UPDATED CITATION for publication

Coston‐Guarini J, Guarini J‐M, Boehm FR, et al. 2018. A new probabilistic approach to estimating marine gastropod densities from baited traps. Mar Ecol. e12509. https://doi.org/10.1111/maec.12509

Citation for pre-print at Biorxiv

Coston-Guarini, J et al. [preprint version 1, 24 July 2017] Estimating Muricid abundances from trapping methods used in Mediterranean Tyrian Purple industry. Bior𝛘iv, “New Results”, http://www.biorxiv.org/content/early/2017/07/24/167387

It's not a very common practice in marine sciences to reconnect with ancient techniques and re-analyze how they work using our present-day mathematical tools. We were intrigued by a problem in archaeozoology which is how to estimate the amount of coastal area needed to supply enough 'murex' gastropods for maintaining the thriving Tyrian purple pigment industry of early Mediterranean civilizations. We now know from archeological sources that these groups have been exploited by human societies as both a source of food and natural pigments, on every continent (except Antarctica) for millennia.

In addition to their economic value, many of these gastropods fulfil important functions for marine ecosystems, such as organic matter recycling and predation. However, their presence has been overlooked in surveys of coastal areas; there is little or no data on these populations’ ecological status and distributions. Nonetheless, information about population state and dynamics are crucial for ecosystem studies; these are not easy to reconstruct, because of spatio-temporal variability patterns in distributions.

Thus, fundamentally, the project is motivated by the long-standing problem of how to estimate population sizes of marine species from the numbers of individuals caught in baited traps. Because trapping techniques used to collect muricid snails for the purple pigment industry resemble methods still used today in artisanal and coastal fisheries, we chose to use one of the most commonly mentioned Tyrian purple sources in the Mediterranean, Hexaplex trunculus.

Engraving of Hexaplex trunculus from one of the earliest known conchology texts (Bonanni, 1684) — Engraving of *Hexaplex trunculus* from one of the earliest known conchology texts (Bonanni, 1684)

The probabilistic approach presented in our article circumvents the problem of estimating population densities of species considered as “rare” or “intermittently” present, because they were observed only occasionally with classic sampling techniques (quadrats, transects) or are unadapted to other approaches used like mark-and-recapture. By combining information from behavioural experiments about how individuals behave toward baited traps with the results of successive capture experiments in situ, it is indeed possible to estimate the population density of a species.

For example, we can, for the first time, estimate that about 0.4 ha of coastal area was needed in Antiquity to collect enough gastropods to produce 1 g of pigment (assuming an even distribution of course!).

Our approach has therefore important implications not only for archeo-zoological and historical ecology work on population dynamics, but also for modern fisheries survey programs of invertebrate stocks, like Buccinum undatum, as well as for biodiversity surveys.

the story behind the paper

pourpre_0001a_HLD_selfie_1858?.png — Victorian souvenirs

Self-portrait of Henri de Lacaze-Duthiers in Murex purple, from about 1858 (Sorbonne University Collections). This is one of his "mucographes" a technique he proposed to commercialise to finance future research.

I first learned of the zoological interest in Tyrian purple, while researching the biography of a French zoologist, Henri de Lacaze-Duthiers. In the 1850s, as a young zoologist, looking to make a name for himself he (re-)discovered that the mythic Tyrian Purple comes from live Murex snails during a series of field trips to Port Mahon (Minorca). He retells in great detail the experience in the 1861 volume, "Un Eté D’Observations En Corse Et À Minorque Ou Recherches D’Anatomie Et Physiologie Zoologiques Sur Les Invertébrés Des Ports D’Ajaccio, Bonifacio Et Mahon" -- just one of many, many texts he wrote on the topic.

Lacaze-Duthiers set about dissecting the snails to identify where the reactive substance was produced by the animal. And, because the precursor is photoreactive, he also experimented with making contact 'photos' on fabrics soaked in the mucus he extracted, hoping to be able to sell the idea to finance his work. However, it never went anywhere because smelly sulphur compounds released during the reaction were difficult to get rid of, even after many, many washings.

The idea to attempt to estimate their population density arose during a field ecology school on Crete Island, which is one of the important centres for the ancient trade in Tyrian purple pigment. Curiously, we were told by other researchers that the murex snails were considered locally extinct, or very rare in the area. After some initial investigations, we realised that one species, Hexaplex trunculus, was present in the nearshore, where it was being collected for private consumption. This lead to the trap tests and method developed in the article that exploits the simple artisanal methods used for centuries to attract and collect these animals. Despite their use as an indicator of tributyltin presence in sediments since the 1990s, these groups of marine snails have been generally overlooked and considered "ubiquitous"; little information has been collected about their population size and distribution. This may change as they are targeted as replacement stocks in the Mediterranean Sea and elsewhere.

--- J. Coston-Guarini