Freshwater pearl mussel (Margaritifera margaritifera (L.)) is an endangered bivalve. Most of its lifetime, it spends partly burrowed in the river sediment, calmly and quietly. If undisturbed, Margaritifera margaritifera can live over a century or two. Every year it adds one more growth layer to shell. These are the growth rings, or annual shell growth increments, that can be used to document its age and growth variations through its lifetime. The study of these increments, their variations and spatiotempral patterns, is the basis for a fascinating science of malacological skeletochronology, sclerochronology.

Margaritifera margaritifera is seriously threatened. This means that it may unfortunately face extinction in the future. Decline of the populations that previously occurred due predominantly to pearl hunting is currently addressed to poor water quality, river habitat degradation, threats to its salmonid host fishes and recent climatic change. Sad but true, it is possible that currently as few as approx. one hundred mussel populations may be globally counted as reproductively viable. In Finland, the situation is somewhat better in the north, where the anthropogenic influence is scarce, than in the south, where only a single population is known as viable to reproduce. Since it is endangered, Margaritifera margaritifera is listed on Annexes II and V of the European Habitats Directive and Appendix III of the Bern Convention. In Finland, Margaritifera margaritifera was protected by law in 1955. The species is still protected today and it is illegal to make any harm, even to touch, these bivalves in the nature. This holds for live and dead individuals, even for empty shells or small fragments of them dead long time ago. All shell samples - that can be collected in Finland under a special licence - are property of the Finnish Museum of Natural History (University of Helsinki).

Importance of studying the skeletal features of Margaritifera margaritifera can be described shortly: the shell growth provides irreplaceable information about the species conservation biology. This is especially due to shell growth increments that provide the ontogenetic age of individuals thus allowing the demographical estimations of the populations and comparisons between the ontogenetic age and shell size. Age-size data is important since the maximum age and shell length as well as growth rate have shown to bear great biogeographical variability with correlation to reproductive output of Margaritifera margaritifera. Moreover, the species exhibits extreme longevity with supercentenarian ages. These factors demonstrate the importance of Margaritifera margaritifera growth characteristics as irreplaceable tools for understanding its ecological preconditions and developing the conservational strategies. Moreover, since the growth variability of the shell growth increments is depending on external - aquatic and climatic - factors, the sclerochronologal records can be used as palaeolimnological proxies for the aquatic parameters they describe.

Taxonomy and philology

Margaritigera margaritifera is a shellfish, but it is also a bivalve and a mussel. It is thus also a mollusc as well as an invertebrate. What else it is, and how it is related to other creatures of life, is illustrated by the phylogenetic tree: Mollusca: Bivalvia: Palaeoheterodonta:  Unionoida: Unionacea: Margaritiferidae: Margaritifera margaritifera

Danish - Flodperlemusling

English - Freshwater pearl mussel

Estonian - Ebapärlikarp

Finnish - Jokihelmisimpukka

France - Moule perlière

German - Flussperlmuschel

Japanese -  Kawashinju-gai

Latin - Margaritifera margaritifera

Latvian - Ziemeļu upespērlene

Norwegian - Elvemusling

Polish - Skójka perłorodna

Russian - ЖЕМЧУЖНИЦА РЕЧНАЯ

Swedish - Flodpärlmusslan

MALACOLOGY is the division of zoology that deals with the study of molluscs. Malacological studies may be focused for example on taxonomy, evolution or ecology of molluscs. Studying any aspect of Margaritifera margaritifera thus falls into the malacological science. CONCHOLOGY is a sub-division of malacology. This term refers to the study of the shells of molluscs. The growth increments are formed and preserved in the shell, likewise, the studies dealing with the growth records of the shells can be termed conchology.

SCLEROCHRONOLOGY is a sub-category of conchology, and thus of malacology. It deals with the study of shell growth increments with some restrictions. Before termed sclerochronology, the increment data ought to be cross-dated. This is an approach that synchronizes the variability of wide and narrow rings in time, between a number of individual series, as serves the basis of high-precision temporal control of the skeletal series (Helama et al. 2006). Sclerochronological data may record growth variations from circadian to annual increments. However, so far the scientific mollusc literature has not presented cross-dating for other types of increments than those bearing annual periodicity. Sometimes used term SKELETOCHRONOLOGY refer to similar study of skeletal increments, usually of other animals than molluscs.

CONSERVATION PALAEOBIOLOGY is the application of palaeoecological techniques to the analysis of the skeletal remains of species that are threatened with extinction. The study of dead-collected (instead of live-collected) shells of Margaritifera margaritifera is thus a practical example of how to apply conservation palaeobiology in order to study the endangered species without reducing the number of individuals in the existing populations and to study populations no longer existing.

TAPHONOMY Since conservational palaeobiology deals with dead shells, it is important to be aware of the differences that may appear between the living comminities (biocoenosis) and the death assemblages (thanatocoenosis) and the features that have altered the shell appearance after the tissue was originally formed (e.g. shell dissolution). Taphonomy is the study of a decaying organism over time and it seeks to better understand 'biases' present in the fossil record.

SUBFOSSIL is an animal or plant remain of a recently died organism and thus a fossil with young geological age. Although there is no strict definition for the term, animal or plant remains that reresent the present interglacial, the Holocene, are often referred to as subfossils.

LIMNOLOGY is the study of inland waters and their physical and chemical, biological and geological aspects. Since Margaritifera margaritifera is a freshwater mollusc, nearly any research aspect dealing with its growth deals also with the limnological variability. Similarly, PALAEOLIMNOLOGY is the branch of limnology studying the past changes in the characteristics of inland waters. Because of its longevity and the possibility of using subfossil shells from geological and archaeological deposits, the growth records of Margaritifera margaritifera may retrospectively give insights into the palaeolimnological variability that have occurred over the past centuries.

ARCHAEOMALACOLOGY As mentioned above, shells of bivalves can sometimes be found from deposits that are not natural but antropogenic. Actually, malacological skeletal parts are the group of invertebrate remains probably most commonly found from archaeological deposits. Studying these remains, in the context of past human cultures and behaviour, brings the science into the field of archaeomalacology.

BIOGEOGRAPHY is one of the basic elements of conservation biology of Margaritifera margaritifera. The data about size and age of the mussels shows that the southern populations grow much faster, but reach lower maximal sizes and ontogenetic ages than the northern populations. This likely due to climate via its influences on the metabolism of the species. These aspects bear importantce on conservation biology since the reproductive output of Margaritifera margaritifera correlates with all three mentioned variables. Also, the regional context ought to be understood prior to any interpretations about the environmental factors other than climate. Conchronological and sclerochornological records can thus be interpreted in terms of invertebrate PHYSIOLOGY. Physiological signal may be preserved also in the inter-annual growth variability and modelled using the autoregressive-moving average methods.

DENDROCHRONOLOGY is research dealing with the study of tree-ring variability. Although the study about tree-rings is more or less applied botany and may as such sound a bit distant regarding the bivalves, it is actually surprising how similar the numerical methods of the two disciplines are. As a matter of fact, many of the sclerochronological techniques and approaches dealing with time-series analysis are identical or slightly modified from dendrochronological knowledge that has longer scientific tradition. Most important such techniques is cross-dating (see above, sclerochronology). Due to their similarity in various respects, sclerochronology has sometimes been referred to as aquatic counterpart of dendrochronology (Helama et al. 2006). While the research that aims to study the relationships between the tree-ring and climate variability is sometimes termed DENDROCLIMATOLOGY, it could be suggested that the studies examining the relationship between shell growth increment and climate variability could be analogously called SCLEROCLIMATOLOGY.

THE OLDEST RECORDED ANIMAL IN FINLAND:
ONTOGENETIC AGE AND GROWTH IN MARGARITIFERA MARGARITIFERA
(L. 1758) BASED ON INTERNAL SHELL INCREMENTS
Helama S. & Valovirta I.
Memoranda Societatis pro Fauna et Flora Fennica 84: 20-30 (2008)

Conservation Paleobiology (by Karl W. Flessa)