Shark Science Blog
Jennif
er V. Schmidt, Ph.D. is an Associate Professor in the Department of Biological Sciences at the University of Illinois at Chicago. She is a molecular biologist using genetic analysis to study the reproduction and development of a variety of organisms, from sharks to mice to humans. Dr. Schmidt’s laboratory has worked on whale sharks since 2001, using species-specific molecular markers to study their breeding and reproduction.
Shark Strandings Investigated
Sharks strand, or beach themselves, with some frequency in certain coastal areas. This phenomenon is seen in other marine creatures as well, and is particularly common in dolphins and small whales. It’s a frustrating occurrence, as the cause for strandings is rarely known, and most stranded animals eventually die. Even in cases where the animal is helped back into the water, it will often strand again. It is usually assumed that stranded animals are sick, or compromised in some way, and have lost their ability to navigate and/or survive at sea, but rarely is the specific cause found.
In one of the most interesting investigations of recent strandings, it was found that mass strandings of whales were likely a result of the use of navy sonar devices in the area. Necropsies of the animals found gas bubbles in their tissues. This is indicative of decompression sickness, what divers call the bends, and typically results from surfacing too rapidly from depth. It is believed that the whales may have dived and surfaced rapidly to escape the discomfort of the sonar on their sensitive hearing. These data led to regulations on sonar use when conducted in areas that whales frequent. In other cases, mass strandings are thought to be a result of poisoning by the toxic algal blooms known as red tides.
In a recent paper by Schaff
er et al, another potential cause of stranding is identified. The authors observed that significant numbers of juvenile salmon sharks (Lamna ditropis) strand along the northern California and Oregon coasts each year. They collected and necropsied 18 salmon sharks, both fresh and preserved carcasses, that stranded and died between 2002 and 2007. Upon initial observation they observed that the meninges, the membranous coverings of the brain, showed signs of inflammation. Various regions of the brain itself also had many inflammatory immune cells, typically a response to some sort of infectious agent, and in some cases abscesses filled with bacteria were seen. When the shark’s livers were available for analysis these also showed signs of severe infection.
In many of these sharks the bacteria could be cultured, and were then subjected to DNA analysis to identify the species. The authors sequenced the 16S ribosomal RNA gene of the isolated bacteria, a region of the genome commonly used to identify bacterial species. This analysis showed the bacteria to be a Carnobacterium, likely the species Carnobacterium maltaromaticum. This is a common bacteria in various marine creatures, though it had not previously been found in sharks. In fish, it typically causes no pathology to its host, but in some cases may result in sickness and death. The bacterium was not seen in any of the control sharks analyzed in this study. Interestingly, all the bacteria isolated from the different sharks (which stranded in different coastal areas) were genetically identical, without even the small genetic differences expected of separate bacterial isolates. This suggests there may be a single source from which all sharks acquire the bacteria.
Meningitis has occasionally been found in stranded sharks before, but its role in the strandings was unclear. The nearly 100% incidence of this disease in the stranded juvenile salmon sharks provides compelling evidence that this is likely to be the cause of the stranding. Meningitis can cause disorientation and confusion, which might lead the sharks to strand inadvertently. Identifying the source of this bacteria, and understanding why it appears specific to juvenile salmon sharks, may help to prevent loss of these animals in the future.
The article is: Schaffer, PA, Lifland, B, Van Sommeran, S, Casper, DR, and CR Davis. (2012) Meningoencephalitis associated with Carnobacterium maltaromaticum–like bacteria in stranded juvenile salmon sharks (Lamna ditropis), Veterinary Pathology (online before print).
It can be found at: DOI: 10.1177/0300985812441033
A Nose By Any Other Name
While we’re on the subject of noses, let’s talk about sawfish. Sawfish are elasmobranchs, the group of animals that includes sharks, rays and skates. While sawfishes appear “shark-like”, with their elongated bodies and tall dorsal fins, they are actually a type of ray. Sawfish are characterized by a long extended snout, or rostrum, edged in sharp tooth-like structures that are actually modified dermal denticles (the scale-like structures that cover the skin of sharks and rays). There are seven living species of sawfish, and none have been well-studied. All sawfish species are considered critically endangered by the IUCN, and six are listed on CITES Appendix I (Pristis microdon, the species studied here, is listed on Appendix II). Sawfish are hunted for their saws which are sold as souvenirs, and for their fins, which are used for shark fin soup, and additional animals die when their saws become tangled in fishing nets intended for other species.
The function(s) of the sawfish snout has long been unclear. It’s been presumed that the rostrum is used for finding prey, either by using it to detect the weak electrical signal all fish emit, or possibly by actually using the saw to rummage through a sandy or muddy bottom to dislodge prey. It has also been assumed that sawfish use the saw to attack swimming prey, much the way billfish such as swordfish use their sword to stun prey. There exists little actual data on the use of the snout for these functions, however, and they have not been scientifically tested.
In a short correspondence by Wueringer et al, these ideas are put to the test - using recently captured Pristis microdon, the freshwater sawfish - with some positive and negative results. Most dramatically, sawfish DO use their saw to strike swimming prey, they slice and dice them right in 
the water column! The authors include a couple of amazing videos in their Supplemental Information, and the article is open access, so you can watch these at the link below. Sawfish also use the saw to trap fish against the ground, immobilizing them and positioning them to be eaten.
The researchers tested the ability of sawfish to detect electrical fields, such as those emitted by fish and other food sources. When electric dipoles, sources of a weak electrical charge, were placed in the tank, the sawfish oriented towards the dipole hanging in the water. What does the sawfish do with the dipole? First he detects its charge….it is a fish? Then he whacks it with his saw! He’s treating the dipole as though it were the swimming fish he believes it represents. The sawfish were never seen using their sensitive saws as digging tools, to root around for prey in the sandy bottom, but they did occasionally scrape the toothed edges of their saws against the bottom, perhaps sharpening them for their next victim.
The article is Wueringer, BE, Squire, L, Kajiura, SM, Hart, NS and Collin, SP. (2012) The function of the sawfish's saw. Current Biology, 22:R150-R151.
It can be found at: http://www.cell.com/current-biology/fulltext/S0960-9822%2812%2900085-1
Photo from www.arkive.org.
Building a Shark Nose
Vertebrates smell using their olfactory system, a complex organ composed of tissues in both the nose and the brain. In mammals where the sense of smell has been well studied, the front line of the olfactory system is a tissue called the olfactory epithelium, the layer of cells lining the interior portion of the nose. These cells are neurons, nerve cells, and they connect to the long olfactory nerve running to the brain, specifically to a portion of the brain called the olfactory lobe, where “smelling” really happens. The cells of the olfactory epithelium contain olfactory “receptors”, proteins that recognize specific odor molecules, and amazingly these receptors are so varied that each recognizes only a few of the millions of possible odors. Researchers Linda Buck and Richard Axel won the Nobel Prize in 2004 for determining how this system functions to allow us to smell so many different things.
Early during embryonic development a gene called Pax6 is expressed in the portion of the embryo that will develop into the olfactory epithelium and other parts of the olfactory system. This gene is essential for olfactory system development, as mice carrying a Pax6 mutation do not form most parts of the system. Many aspects of embryonic development are highly conserved across vertebrate species, and studying developmental processes in basal (i.e. evolutionarily "earlier”) vertebrates can often tell us how these systems first came to be.
Sharks are famous for having a highly developed sense of smell, and are known to have large olfactory bulbs, but how their olfactory system develops has not been well-studied. Identification of the shark Pax6 protein showed that it is an astounding 95% similar to that of the mouse, which suggests that Pax6 function is likely to be well conserved between these two very different species.
In a recent paper, Ferreiro-Galve et al studied the expression of Pax6 in a favorite shark model for embryonic development, the lesser spotted dogfish, Scyliorhinus canicula. (Interestingly, the genus Scyliorhinus comes from the Greek, meaning – for reasons that are unclear to me – ‘shark nose’.) Colorimetric stains specific for Pax6 showed that the gene is active in the developing olfactory epithelium, olfactory nerve, and the olfactory bulb of the brain. See the figure, where the purple color indicates Pax6 gene expression in the highly folded olfactory epithelium of the dogfish. (From Ferreiro-Galve et al.)
This pattern is remarkably similar to that seen in mice, suggesting that Pax6 already played a role in building the olfactory system in the common ancestor of sharks and mammals, more than 450 million years ago. Sometimes the outward differences between species can detract from underlying similarities conserved across eons – when something works well evolution tends not to mess with it.
That paper is Ferreiro-Galve, S, Candal, E, and Rodríguez-Moldes, I. 2011. Dynamic expression of Pax6 in the shark olfactory system: evidence for the presence of Pax6 cells along the olfactory nerve pathway. Journal of Experimental Zoology, 314B. It is available here (http://onlinelibrary.wiley.com/doi/10.1002/jez.b.21448/abstract), but unfortunately is not open access.
The Far-Reaching Hand of Anthropogenic Change
More shark history this post. Not fossilized extinct sharks this time, but the use of historical records to trace the extinction - more correctly extirpation, more on that later - of a shark population in recent history.
The Saint Peter and Saint Paul Archipelago is often called Saint Paul’s Rocks, and it is just that, a largely barren outcropping of small rock islands. The archipelago sits atop the Mid-Atlantic Ridge just a scooch north of the equator, putting it midway between Brazil and the west coast of Africa. It’s hard to imagine a more remote oceanic locale, and yet a new paper by Luiz & Edwards shows that even this place is not untouched by human effects.
Despite the remoteness of the Rocks, they have been visited periodically by ships. Seamounts draw fish, and were historically a popular stop for replenishing food stores on long ocean voyages. Initially the visitors were ocean explorers and naturalists - Charles Darwin stopped there aboard the Beagle in 1832 - and more recently there have been dedicated scientific missions and an increasing number of fishing vessels. These occasional visitors recorded their observations at the Rocks, and one of the most remarkable aspects of this paper is the authors’ collection of historic reports about the islands. Some of the comments have to do with the lack of land animals or even vegetation, but the most compelling records concern the vast numbers of sharks that were observed through the 18th, 19th and early-mid 20th centuries.
A few examples are below, and there are many others in the paper, which is worth reading for these excerpts alone. (The quotes are taken directly from Luiz & Edwards, and original references can be found in the manuscript):
HMS Beagle: February 1832
‘‘While our party were scrambling over the rock, a determined struggle was going on in the water, between the boats’ crews and sharks. Numbers of fine fish, like the groupars [sic] (or garoupas) of the Bermuda Islands, bit eagerly at baited hooks put overboard by the men; but as soon as a fish was caught, a rush of voracious sharks was made at him, and notwithstanding blows of oars and boat hooks, the ravenous monsters could not be deterred from seizing and taking away more than half fish that were hooked.”
SY Scotia: December 1902
‘‘Dec. 10th, St. Paul’s Rocks. Sharks innumerable. Secured eight specimens, and took dimensions and weight of each . . . Several fish seen but none caught, as the sharks took every bait.’’
USS Atka: March 1955
‘‘The numerous sharks, which swarm in the waters of the cove and around the Rocks, speedily attack hooked fish and either snatch the whole fish off the line or leave only a half fish or head on the hook for the fisherman. On the ATKA, the chief medical corpsman hooked a beautiful tuna-like fish from the fantail several hundred yards off the Rocks, but when he hauled in his catch all that remained was an enormous head fully a foot high . . .’’
RRS Bransfield: May 1971
‘‘. . . the ship’s launch was used to catch fish just off-shore from the Rocks. . . . Difficulty was experienced in obtaining these specimens as fish once hooked were frequently taken by marauding sharks before they could be brought on board.’’
The sharks that were so abundant at St. Paul's Rocks were likely of two species, the Galapagos shark (Carcharhinus galapagensis), and the silky shark (Carcharhinus falciformis). Galapagos sharks tend to have restricted ranges, keeping close to reefs and island shores and seldom venturing into open water. Silky sharks in contrast are more pelagic in their behavior, but are often found at the edges of reefs and are known to feed at night.
Beginning in the late 1900s, observer's reports from St. Paul's Rocks carry a different tone.
Cambridge Expedition: September 1979
‘‘It was notable that during the day sharks hardly interfered with line fishing activities and were only rarely seen at the surface. . . . suggest that the shark population may have declined somewhat in recent years; our observations are in agreement with such a conclusion. In this respect it is perhaps worth noting that the Rocks have recently been subject to occasional visits by Brazilian fishing boats; one of these recorded capturing two tons of sharks by accident in one evening while fishing for commercial species.’’
Segredos Submersos Expedition: November 1993
‘‘All dives we made were magnificent, but the lack of sharks was noticeable. We carried luparas (sticks with explosive tips) and electric end sticks in order to repel the sharks we expected to find, . . . truly, we never had to use these.’’
Scientific surveys beginning in 1998 that were specifically directed at assessing the fish populations at the Rocks failed to find any sharks. What happened between the 1970s and the 1990s? The Cambridge report from 1979 gives a clue to one change that took place, in the mid-1950s the Brazilian government opened the region to commercial fishing, and since 1988 these waters been fished on a daily basis. Sharks are not the target of this fishery, the region is rich in tuna and other desirable pelagic fish. Sharks are caught as bycatch at high levels in longline fisheries however, and the article documents significant rates of shark bycatch through the 1970s. By the 1980s there were no longer any sharks to catch.
Species declines due to fishing are difficult to gauge in remote areas, particularly when there is little information about baseline numbers prior to human activities. This paper does a great job of using historical reports to demonstrate the pre-fishery abundance of sharks at St. Paul's Rocks, allowing the full magnitude of these species' decline to be understood. Although reliance on such accounts can be fraught with error, the authors applied rigorous criteria in selecting reports to use, and in gauging their reliability. And the fact remains, since 1993 no sharks have been seen at St. Paul’s Rocks. C. galapagensis and C. falciformis have been extirpated (the word means to become extinct within a portion of a species’ range) in this region.
The paper is: Osmar J. Luiz and Alasdair J. Edwards. (2011) Extinction of a shark population in the Archipelago of Saint Paul’s Rocks (equatorial Atlantic) inferred from the historical record. Biological Conservation, In press.
400 Million Years and Counting
There’s an often-reported statistic that sharks have remained unchanged for 400 million years. What does this really mean? Our familiar sharks weren’t around 400 million years ago, and what do we really know about the extinct shark species that have existed between then and now?
Fischer et al now report on new shark fossils from the middle to late Triassic period (250-200 million years ago). These fossils were found at a site called Madygen, in Kyrgyzstan in central Asia. During the Triassic period, Madygen is believed to have been a large inland freshwater lake.
Unlike bony animals, the cartilaginous skeletons of sharks fossilize poorly, so most extinct shark species are identified from little more than their teeth. The shapes and sizes of the animals belonging to these teeth must be inferred from similar species alive today, and hoping to understand anything about the lives and behaviors of these sharks is usually impossible.
Fischer et al found fossilized shark teeth at Madygen that were different than those that had previously been described, and eventually determined that they belonged to three new species of freshwater shark. The term ‘freshwater shark’ sounds odd today; all but a few of the 400+ species of selachians (sharks, exclusive of rays and skates) currently alive are marine animals. The bull shark can live in both salt and fresh water, allowing it to move far up into river systems, and there is a genus of ‘river sharks’ in Asia, but most sharks live in the ocean. Freshwater elasmobranchs appear to have been abundant in shark history, however, based on large numbers of fossil shark teeth found in the deposits of inland lakes.
In addition to teeth, the embryonic egg cases of sharks can also fossilize, and while the embryonic sharks within them remain only as tiny teeth, studying them can sometimes tell us about the lives of these animals. Madygen is a particularly rich site for fossil shark egg cases and the teeth of the developing young they once held. The researchers found many egg cases, from two different species, in an area that at one time encompassed the shallow near-shore waters of this ancient lake. Few adult teeth were found nearby, however, suggesting only young sharks inhabited the area -- a shark nursery.
Many shark species today use shallow near-shore ocean habitats as juvenile nurseries, where young sharks can grow, with abundant food and reduced risk of predation, until they are large enough to join the adults of their species. It seems things weren’t all that different for sharks more than 200 million years ago.
The paper is: A selachian freshwater fauna from the Triassic of Kyrgyzstan and its implication for Mesozoic shark nurseries. (2011) Fischer, J, Voigt, S, Schneider, JW, Buchwitz, M and Voigt, S. Journal of Vertebrate Paleontology, 31:937-953.
It can be found at: http://dx.doi.org/10.1080/02724634.2011.601729
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