Goose Barnacles as Forensic Tools
Ocean debris is particularly important for oceanography, biogeography and forensics. The biofouling communities on floating objects like logs, pumice, plastic or even corpses can help reconstruct where the objects came from and their duration at sea.
Understanding biofoulers also helps predict the spread of invasive species. These biofouling communities are often shaped around Lepas, a genus of goose barnacles found across the world’s oceans.
Thomas Mesaglio of the University of New South Wales is investigating how marine biofouling communities ─ especially those that develop around the goose barnacle genus Lepas ─ can be used in forensics to reconstruct the drift path and duration at sea of beach-cast objects. His project aims to:
1) Review the pelagic biofouling community and understand how it changes over time, with a focus on goose barnacle communities.
2) Develop a greater understanding of the goose barnacle genus Lepas focused on its morphology, growth and life history by combining field observations with laboratory studies.
3) Determine whether Lepas can be used as a useful forensic tool for tracking drift objects, combining calculations of Lepas growth rates with a study of shell-derived oxygen isotopes to estimate the approximate water temperature, origin and minimum duration at sea of Lepas-covered drifters.
Background
Biofouling community
Historically, ocean drifters have mostly been natural floating objects like pumice, seeds, and wood (Thiel and Gutow 2005a, Goldstein et al. 2014). However, in the past few decades, anthropogenic drifters, predominantly plastics, have increased in abundance and distribution to become the dominant debris type globally (Goldstein et al. 2013, Eriksen et al. 2014). Together, the rapid increase in global shipping and quantity of drifting debris has increased the transport of marine organisms across the planet (Barnes et al. 2009, Murray et al. 2018). As a serious threat to marine biodiversity (Molnar et al. 2008) and a part of the IUCN’s Red List criteria (Keith et al. 2013, Rodríguez et al. 2015), the increased spread of invasive species magnifies the importance of understanding drifter communities.
Lepas
The barnacle family Lepadidae is one of the most abundant biofouling taxa. Referred to as goose barnacles, lepadids are obligate epipelagic rafters, which means they are restricted to living on floating substrates near the ocean surface (Thiel and Gutow 2005b). Lepadids are a cosmopolitan family and are able to survive on drifters for long periods of time and travel large distances (Iljin et al. 2013). Because of their pervasiveness in the global epipelagic environment, many studies of biofouling communities are inherently of Lepadidae, with the genus Lepas of particular interest. Despite the dominance of Lepas as a biofouler, its ecology is poorly understood. Besides studies of how Lepas anatifera cements to substrates (Lacombe and Liguori 1969, Walkera and Youngson 1975, Jonker et al. 2015), there are few studies on their growth rates, life history, or interactions with predators or other biofoulers.
Isotopic analysis
After the disappearance of Malaysian Airlines flight MH370 in March 2014, a wing fragment covered in L. anatifera washed ashore 505 days later on the western Indian Ocean island of Réunion (Poupin 2015, ATSB 2017), strongly supporting the belief that the flight had terminated in the eastern Indian Ocean. An oceanographic analysis based on the Global Drifter Program (Trinanes et al. 2016) estimated the possible origin of the wing fragment in relation to the last recorded position of the flight. To constrain these oceanographic hindcasts, the Australian Transport Safety Bureau recommended that the attached Lepas be studied for their growth rates and their temperature of calcite formation in an attempt to retrace the path of the fragment. A 2016 study consequently established a linear relationship between sea surface temperature and the delta-O-18 (ratio of oxygen-18 to oxygen-16) content of Lepas shells allowed them to conclude that Lepas faithfully record the water temperature during formation of their shell in the oceanographic conditions of the Indian Ocean.
Methods
NSW beaches will be combed for shore-cast drifters to achieve a better understanding of the pelagic biofouling community, with a strong focus on lepadids.
An offshore mooring will be installed 3 km off the North Bondi headland. Designed as a biofouling substrate, the mooring will be sampled monthly from a boat launched from SIMS, allowing observations of live biofouling communities and how these communities change over time. Also Lepas will be reared in the SIMS Ian Potter Research Aquaria for six weeks using three temperature regimes to understand how temperature affects growth and allow the calculation of a growth curve.
Finally, an isotopic analysis will be conducted at UNSW’s Mark Wainwright Analytical Centre. The delta-O-18 content of Lepas shells will be measured and these data validated against the known sea surface temperatures recorded at each sampling site to establish the accuracy of the isotope thermometer, and then checked against each specimen’s size, and thus age, to further confirm the accuracy of the settlement time estimation.
Outputs
In combination with delta-O-18 data, calculating the age of any attached Lepas and referring to known growth rates allows the calculation of minimum duration at sea and a prediction of place of origin for forensically important objects like corpses or aircraft debris, creating a novel, forensic tool with strong potential for solving crimes.
Understanding pelagic biofouling communities and the factors facilitating long-distance drifter transport will help identify the threats posed by the emergence of plastic as a cosmopolitan, hyper-abundant debris type and extreme events like tsunamis, the combination of which can facilitate the transport of debris, and thus invasive species, across entire oceans.
This study will represent the finest temporal resolution study ever conducted on mooring biofoulers, as well as the first ever convergence of drifter, mooring and laboratory studies for not only Lepas, but biofoulers in general. The combination of data from all three analyses will allow cross-checking of all findings against each other and strengthen the conclusions drawn.
