Research on shark anatomy

The first thing I notice when picking up the Short Nose Spurdog Shark is the roughness of its skin. 

This is because its skin is made up of dermal denticles, which are very similar to their teeth, with pulp cavity, dentine and enamel.

The shape of their dermal denticles increases the sharks swimming efficiency and allows them to glide silently through the water.

With a closer inspection, we see that these sharks also have a spine at the top of their first and second dorsal fins, which is used for protection against other larger predators. These spines are also modified denticles.

Before we get down and dirty with the dissection, we need to take over 100 different measurements of the shark.

This can be tedious and time consuming at times, but is important so that we can determine growth rates and size differences in genders.  We start off by measuring the whole shark, from snout to caudal fin tip, from here we work our way down the shark taking measurements from each fin and the space between them. The sharks can often be very squishy, as they have been frozen and defrosted a couple of times, so sometimes we are unable to get accurate measurements, in this case we would rather have no data than wrong data.

Now all the measuring is done, we take a few photos for our records and get the total mass of the shark.

As the interns start to cut open the shark, they notice just how tough the skin is. A fair amount of pressure has to be applied to make a starting incision. After each dissection we need to replace the scalpel blades as the skin actually blunts the blades.

 

With the organs exposed we notice just how large the liver is, it is by far the largest organ, weighing in at 32 grams. It’s so large because sharks actual use their liver for buoyancy, as opposed to bony fish who have a swim bladder for this purpose.

Once we’ve weighed the liver, we move on to its digestive system, cutting out the stomach and the intestine. We see that the intestine is a lot smaller than we would expect, but this is because it has a more efficient structure, called a spiral valve, which increases the surface area for the absorption of nutrients.

After weighing the intestine, we squeeze out the stomach contents just to make the interns squirm, but also to check for any remnants of fish. Our stomach was empty, so we get the mass of it and move onto the reproductive organs.

Our shark is lacking claspers, so from this we already know it’s a female shark. Surprisingly we find that the ovaries are positioned at the top end of the shark near the head, and likewise for males with the gonads. 

The ovaries of our shark are developed and containing eggs, so we can deduce that she is mature and was actively mating.

To help with future studies on this species, we next take genetic samples and freeze them for later use. We take 3 samples from the left pectoral fin, as well as a sample of the liver, kidney, muscle and vertebrae.

For the final part of the dissection we remove the head, so that we can later do a thorough check for parasites.

With the dissection part over, we then enter in all the data and photos to the computer and file away our data sheets so they can be checked over at a later date.

All that is left to do is to throw away the shark bits and pieces and give our tools and table a good cleaning!

p.s. all shark used for those dissections are obtained for discarted bycatch from commercial demersal trawlers. This does not create demand and thus increase the exploitation of the oceans.

Ben Southall