Publishable Summary - Periodic Report EGOMARS
Project Objectives
The coastal zone, where fresh and saltwater meet, hosts some of the most dynamic, diverse and productive
ecosystems on Earth. Ecological and hydrological land-ocean connectivity are important drivers of these ecosystems.
This project aimed at advancing the understanding of hydrological-ecological coupling, by studying relationships of
key fauna with coastal groundwater hydrology in the subtropical realm on the Yucatan Peninsula in Mexico. Highresolution sclerochronology and concurrent acoustic telemetry of the iconic Queen Conch Strombus gigas (a marine
snail) were used to understand habitat usage of this endangered and socio-economically important species in the
Caribbean Sea.
The study was carried out in collaboration between the European Institute of Marine Science (Institut universitaire
européen de la mer IUEM, Plouzané, France), Centro de Investigación y de Estudios Avanzados del Instituto
Politécnico Nacional (CINVESTAV, Mexico) and Parque Xel-Ha (Mexico), a commercial tourist park facility where field
studies were carried out. Xel-Ha comprises a small, typical ground-water fed coastal inlet (‘caleta’), which provides a
protected home for a large population of queen conch. We have learnt new aspects of the physical environment of
groundwater-fed coastal ecosystems, how animals respond to it and how they use this ‘special’ habitat.
Work Performed and Main Results
Xel-Ha – ‘the place where waters are born’. Xel-Ha means ‘the place where waters are born’ in the local Mayan
language. Waters are ‘born’ in the southern arm of the caleta, where an underground spring discharges into Xel-Ha.
The freshwater flow results in a vertical stratification between low-salinity river water in the upper layer and
saltwater on the bottom. The spring is flowing strong all year round, and even though the wind and the tides affect
the water level and the mixing of water in the caleta, this stratification is maintained throughout the year. The
salinity of the bottom layer where the conchs live is similar to the salinity of the ocean, but once every day during
the early morning hours, its oxygen concentration is very low. This is due to the large oxygen demand by organisms
living below the freshwater layer on one hand, and the inhibited flux of oxygen from the atmosphere to the bottom
layer on the other hand. Despite these periodic low oxygen levels, a large population of conchs live on the seafloor
underneath the freshwater ‘blanket’.
How do queen conchs use their habitat? These marine snails - like all snails - move slowly. But where do they go
over longer periods of time, for example over a one month or longer period? We answered these questions by
tagging animals with small acoustic transmitters, and by recording their movement with acoustic receiver stations
installed throughout the caleta. This ‘big-brother-is-watching’ study revealed two important behavioural patterns.
Caribbean
Sea
Figure 1: (left) Queen conch tagged with acoustic transmitter. Acoustic receiver station in
background; (right) Typical home ranges of three individual queen conch in caleta Xel-Ha (red, green
and yellow). Receiver stations are marked with black dots.
For most of the time, an individual queen conch has a small home range (the space it uses). Over a six month period,
the typical home range is 2000 m2 (or an area of 40m x 50m). This has important ecological implications: a small
home range of a fast-growing animal indicates that it does not need to go far to find food, in other words its
movement is not limited by food availability. Over longer periods of one or two years, the conchs’ story changes.
Even though these snails are slow, they are capable of moving large distances. As they grow older, they move from
the more protected inshore regions towards the mouth of Xel-Ha, and many of them leave to the offshore ocean.
This has important implications for fisheries management: Xel-Ha is a nursery ground for offshore populations of this
threatened species.
What can the shells tell us about the life of the animals? Shell-building animals like Strombus gigas incorporate
chemical and isotopic signatures of the water they live in in their shell as they grow. By sampling these signatures
along the growth axis of the shell, we can unravel the changes of environmental conditions during the life of an
animal. For example, the oxygen isotope ratio embedded in the shell depends on both the temperature and the
salinity of the water at the time of carbonate precipitation.
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Figure 2: Measured temperature of bottom water in caleta Xel-Ha (grey), together with oxygen isotope
ratios in the shells of three queen conch (blue, red and black),representing life from birth (apex) to
maturity (lip).
On the bottom of the caleta where the conch live, variations in ambient water salinity are small, and therefore
(annual) variations in temperature dominate the oxygen isotope signature. We can match the shell isotope record
with measurements of water temperature, and from this comparison we can calculate the age of an animal at the
time of collection (how many summers and winters it has lived for). The isotopic records show that the animal shells
grow in their conspicuous spiral pattern for 1.5-2 years, after which they cease to grow in length and instead develop
a thick shell lip. This growth pat-tern is similar to that of conch living in ocean environments, suggesting that the
somewhat stressful low-oxygen conditions in Xel-Ha due to persistent groundwater flow do not significantly affect
their growth.
Conclusions and socio-economic impact.
Throughout the Caribbean region, the queen conch is a highly important and endangered fisheries resource.
Successful fisheries management, e.g. in the form of marine protected zone design depends on a good scientific
understanding of the habitat usage of the target species. This study clearly demonstrates for the first time, that
groundwater-fed estuaries are nursery grounds for Strombus gigas, which efficiently supply offshore populations.
This suggests that these previously unrecognised habitats of this iconic species under the influence of groundwater
discharge deserve special consideration in fisheries management in the region. The results of this study contribute to
an improved understanding of coastal resources on Mexico’s Yucatan coastline, and are now taken up by the
scientific and commercial partners of this project in Mexico in cooperation with local fisheries management
authorities.
Contact: Dr Thomas Stieglitz, Tropical Water & Aquatic Ecosystem Research TropWATER, James Cook University,
Townsville, Australia. email: thomas.stieglitz@jcu.edu.au