Introduced species are defined as species suddenly found far away from their native range; this sudden appearance being due to human-mediated transportation. These species therefore have strong interactions with human activities. Concerning marine introduced species, they are often brought to different places by marine transportations (on ship hulls or ballasts) or exchanges between shellfisheries. As a consequence, they are mainly found in harbours and aquaculture basins.
These species are also called invasive species, especially when populations grow and disperse very fast, outcompeting native species.
Two examples of case studies carried out in the network on invasive species are presented to you:
Undaria pinnatifida
credit picture: Y. Fontana, Station biologique de Roscoff
The case of the brown alga, Undaria pinnatifida is very interesting as this species has been cultivated in Japan for many years and was introduced by mistake in France in the 70’s. It has since been able to adapt to three different types of environments:
- The harbours (which is a classical zone of settlement for introduced species as there are a lot of human activities there)
- In aquaculture farms where this alga is cultivated by humans for its nutritional interest and is then called "Wakame" (see picture below). However, it can also become a problem for marine farms by increasing labour and harvesting costs due to fouling problems on oyster racks and mussel ropes, for example.
Undaria pinnatifida commercialized under the name of "Wakame"
credit picture: http://www.delicioso.co.uk/shop/Sea%20Products/
- On the subtidal rocky areas where populations have become independent.
This remarkable capacity of adaptation to different environments is of great interest for fundamental research.
A few individuals of the species Crepidula fornicata pilled up on top of each other
credit picture: Franck Gentil, Biological Station of Roscoff
The mollusc Crepidula fornicata comes from the eastern coast of the United States of America (from the Canadian frontier to south Florida). It has been introduced in Europe (in the late 1800’s) and on the western coasts of the USA most probably because of oyster transfers for shellfish culture purposes. This species is now widely spread on the coasts of Europe and is an oyster-pest in Great Britain, reflecting the nuisance it can represent for oyster farms.
Proliferation of Crepidula fornicata
credit picture: http://www.ifremer.fr/francais/rapp2003/circulation-ocean.htm
Crepidula fornicata seems to compete with other filter-feeding invertebrates (as mussels or oysters) for food and most certainly does for space; it often occurs in enormous numbers. In fact, the individuals of this species have a very efficient reproduction system as they are protandrous hermaphrodites: after having pilled up on other individuals, they start their life as male and then become female. They can also tolerate a wide range of environmental conditions. By settling in large numbers in new environments, they modify the nature of the substratum (which is also a problem for native species), slow the bottom currents and seem to favour silting. The invasion of ecosystems by Crepidula fornicata can change the nature of communities that usually settle in these places and therefore the global functioning of ecosystems.
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A larva of Crepidula fornicata, ready for metamorphosis. Length is about 1 mm. Note the velum, a ciliated swimming and feeding organ that looks like an enormous pair of ears. | A newly-metamorphosed juvenile of Crepidula fornicata. The velum is gone; the animal has abandoned its free-swimming planktonic lifestyle for the serious business of growth and reproduction as a bottom-dwelling adult. |
credit pictures and commentaries: Anthony Pires from Dickinson Biology Faculty, http://www.dickinson.edu/departments/biol/pires.html
As larval development and metamorphosis (the transition between the free larval form and the fixed adult form) are a critical phase of the species life cycle, they are also being studied within the network, Marine genomics Europe, through different aspects of genomic studies. In fact, during the acquisition of competence (i.e. when the larva acquires the capacity to metamorphose without however doing it) and during metamorphosis, great physiological and metabolic changes occur (e.g. the development of new organs, the disappearance of others). Organisms are therefore more vulnerable at these stages and it is therefore interesting to know what proportion of larvae actually become adults. Understanding the way these mechanisms are regulated inside the organisms is another type of study going on in the network. Scientists are seeking to understand what enables populations to settle, survive and, in certain cases, proliferate at an important speed in new environments.
It has been demonstrated, for example, that larvae seem to be attracted by the presence of adults of their species and settle close to them, therefore favouring the maintenance of the population at the same place.
Demographic studies, Population genetics and Phylogeography studies are being carried out on introduced species to study intraspecific genetic variation and other characteristics such as spatial distribution and life history traits. The description of the different kinds of studies performed on introduced species is illustrated here by those performed on Crepidula fornicata.
Firstly, in order to understand the process of dispersal of introduced species, demographic studies are carried out in the natural environment to see how the individuals reproduce in the wild, how they grow and spread. This is part of learning about the life history traits of the population, a key characteristic to know how the population can evolve in time and space. Larvae are counted with the help of a dissecting microscope.
Population genetics studies are then carried out and genetic markers have been developed concerning the study of Crepidula fornicata, for example. The genetic relations between the different populations are being studied thanks to population genetics and phylogeographic methods:
Genetic markers were developed for these purposes. Examples of the genetic markers used in this type of study are microsatellites and mitochondrial markers. Microsatellite development relies on the construction of DNA libraries. Microsattelites and mitochondrial markers are then compared between individuals by doing genotyping and sequencing, respectively, and statistical analysis. This comparison enables to establish genetic relationships between individuals and populations.
Functional genomics is being used to identify genes implied in metamorphosis. In this purpose, a microarray is being constructed for Crepidula fornicata representing the expressed genetic material of individuals in different conditions: in larvae alone or with adults, at different times after having provoked metamorphosis, etc. cDNA libraries were constructed prior to the establishment of this microarray. (See transcriptomics).
Comparative genomics is also being used and different biological models are being compared to see if the genes that are identified in each species as being implied in metamorphosis are of the same family.
The microarray that is being constructed for Crepidula fornicata will also be used to compare the levels of gene expression in Crepidula fornicata with other species of Crepidula that have different modalities of development. The objective is to see whether or not the genes are conserved across this set of species for which the larval state is shorter or absent.
Within the network, scientists in the Biological Station in Roscoff (partner #2) are collaborating with the Plymouth Marine Laboratory (partner #41) on the subject of invasive Tunicates as Styela clora and Corella eumyota for instance. Collaboration also exists, for example, with the Centre for Advanced Studies in Ecology and Biodiversity, CASEB (http://www.bio.puc.cl/caseb/) which is part of the Pontificia Universidad Catolica de Chile, PUCCH (partner # 43). The CASEB are working on ecophysiological aspects of the study of the different species of Crepidula: factors that influence the length of incubation of the eggs, the length of the larval state, etc. (This collaboration is part of the Laboratoire International Associé, LIA DIAMS (http://www.sb-roscoff.fr/EGPM/LIA.php).)
Introduced species like Crepidula fornicata or Undaria pinnatifida are or have the potential to be a nuisance to their environment as they tend to spread in the environment to the disadvantage of the native species, some of them being economically important species as oysters.
Molecular markers, genomics and population genetics can be helpful not only to analyse introduction pathways but also local colonization patterns of introduced species (which is part of their life history traits). In fact, more has to be understood about mechanisms of adaptation and plasticity of these species.
The massive settlement of Crepidula fornicata being a problem to shellfisheries’ industry, there are projects of eradication of this species. There is therefore an urge to understand the mechanisms underlying invasion. Since fighting against the settlement of Crepidula fornicata, has revealed to be almost impossible, scientists are trying to understand what role is played by the larvae, which is one of the reasons why larval development and metamorphosis are being studied within the network. Being able to predict what species will actually settle in the environment after eradication would be of great interest for environmental management politics. In fact, economically interesting species might not be the ones to resettle in the environment because of the changes provoked by invasive species. The aim of the research performed on this subject is also to increase public awareness on the impact of human actions, as commercial exchanges and the introduction of exotic species, on the environment and ecosystems.
Contributed by Stephanie Ries
