Saturday, June 11, 2011

Driveway shark & spiny dogfish

Driveway shark
This morning when I saw  that an unknown artist had left a chalk shark on my driveway overnight, I realized it must be time to write about spiny dogfish. 

These sharks are recognized by divers as more annoying than dangerous. The 3' to 4' spiny dogfish has an unnerving habit of veering abruptly toward divers, but it does not attack people. It eats fish, not the seals and sea lions that a larger shark might occasionally mistake a human for.

Spiny dogfish shark
In addition to its sharp teeth, the spiny dogfish has two slightly venomous spines on its back, one in front of each fin. If one of the spines punctures flesh, a gland releases venom that can create a painful wound in humans--this is most likely to happen when one of these sharks is hauled aboard a boat.

Spiny dogfish mamas carry their 4 to 6 young for almost two years--probably the longest of any vertebrate. She doesn't pass along nutrients to them that entire time, however. Each hen-sized egg has its own large yolk sac from which the embryo takes nourishment. As the young develop, their sharp spines are covered with pads of tissue that protect the mother.

These sharks were once the most abundant shark species in the world, but you know how this story goes. First we overfished their food species, then we overfished spiny dogfish themselves. Some fishery management policies are now in effect, but it remains to be seen whether the species can recover.

Have you ever seen a shark--spiny, driveway, or otherwise? 

12 comments:

  1. I have never, to my knowldge, swum freely with a shark.  At an aquarium in Curacao. thhey have a tank divided by a plexiglass wall about 2 inches thick, with  inch diameter holes cut in it here and there.  On one side - nurse sharks about 6 ft long.  On the other side - you  with scuba gear and a handfull of dead fish.  You poke the fish through the hole and wiggl it, and the shark cruises by and rips it out of your hand.  Be careful not to stick fingers through the hole.

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  2. Your driveway artist was pretty close---a friend who knows you, no doubt!   How long and slender and sleek,  the real thing--- 

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  3. This is the species we used for our shark dissection class at the 4-H center where I taught until recently, so I'm very familiar with their insides! Supposedly the ones we used were all bycatch, but I still wondered whether it was 100% ethical to be using a vulnerable species like this to teach dissection.

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  4. A very interesting post. I've never heard of this type of fish before. I've never seen any kind of shark myself.

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  5. You amaze me with your knowledge of flora and fauna!  I've never seen a spiny dogfish but did see a few sharks at the San Francisco Aquarium.  It's depressing to know how humanity has wiped out whole species in the past and is still doing it.  :^(

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  6. Ha! I really think it was a random artist. My driveway made a nice canvass, I think--black asphalt instead of cement.

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  7. Thanks for the post! I wasn't able to reply to the Monday Nature Quote but I liked it. I've only seen sharks in an aquarium and maybe off the coast of CA--where we were swimming, but it was in the morning and we swam in the afternoon. Plus it was a reef shark, that my brother told us wasn't dangerous.....maybe he just wanted to swim. haha.

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  8. I've not swum with sharks, although wanting to. We have some beauties here in Oz. While I was doing my final year at university, some
    of us students got the chance to go on a boat to Port Lincoln in South Australia and
    participate in tagging seals on the small islands near there. My job was
    to hold a pitchfork to keep the mammas at bay when we grabbed their kids
    for weighing, measuring length and tagging. I quickly discovered a
    better method - put a hessian bag on the pitchfork and stop mamma from
    seeing baby. She forgot what was happening if she couldn't see her baby,
    and was quite calm. We could then get on with the business and give baby
    back without trauma to either species.

    The shark connection is that the
    area had been the location for several US shark-horror movies (hate
    those things - they give entirely the wrong idea about sharks and
    encourage slaughter of a species that is already in trouble). With all
    those shark lunches swimming around, I was looking forward to at least seeing one
    - but didn't. Have never dived with a large shark, alas - but I nearly
    sat on a wobbegong once while snorkelling at Byron on the wreck off main
    beach. I don't know which of us got the bigger fright. :)

    I often see wobbegongs snoozing under ledges in rock platforms during the day. Have startled a couple (and vice versa) while snorkelling in such areas. They look much bigger underwater than they really are! They are really pretty, with carpet-like patterns - hence the common name, "carpet shark".

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  9. Whoa, I'd never heard of a wobbegong and had to go look it up--I can see how it would be easy to mistake for seafloor and sit on! Such interesting info--thanks for sharing it. 

    I saw a big ol' shark once while swimming in the wide open Pacific. We'd stopped the ship dead in the water for a swim break and nearly the whole crew was in the water. I was underwater and have an image like a snapshot seared into my memory of two crewmates in the beautiful clear blue water, positioned between me and that shark--and then the  guys immd starting to swim upward, and the shark's head swinging toward us at their movement. Needless to say, I followed them up (anticipating a chomp on my fins the whole way). On a subsequent voyage with a different crew, the captain and I laughed at how the two of us stuck close to the ship on the swim breaks, while our crewmates swam off--it was obvious who had once found themselves swimming with a shark that appeared like magic out of the blue.

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  10. Nerve-racking! Just remembered a positive swimming-with-sharks story. We have whale sharks (a slow-moving, filer-feeding shark) migrate up the west coast of Australia to Ningaloo and beyond, and an acquaintance of mine went up there and hired a charter boat to take her (and a small group of other humans) out snorkelling with these sharks. She said it was beyond awesome to be dropped off in front of such a huge creature (they are harmless, unless you're plankton) and have it swim past!
    I saw three whale sharks in the Atlanta Aquarium last year - less awesome than to be in the water with them, but still jaw-dropping.

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  11. Oh! I would LOVE to swim with whale sharks! Have you heard of the afueras happening off of Mexico? Check this out: http://www.livescience.com/13137-whale-shark-swarms-yucatan-mystify-scientists.html

    Oooh, I want to go!

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  12. Wow, you should go - close to you than Ningaloo! I hadn't heard of that, but am not surprised - I'd read they do mass migrations elsewhere, so logically would do mass feedings too.

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