Tuatara: Volume 7, Issue 3, June 1959
Preparation of Skeletons by Marine Animals
Preparation of Skeletons by Marine Animals
This note describing an easy, quick and successful method for reducing fresh or unpreserved zoological specimens to the state of clean skeletons is published for the benefit of those interested in both wholesale and small-scale results which are not time-consuming to achieve.
On parts of the New Zealand coast where there is little wave action but where the water is freely exchanged, as in harbours, with each tide, small marine carnivores accumulate at certain seasons of the year and rapidly eat the flesh from any offal such as fish heads or bodies left in the water. At first only the softest parts are eaten (muscles, nerves, fat, gills and blood vessels), the animals crawling in beneath the skin from cut surfaces of the body; subsequently the lining of the gut and the tougher connective tissue and the orbits and skin are eaten, and finally the ligaments between bone or cartilage and remnants of skin are removed so that all that is left is a disarticulated skeleton. With little modification these circumstances can be exploited for the controlled preparation of skeletons.
The method may be applied to any sizeable fresh or unpreserved vertebrate specimen. A suitable way of ensuring that the specimen to be cleaned is not carried away by currents or large carnivores is to place it in a tethered frame covered with wire netting; the mesh size should be sufficiently small to prevent isolated parts of the skeleton such as girdle and gill arches from falling through. For some purposes, such as class material for teaching the skull foramina of the dogfish or mammal, the flesh-covered specimens can be placed as bait in an ordinary crayfish pot to be dropped below tidemark and then recovered when clean. For delicate or critical material it is probably best to use individual net or glass containers which can be recovered and opened without disturbing the arrangement of the collapsed skeleton.
The order in which the soft parts are progressively eaten varies with the specimen subjected to cleaning, but even the less accessible softer parts are usually eaten before the more accessible tougher parts such as skin, mouth lining and fibrous skeleton. With experience, the process can be allowed to continue until the desired stage is reached, and for specimens to be page 120 mounted can be stopped when the skeleton is free of all but the ligaments. By careful stopping at the earlier stages even such notoriously difficult material as selachian visceral arches and teleost skulls can be maintained intact.
The most effective scavengers are the voracious sea lice, Cirolana (Isopoda) of which there are two species identified by Dr. D. E. Hurley as C. rossi, the larger one, and C. arcuata, the smaller. The rate of skeletonising by these carnivores can be phenomenally high. Fish caught in set nets are often attacked by sea lice which enter at the gills and mouth and eat their way beneath the skin back to the base of the tail, and in this way they are reduced overnight to nothing but skin and skeleton. This is a fraction of the time required for the full action of even the most vicious chemical macerating fluids.
Since the method is an ecological one, a similar ecological situation in other parts of the world will have its list of scavenger species to help in the preparation of skeletons. In South California the local sea louse used is Cirolana harfordi. In Otago Harbour, cockabullies (Tripterygion), the whelk (Buccinulum), and a number of species of small crabs have been found during skeletonisation at low-water mark, doing the final cleaning of specimens after the departure of the sea lice which work mainly at night. After the biological process has been stopped, little more cleaning may be required than can be done under a running tap with a pair of forceps to remove sediment and scraps of tissue. Further treatment by leaving skeletons in cold, fresh water for further maceration by bacteria may be tried but has not proved satisfactory for cartilaginous skeletons, and is not necessarily desirable if the action of scavengers has been really effective. Cartilaginous skeletons soon become soft and fall apart, so that if the scavenger fauna is not rich enough for them to be cleaned within a few days, leaving them longer will not be successful.
The advantages which this method of skeletonisation has over other biological methods such as bacterial decomposition, cleaning by insect larvae or ants, are: (i) it is quick; (ii) it can be observed and readily controlled without unduly disturbing the process by digging for the specimen; (iii) the skeleton is not discoloured by earth staining or faeces; (iv) the specimen is continually washed.