p1: What is a crustacean? When we think about crustaceans, we immediately
p1: think about lobsters, shrimps, possibly sowbugs, but we forgot that the part of
p1: the diversity of crustaceans is made up of smaller organisms that are less known to
p1: the general public and are indeed very diverse. Before focusing on development,
p1: we will start by looking at the phylogenetic status of crustaceans.
p1: In the past, crustaceans were considered as a homogeneous group. Most of those
p1: representatives were aquatic.
p1: The group was then thought to be the brother group of the hexapods, which
p1: are essentially terrestrial.
p1: Crustaceans have often been considered to be the "insects of the sea".
p1: Today molecular phylogeny tells another story, with the Hexapods placed within
p1: crustaceans, and not as the sister group. As a result, crustaceans are a paraphyletic
p1: group relative to Hexapods, and the whole group is now called Pancrustaceans.
p1: In this course you will see several examples illustrating the groups
p1: Branchiopoda, Copepoda, Malacostraca and Cirripedia.
p1: Now we will look at a very special species of crustacean in particular.
p1: Artemia franciscana is a Branchiopod species of American origin and has been introduced
p1: in many parts of the world. It is known because its larvae can be obtained en masse
p1: and used to feed very young babies of many species; without Artemia, there would be no
p1: aquaculture as we know it today. Artemia are commonly called brine shrimp and are
p1: very small organisms between 5-15mm.
p1: Here, we observe tandem swimming that precedes mating and we can see young
p1: cohabiting with adults. As we can see there are anatomical differences between the
p1: adult form and the young, and therefore we call them larvae in the first stages
p1: and then juveniles the ones that follow, resembling adults.
p1: They live in salty water but never in the ocean or sea. Typical habitat consists of
p1: salt lakes such as the Great Salt Lake in the United States, coastal lagoons or
p1: salt marshes that are often saltier than the sea and that are free of predatory fishes.
p1: The particularity that make the species interesting is the possibility to obtain
p1: live lava at the right time. We can therefore use Artemia as a food source, but we can
p1: also study this organism in its own right, and in particular its development.
p1: That's exactly what we are going to do now.
p1: Artemia franciscana is gonochoric, meaning there are both males and females, but in
p1: Europe there is still sometimes a native, parthenogenetic species, meaning with
p1: only females. In any case, the development of brine shrimp begins in the mother with
p1: the fertilization of oocytes, either following mating with a male who brings in spermatozoa
p1: or by parthenogenesis. The zygote obtained can follow two modalities depending on the
p1: environmental conditions. When conditions are favorable, development leads directly
p1: to a small swimming larva.
p1: When conditions are bad, development stops in the mother at the blastomeric stage
p1: and the embryo is covered with a thick brown shell that forms embryonated eggs
p1: called resting eggs. In Artemia this egg is smooth and round. Once laid, these eggs
p1: can dry out and still be alive. In this stage there is no respiration and the embryo is
p1: in a suspended state of life. This is called "cryptobiosis".
p1: Eggs in this form can be transported by the digestive tract of birds, or on
p1: the skin of animals, but also by the wind. Resting eggs thus allow the species
p1: to survive in a temporarily hostile environment, but also allows dispersal
p1: from one point to another. When the water returns and the conditions are favourable,
p1: the embryo resumes its development and reorganises itself without cell division
p1: to quickly form a larva.
p1: Hatching takes place in two stages: during the pre-hatch, the larva remains enclosed
p1: in a thin membrane. The second stage is a true hatching which allows the
p1: release of a swimming larva similar to that resulting from a development
p1: without diapausis, the resting egg stage, except for the colour. As we can control
p1: hatching, we can get larvae whenever we want. Now let's study it.
p1: The first swimming lava has a constant organisation with a single eye and
p1: three pairs of appendages; the first is with one branch (uniramed) and the
p1: next two have two branches (biramed). The larva has a smooth posterior region
p1: and a terminal part called a telson. Compared to an adult, the size and
p1: animal morphology change a lot. However, some structures persist, even if they have
p1: changed: a naupliar eye, a first antenna, a second antenna, a mandible, and also
p1: a telson. The body has many more segments that have been added between the mandibles
p1: and the telson. We will try to understand how larval growth occurs by comparing
p1: close stages. If we look in detail at a larva, we see that several morphogenesis
p1: stages follow one another spatially. The least advanced stages are posterior
p1: and the most advanced are anterior. Indeed, in the anterior region, we see
p1: well-individualised segments with visible appendages, and then later on we see only
p1: rough segments. On a larva at a given stage there is therefore a morphological gradient.
p1: Indeed the genes that indicate early cellular proliferation such as the
p1: caudal gene, and the determinants of segmentation, such as the engrailed gene,
p1: are expressed before segmentation is implemented and make it possible to
p1: identify two other regions. This type of development leads to an addition of
p1: segments and appendages with each moult that gradually transforms the organisation
p1: of the larva and later brings a regular increase in size,
p1: even after the acquisition of a first reproduction.