, when constantly threatened with being eaten, the fish not only grow a larger false ‘eyespot’ near their tail–but also reduce the size of their real eyes. Small prey fish with bigger “false eyes” on their rear fins dramatically boosted their chances of survival on the reef, they found. Relationships between eyespot size and eyeball size and body length. The relationship between standard length and eyespot diameter (A) and standard length and eye diameter (B) in presence and absence of predators. All prey fish exposed to predator cues over a 6 week period had significantly larger eyespots (F,H) and smaller eyes (F,G) than fish from the control treatments (C–E). The changes were not evolutionary—over a succession of generations—but rather a relatively rapid response by individual fish tracked over a period of six weeks. The enlargement of the eyespots results in a fish that looks like it is heading in the opposite direction–potentially confusing predatory fish targeting them to be eaten, says Oona Lönnstedt, working toward her Ph.D. at CoECRS and James Cook University. These spots are known as ocelli, and for decades scientists have debated whether false eyespots, or dark circular marks on less vulnerable regions of the bodies of prey animals, played an important role in protecting them from predators–or were simply a fortuitous evolutionary accident. The widely accepted theory is that these spots, found in the juveniles of many species, tend to cause predators to strike at the eyespotted tail or fin rather than the much more vulnerable head region. Researcher and lead author the paper, Oona Lonnstadt. The CoECRS team has found the first clear evidence that fish can change the size of both the misleading spot and their real eye to maximise their chances of survival when under threat. “It’s an amazing feat of cunning for a tiny fish,” Lonnstedt says. “Young damsel fish are pale yellow in colour and have this distinctive black circular ‘eye’ marking towards their tail, which fades as they mature. We figured it must serve an important purpose when they are young.” “We found that when young damsel fish were placed in a specially built tank where they could see and smell predatory fish without being attacked, they automatically began to grow a bigger eye spot, and their real eye became relatively smaller, compared with damsels exposed only to herbivorous fish, or isolated ones. “We believe this is the first study to document predator-induced changes in the size of eyes and eye-spots in prey animals.” When the researchers investigated what happens in nature on a coral reef with lots of predators, they found that juvenile damsel fish with enlarged eye spots had an amazing five-fold increase in survival rate compared to fish with a normal-sized spot. “This was dramatic proof that eyespots work—and give young fish a hugely increased chance of not being eaten,” says Lonnstedt. Comparison of depth to length ratio. The relationship between standard length (SL) and body depth (BD) of P. amboinensis when in the presence and absence of predators (A). Fish had significantly deeper bodies when exposed to predator cues (B) compared to the shallow bodied controls (C). “We think the eyespots not only cause the predator to attack the wrong end of the fish, enabling it to escape by accelerating in the opposite direction, but also reduce the risk of fatal injury to the head,” she explains. The team also noted that when placed in proximity to a predator the young damsel fish also adopted other protective behaviours and features, including reducing activity levels, taking refuge more often and developing a chunkier body shape less easy for a predator to swallow. “It all goes to show that even a very young, tiny fish a few millimetres long have evolved quite a range of clever strategies for survival which they can deploy when a threatening situation demands,” Ms Lonnstedt says. Their paper Predator-induced changes in the growth of eyes and false eyespots by Oona M. Lonnstedt, Mark I. McCormick and Douglas P. Chivers appears in the latest issue of the journal Scientific Reports. ABSTRACT The animal world is full of brilliant colours and striking patterns that serve to hide individuals or attract the attention of others. False eyespots are pervasive across a variety of animal taxa and are among natures most conspicuous markings. Understanding the adaptive significance of eyespots has long fascinated evolutionary ecologists. Here we show for the first time that the size of eyespots is plastic and increases upon exposure to predators. Associated with the growth of eyespots there is a corresponding reduction in growth of eyes in juvenile Ambon damselfish,Pomacentrus amboinensis. These morphological changes likely direct attacks away from the head region. Exposure to predators also induced changes in prey behaviour and morphology. Such changes could prevent or deter attacks and increase burst speed, aiding in escape. Damselfish exposed to predators had drastically higher survival suffering only 10% mortality while controls suffered 60% mortality 72 h after release. Sources From materials released by the ARC Centre of Excellence for Coral Reef Studies (CoECRS). Featured Image credit: Indonesian Biodiversity Research Center Images this page: Oona Lonnstadt, top.

A Snail’s Babysitter

Whelks, Anemones, and Sea Urchins I am back to continue with my posting after an unexpected absence due to bodily self-decomposition.  A word to the wise, don’t get old.  Or if you do, don’t let your body know.  It might just not like the process.   Anyway, on with my  tales from the slimy lagoon… In an earlier discussion, I mentioned that aeons ago I saw large female whelks depositing egg capsule masses on one of my research sorties to “my” intertidal study site near Homer, Alaska.  I found this to be very interesting, at the time I was casting around for some research to do, and here a potential easily-done project dropped into my lap. Normally I don’t trust to luck, but I wasn’t about to overtly examine the buccal anatomy of this presentation equine.  I was able to identify the animals, but, at that time, there was no record of them depositing egg capsules in a mass or otherwise.  In point of fact, virtually nothing was known about the natural history of these beautiful whelks, an artifact of being found in an out-of-the-way place where the accumulated knowledge of such critters was minimal.  In fact the only reason I knew the whelks were at this area was that I had taken some students down to the site the previous autumn on a class field trip. Neptunea pribiloffensis whelks on the study beach. The substrate is sandstone, and the “fuzzy” clumps are masses of a feather duster worm which is one of the common prey of the whelks. Figuring that the presence of essentially unknown animals that I was interested in learning about would lead to an easy publication, the following spring I decided to do a little bit of basic research on the snails, and went down to the site to make some field observations as well as to collect a few animals for gut analyses.  Having examined some other Neptunea, including some specimens for this species, I knew I had to look at the gut contents to determine what they were eating rather than simply examining their feces, which was a technique I had perfected for some other snails for my doctoral research.  Fecal analysis is a much preferred technique when compared to gut analysis, as the animal is not harmed in the process.  However, fecal analyses require that the animal’s foods leave some indigestible and identifiable trace in all the feces, and that was not the case with these animals.  They could eat a wide variety of things, including carrion and animals possessing no hard parts at all, as well as some polychaete worms having chaetae, which would be passed through the gut undigested.  It was during a trip for the collection of some specimens for the dietary study, that I noticed the ovipositing females. Several female Neptunea pribiloffensis depositing egg capsule masses near a large sea anemone. Obviously, this was an immediate serendipitous chance for some more and different research.  As with every other aspect of Neptunea pribiloffensis life, virtually no aspects of their reproduction were known.  I had budgeted a couple of days of “research” time on the beach.  I figured I would need about fifteen minutes to collect all the animals I needed for the gut content work, but the site was beautiful and in the spring the weather was often gorgeous.  I had been told that when the Russians owned Alaska, their anecdotal name for the Homer region was “летом земля or Summer land” for the nice climate- a distinct contrast to effectively everywhere else in the region.  Consequently, I truly considered it a terrible hardship to have to make the four or five hour drive to Homer to do field work.  Given how low the tides needed to be for my research, the field work time each day didn’t amount to much time being spent, which meant my assistants and I had plenty of time to work up our samples in the motel we stayed in while working there. After wandering around the study area on the couple of days I had budgeted for that research, for a total of maybe five hours of field work time.   I came away from the site with some facts in hand.  First, there were a number of old egg capsule masses in the area.  Second, the new egg capsule masses were being deposited near the old ones.  Third, most egg capsule masses were being deposited near individuals of large sea anemone, Urticina grebelnyi, referred to at the time as Tealia crassicornis. Egg capsule masses near a large sea anemone. Note the whelk to the upper right. And my experimental  marker is indicated by the arrow. A couple of fundamental questions immediately presented themselves.  Is there any benefit for the whelk to place its egg capsule masses near the anemones?  Likewise, is there any benefit for the anemone to have a whelk egg capsule mass near it?  Today, it seems obvious that the answer to either or both of these questions would almost certainly  be yes, but in the late 1970s very little was known about boreal marine symbioses, in general, and specifically interactions between spawning whelks and anything, let alone anemones.  At the time, there were no hard data either supporting or rejecting a hypothesis of benefit to either party for such an interaction.   And here I was, standing plumb in the middle of a wonderful opportunity in a beautiful area with the chance to address this question. So!!!  Boy-Scientist, at the ready!  I grabbed some buckets, my camera, my voice-activated tape recorder, a meter-stick, and kazango!  I was research bound! Obviously, I didn’t go into this situation as a naïve biologist.  I had just spent several years working at a laboratory where many researchers were studying a wide variety of marine research topics.  As one might expect, there was a lot of cross-pollination of information and ideas.  For example, one of my acquaintances during that time was completing the scientific description of one of the larger, previously unknown, sea anemones from that region.  He told me that it would be called “Tealia piscivora”, a name meaning “the fish-eating Tealia”, an apt name because specimens had been found with their gut cavity full of fish; herring, as a matter of fact.  Those data told me that the nematocysts of a sea anemone closely related to the one I was seeing in Alaska could pack a really potent sting.  And, therefore, the anemones might well be able to protect the snails’ developing progeny. Two egg capsules (white arrows) near a protective anemone are intact. The green arrow indicates my experimental marker. Also, I knew from other researchers that individuals of the sea urchin species, Strongylocentrotus franciscanus, would eat the egg capsules of other whelks, and that those whelks protected their spawn by attacking any urchins that approached their egg capsules.  Given that the “green sea urchin” Strongylocentrotus droebachiensis was common in this Alaskan intertidal habitat, not only was it was possible, indeed, it was likely, that it would eat the egg capsules and the eggs they contained if given the opportunity. An egg mass that is about a year old. The top has been eaten off by sea urchins. My working hypothesis was that the snail obtained some benefit from depositing its egg capsules near the sea anemone.  I anticipated that I would find that the sea anemone protected the egg capsules from predation by the sea urchin, and perhaps other predators as well.  I thought it was also likely that the sea anemone would obtain some benefit from the situation; potentially it could benefit by eating sea urchins that would be attracted to or eating the snail eggs. I immediately set about collecting some animals and egg capsule masses, and setting up some experiments both in the laboratory and in the field.  Some of the experiments were long-term, running about a year in the field and lab, others were of shorter duration.  When I was finished with all of the work, I thought would be able to answer many of the questions necessary to be able to assess the hypotheses. An experimental egg capsular mass is completely gone after the anemone’s removal. All that is left is my marking washer. This Snail Has Babysitters!! I found a series of statistically significant results. First, the snails were more likely to deposit their egg capsular masses near the sea anemones.   It takes about a year before the snails hatch from the capsular masses.  At hatching times the capsular masses near sea anemones were bigger, had more capsules remaining in them, and fledged more juveniles than those capsular masses a short distance away from the anemones. The anemones could deter predation on the egg capsule masses in the laboratory experiments and certainly appeared to do so in the field.  My lab tests showed that the anemones can protect the capsular masses from the sea urchin.  Finally, the sea anemones can eat the sea urchins.  In the lab tests and field observations indicate the major cause of capsular mortality is urchin predation.  Lab and field experiments and observations support the hypothesis that the anemone babysitter protects the capsular masses from predation by urchins by eating the approaching urchins. Newly hatched whelks fresh out of the capsule. All six came from one capsule. And each “corncob” like mass would average about 50 capsules. The scale is mm. This neat little series of interactions started me down the road investigating a number of significantly more interesting anemone interactions that just happen to have some of the most beautiful animals in the world as the actors in the various plays.  More on that in the near future. Reference: Shimek, R. L.  1981.  Neptunea pribiloffensis (Dall, 1919) and Tealia crassicornis (Müller, 1776), On a snail’s use of babysitters.  The Veliger.  24:62-66.