Sunny the Sunfish

Glenn Moore's blog | Created 1 decade ago

Ocean Sunfish Mola mola (Linnaeus, 1758)

This magnificent Ocean Sunfish (nicknamed ‘Sunny’ due to the sunglasses used for scale in the photo) was washed up at Jay’s Beach in Augusta in August 2010.  This individual is a juvenile by sunfish standards, at around 1.5m nose to tail and 2.2m between fin tips, as they can reach over 4m in length and over 2,000kg.

The cause of death is unknown, but Ocean Sunfish individuals occasionally wash up on southern beaches, especially during storms. Over the last few years several mass strandings of the smaller Slender Sunfish Ranzania laevis (Pennant, 1776) have been recorded on the southern coast. Unlike other sunfishes, the Slender Sunfish is known to school.

These unusual-looking fish are closely related to leatherjackets, triggerfishes, boxfishes, pufferfishes and porcupinefishes, in the order Tetraodontiformes (meaning four-toothed). Ocean Sunfishes are easily recognised by their unique blunt-ended body in which the tail is reduced to a rudder-like lobe. The dorsal and anal fins are tall and triangular, and the small rounded pectoral fin is just behind the tiny gill opening. The thick, leathery skin is covered with horny, rough-textured modified scales that resemble shark skin denticles.

The mouth is small and the teeth are fused into a beak-like structure, with a single tooth plate incorporated into the jaw bone of each jaw (top and bottom, left and right, making a total of four plates). Despite these sturdy teeth, adults mainly feed on soft-bodied animals such as sea jellies, salps and comb jellies, as well as small crustaceans and juvenile fishes.

Ocean Sunfish are primarily pelagic in most tropical and temperate seas worldwide, usually far offshore in oceanic waters. They are sometimes seen slowly drifting along at the surface, often on their sides. In this position their fin tips may break the water surface and are often mistaken for shark fins. Recent tagging experiments have revealed that they spend the majority of their time in the top 50 metres of water, but occasionally dive to depths between 400-800 metres, especially during the day.

Caring for Sunny

Death is not the end of the story for Sunny. Thanks to Gilbert Stockman from DEC who quickly arranged for the specimen to be frozen by Alan Adams, and subsequent transport, Sunny is now a part of our collection at the Western Australian Museum. This process began by preserving in formalin. We needed to ensure that all of its body was ‘fixed’ by formalin, so it required injecting litres of formalin solution into the tissue and organs.  Sunny then spent many months in a bath of formalin – it takes a long time to preserve a fish of this impressive size.

We decided to store Sunny in a glycerol solution, in the hope that we can have it on public display at some stage in the future. This is a much more complex task than our usual method of storage (in ethanol). Due to the chemistry of the cells of the body, we need to impregnate the tissue slowly, by increasing the concentration of glycerol in stages.  This is very exciting for us because it is quite a cutting edge approach to museum practices and gives us the opportunity to study the chemical process of this method for the first time.  Before starting the impregnation, Sunny was weighed – a mighty 155kg! Sunny was then placed into a custom built tank, which was filled with 25% glycerol.

We check on the fish regularly, and take measurements of the density of the glycerol. This gives us a measure of how much the glycerol has impregnated the body – as glycerol is absorbed into the body, the density in the surrounding glycerol bath will be reduced. When the density readings have stabilised, it means the body has absorbed as much glycerol as possible at that concentration and it is time to step-up to the next concentration. We will repeat this many times and step up through 35%, 50% to the final solution of 65%.  Of course, as Sunny absorbs glycerol, it will get heavier and using some fancy chemical mathematics, we can predict the rate of absorption and therefore the fish’s weight. However, we can also check these predictions very simply, by just weighing the fish.  Sounds simple enough, but getting Sunny carefully and safely out of the tank and onto some suitable scales is a big job.  We recently did this for the first time and much to our delight, Sunny’s weight had increased just as the calculations had predicted and has put on around 16kg.

There are other rewards in using glycerol.  Normal preservation methods cause specimens to shrink and go very stiff, and to fade in colour intensity. However, in glycerol Sunny has plumped up and all the fins and body parts are flexible.  Furthermore, Sunny’s colour has improved and much of the original pattern of spots can be seen again.  These effects of glycerol are wonderful and the result is a specimen that is more life-like than most museum specimens.

Apart from ending up with a beautiful specimen, when the process is complete we will publish the results of our research to document the chemistry of this process for the first time.  We hope that the work we are doing here at the WA Museum will assist and encourage other museums to use glycerol to display large and iconic specimens.