Small spotted catshark skulls

I've been cleaning a few small spotted catshark (Scyliorhinus canicula) skulls lately, and took some pictures to illustrate various stage of the cleaning process.

The heads prior to preparation. All of them are under 10 cm long.

Step 1: The heads are soaked in very hot (but not boiling!) water for a few hours. The water may need to be changed a couple of times as it cools and gets cloudy.

Step 2: Scraping. As the heads soak in hot water, the tissues are scraped using a scalpel and a fine pair of tweezers. 

During the scraping process, care should be taken not to disconnect the hyomandibulars, ceratohyals and basihyal if those are to be kept with the skull. On larger skulls those might be disconnected for easier handling, but on small skulls, it is easier to keep them attached. The nose area is also very delicate due to the small cartilage flaps in the nasal capsules, and to the rostrum which consist of 3 small unconnected cartilage rods in this species.

Step 3: Whitening and further scraping

Once the initial scraping is finished, the skulls are transferred to a mix of diluted H202 and ethanol. This step will whiten the specimen, and soften the remaining tissues which can be scraped further, leaving a very clean specimen. They are then rinsed thoroughly in hot water and transferred to 95% ethanol.

Step 4: Drying. 

The most common technique used to dry cartilaginous material is freeze drying. It can take some times, but restricts warping of the cartilage to a minimum. I'm still new to this technique however, and these particular specimens were dried in very strong ethanol for a couple of weeks. Note that the hyomandibulars separated from the chondrocranium on the above specimens. This will happen if the specimens are scraped and/or handled too much during this step, and they'll need to be glued back later on.

Finished specimens.

Multiple view of a finished specimen. The hyoid arch was kept in place. In order to avoid warping, small perspex sheets were used to hold the rostrum during the final air drying stage.

I'm basically using the same process, with only minor variations on all the chondrichthyan specimens I'm working on, but need to look further into the freeze drying technique in order to reduce warping.