Tuesday, June 30, 2015

Answer: A couple of fishy questions...

This was fun,
but then again, 

sharks are always interesting... 

Here's that picture again... 

The Challenges are: 

1.  What kind of shark is right in front of me?  Am I crazy?  Should I be worried about this apex predator?  By the way, please don't tell my mother about this... 

As a few of you pointed out, there's no EXIF data here.  Not terribly surprising, it's underwater (GPS signals don't work underwater).  But there are a couple of clues. 

If you look at the lettering on my wetsuit, you can just barely make out "Stuart's Cove," which is the name of the place that rented me the wetsuit... AND organized the dive. 

The simplest solution here is to do a search for: 

     [ Stuart's Cove shark dive ] 

which leads pretty quickly to their "Shark Adventure" page, which tells us that this is probably a Caribbean Reef Shark  (Carcharhinus perezii).  

But it's possible that another shark might have wandered into the adventure, so I thought I should double check by looking for: 

     [ Caribbean reef shark identification ] 

Which gives us this set of results: 

The second image in this set leads to the ReefQuest Centre for Shark Research page on identifying sharks in the Caribbean (Stuart's Cove is in the Bahamas), and on that page they have this lovely diagram with features describing the reef shark (snout short and blunt; pectoral fins moderately long and narrow; caudal fin margins dusky, etc.).  If you check out that page, the features exactly match up. 

Their ID page goes on to say that "Large, robust individuals are often misidentified as Bull Sharks (Carcharhinus leucas)..."  but that the Bull shark has a shorter snout and a broader first dorsal fin.  (Look at images of the Bull shark for comparison.)  

The Caribbean Reef Shark is most likely to be confused with the Dusky shark (Carcharhinus obscurus) and Galapagos shark (Carcharhinus galapagensis), but divers are unlikely to encounter either of those within the Bahamas.  

How dangerous is a Caribbean Reef shark?  The ReefQuest Centre says that they're only two stars (out of 5)  "According to the International Shark Attack File, 22 attacks are attributed to this species, of which 11 were provoked and none were fatal." And none of the attacks were on scuba divers.  

That mention of the "International Shark Attack File" was just perfect click bait for me (sorry about the pun).  So I did the obvious search and found that the Attack File is kept by Ichthyology Department at the Florida Natural History Museum.  That file is a compendium of all shark attacks worldwide.  Interestingly, 2014 was a slow year for shark-human interactions, only 3 deaths worldwide were reported, with no attacks in the Caribbean whatsoever.  (On the other hand, there are already 7 attacks in the Carolinas so far in 2015, none fatal. They seem to be from Bull sharks attracted by bait fish in the shallow water.) 

And although the sharks in the Bahamas were pretty big, one of my dive buddies picked up a small tooth that was knocked loose from the passing sharks (they lose teeth all the time; luckily, they have a robust tooth replacement mechanism).  As you can see, it's not especially fearsome. 

Caribbean Reef shark tooth.  The pointy end sticks up, while the broad base is anchored in their gums.  

Bottom line:  You can tell my Mom; these sharks are pretty safe to be around, especially as a scuba diver.  (A pretty good video can be see below...  And no, I'm not getting paid by these folks.)

2.  Speaking of kinds of sharks, one of the strangest sharks in the ocean today seems to be a holdover from the Cretaceous period.  What kind of shark is that?  Just from that description, can you figure out the genus and species? 

For this Challenge, I first looked up when the Cretaceous period was--that part is easy:  

    the Jurassic was 201 - 145 million years BCE
    the Cretaceous was 145 - 65 million years BCE 

So I did my first query:   

     [ shark Cretaceous living fossil ] 

And I found a couple of possible candidates.  Several articles in the first list of 10 hits all refer to Cow sharks, Goblin sharks, and Frilled sharks all as "living fossils."  

So I checked into each of these.  

Cow Sharks, (in the Hexanchiformes order) are considered the most primitive of all the sharks, as their skeletons resemble those of ancient extinct forms, with few modern adaptations. Cow sharks are represented in the fossil record by their characteristic cockscomb-shaped lower teeth, dating as far back as the early Jurassic Period, about 190 million-years ago. Articulated cow shark remains are known from the late Jurassic, about 150 million years ago (which means they didn't really arise during the Cretaceous, they were already around during the Cretaceous). 

On the other hand, the eel-like Frilled Shark (Chlamydoselachus anguineus) is probably about as old as the Cow sharks, but its unusual three-pronged, trident-shaped teeth are known only as far back as the late Cretaceous, about 95 million years ago.  Still, there is some controversy about this, with some arguing that this shark  is a relatively recent species, with the earliest known fossil teeth belonging to this species dating to the early Pleistocene epoch (only 2.5 million to 11K years ago).  

Goblin sharks, on the other hand dates back to the Aptian age of the Cretaceous period, and are strange, strange beasts.  This is video of a Goblin shark swimming around in its normal swim posture. 

And a short video of a Goblin shark actually feeding: 

In any case, the Goblin shark, with it's very Alien-like, strange, extendable jaws, is clearly a Cretaceous fish.  

3.  While we're on the subject of large marine predators, I remember reading that there was an order of now-extinct marine reptiles that dominated the seas during  late Triassic and the Jurassic periods.  These giant predators were warm-blooded, and sometimes suffered from the problems of coming up too fast from the briny deep.  What kind of animals were these?  And how do we know they suffered from too rapid ascents?  

This is one of those cases when you might need to know a technical term in order to get a good search to work.  To start, I did a search for such a term: 

     [ coming up too fast from ocean ] 

that quickly told me that I needed to search for "the bends" or "DCS"  (decompression sickness).  Then, a search like: 

     [ "the bends" marine reptile Triassic OR Jurassic ] 

There are lots of hits there, but I wanted to find something fairly authoritative.  That's when I spotted (at position 7 in the results list) the paper "Adaptations for marine habitat and the effect of Triassic and Jurassic predator pressure on development of decompression syndrome in ichthyosaurs."  It caught my eye because it was from the journal Naturwissenschaften (2012 Jun; 99(6):443-8. doi: 10.1007/s00114-012-0918-0).  This is very respected journal, and so I thought a paper published there would be fairly interesting.  

Indeed. The abstract says it all: 

Decompression syndrome (caisson disease or the "the bends") resulting in avascular necrosis has been documented in mosasaurs, sauropterygians, ichthyosaurs, and turtles from the Middle Jurassic to Late Cretaceous, but it was unclear that this disease occurred as far back as the Triassic. We have examined a large Triassic sample of ichthyosaurs and compared it with an equally large post-Triassic sample. Avascular necrosis was observed in over 15% of Late Middle Jurassic to Cretaceous ichthyosaurs with the highest occurrence (18%) in the Early Cretaceous, but was rare or absent in geologically older specimens. Triassic reptiles that dive were either physiologically protected, or rapid changes of their position in the water column rare and insignificant enough to prevent being recorded in the skeleton. Emergency surfacing due to a threat from an underwater predator may be the most important cause of avascular necrosis for air-breathing divers, with relative frequency of such events documented in the skeleton. Diving in the Triassic appears to have been a "leisurely" behavior until the evolution of large predators in the Late Jurassic that forced sudden depth alterations contributed to a higher occurrence of bends.

In other words, marine dinosaurs (the mosasaurs and friends) got the bends back in the mid-Jurassic until the late Cretaceous.  Oddly, similar diving animals in Triassic (which was earlier, before the Jurassic and Cretaceous) didn't have this.  The authors think this is because that's when bigger and badder predators evolved, forcing these later marine animals into "emergency surfacing" (that is, coming up too fast and developing the bends).  

Now, I hear you asking, "I don't know what this Naturwissenschaften thing is... Why should I believe you when you say it's authoritative?"  

One easy way to check for the acceptance of a journal like Naturwissenschaften is to do an advanced search with Scholar.  You can quickly scan the articles and check things like "how long have they been publishing?" and "do other authors cite work published in Naturwissenschaften?"  

I went to Scholar.Google.com and did a search for [ mosasaur ] just to get started.  THEN, I clicked on the down arrow (upper right) 

That then opens up menu to select Advanced Search... 

Choose Advanced Search, and then put Naturwissenschaften into the "Return articles published in.."  field below:  

That then returns articles published ONLY in the journal Naturwissenschaften.  (To get a complete list of all the articles that Scholar has, remove the mosasaur from top search query field.) 

And the results. Note that there are around 97,000 papers published, and the top several papers have MANY MANY citations.  These are the hallmarks of well-respected journals--longevity, citations by others, authors from established institutions, and so on.   

If you check the Wikipedia article about Naturwissenschaften, you'll find that they've been publishing since 1913.  The original subtitle was Wochenschrift für die Fortschritte der Naturwissenschaften, der Medizin und der Technik (Weekly Publication of the Advances in the Natural Sciences, Medicine and Technology), and is now published completely in English, having changed over from German in the 1990s.  

As some Regular Readers noted, there is some controversy about this finding.  But that's what makes it interesting science.  The observation that there's a big change over time leads one to suspect an underlying cause.  Pity we didn't have a camera there to document the chase!  

4.  If you're really into the topic, for extra credit, in the picture above why does this shark's eye seem to have a white crescent moon in it?  (No, it's not a cataract, nor is he bug-eyed.)  It's a normal sharky thing.  But what's that white thing called?  

I did a straight-forward search: 

    [ shark eyelid ] 

and found a fascinating article on their Nictitating membrane.  This is a transparent (often just translucent) membrane that covers the eye of fish, amphibians, reptiles, birds and mammals.  This membrane protects the eye when the eye is potentially exposed to a hostile environment--in the shark's case often when they're about to strike a prey (which might flop around in protest, potentially striking their eye).    

From the description, this is pretty clearly the Caribbean Reef shark's nictitating membrane.  

In reading about this membrane, I was surprised to learn that people have a vestigial nictitating membrane called the plica semilunaris.  (Image from the Wikipedia article, originally from Grey's Anatomy. No, he's not trying to poke this guy's eye out--he just wants to show you the inner eyelid structures.)   

What do you know--I do have something vestigially in common with the sharks!  

Search Lessons

Let's start from the top: 

1.  Use whatever text you've got!  As in many of the Challenges, starting with the text  you can see will often get you a long way.  In this case, we found the dive operator, and then a place, and  a strong suggestion about what kind of sharks these are.  

2. Verify what you've read.  We know it's tough to positively identify animals, but in this case we had a strong lead ("Caribbean Reef shark"), and then turned to identification guides to get to a positive ID.  I checked a couple of different identification sources AND looked at the images of possible confusingly similar sharks until I knew I could spot the differences.  

3. Check the dates.  The Cow, Frilled, and Goblin sharks are all "living fossils," in one form or another, but they didn't all come into being in the Cretaceous AND stay more-or-less the same until now.  Details matter in these Challenges! 

4. Sometimes you need to find the right technical term to get started.  In this case, we had to search for a specific term (or phrase) for "coming up too fast from bottom of the ocean..." and we learned about "the bends" and "DCS."  Once you've got those terms, the rest of the search is easy.  

5. Check authoritativeness.  In this example I showed you how to use Scholar to identify the credibility of a publication that you might not know.  Obviously, there's more to authoritativeness than just this, but this is a method you might not have thought about. 

Great comments for this week's Challenge.  Well done, troops! 

Search on! 

1 comment:

  1. A widely used measure of scientific/technical journal authoritativeness is the citation-based "impact factor", which was devised in the 1960s by Eugene Garfield, the founder of the Institute for Scientific Information (which is now part of Thomson Reuters). Impact factors have been calculated annually since 1975 for those journals that are indexed in the Journal Citation Reports.

    Here's Thomson's description of it: http://wokinfo.com/essays/impact-factor/

    Like any simple metric of a complicated system, the impact factor has been criticized, and improvements have been proposed (including a recursive metric not unlike Google's PageRank algorithm, which is mentioned in the Wikipedia page: https://en.wikipedia.org/wiki/Impact_factor)

    Here's a page of the 2008-2014 impact factors of journals that begin with the letter "N": http://www.citefactor.org/journal-impact-factor-list-2014_N.html

    This list contains the traditional high-flyers -- New England Journal of Medicine (2013-14 Impact factor = 54.42) and Nature (42.351) -- as well as Naturwissenschaften (1.971). Naturwissenschaften's impact factor is modest, but there are many journals with lower ratings.