p1: The annelid species we are working with, Platynereis dumerilii, is a marine worm
p1: found worldwide in temperate seas. For the past several decades, researchers have
p1: no longer taken animals directly from the sea, but are able to raise them in the lab.
p1: Platynereis' full life cycle is completed easily and successfully in laboratory
p1: settings like here at the Institut Jacques Monod in Paris. We do not need to be
p1: close to the sea to study a marine animal!
p1: At the Institut, we breed the worm in a thermostatic room at 18 degrees.
p1: We raise them all year long in tupperware -like boxes with less than a litre of fresh
p1: seawater in it, which we change every two weeks, thanks to the help of the
p1: animal facility members.
p1: In the wild, Platynereis immature worms live within a silk tube they secrete on algae.
p1: In the lab in their boxes they also secrete a tube and live inside it most of the time.
p1: They are quite inactive and crawl out of the tubes only to grab some food.
p1: They are fed three times per week with ground fish food and organic mashed spinach.
p1: Platynereis worms are very sensitive to detergents, and for this reason are also
p1: used for ecotoxicology studies.
p1: Like many other marine animals such as corals, sea urchins, and even fishes,
p1: Platynereis' life cycle is synchronized with the lunar cycle, which we call a
p1: circalunar life cycle. Each worm will reproduce only once in its life, and the
p1: timing of this reproduction is tightly regulated by this lunar life cycle.
p1: In the culture room, we keep the worms, most of the time in a daily artificial
p1: illumination regime. We try to reproduce natural "summer" conditions and so we have
p1: a cycle of 16 hours of light and eight hours of darkness. On top of that,
p1: to induce sexual maturation, we need to mimic the lunar stimulus to initiate
p1: the reproduction phase. We thus artificially reproduce moonlight using
p1: a low-light lamp that we switch on at night for seven days on a 28-day cycle.
p1: A couple of days after this week of full moon, worms start to mature.
p1: We are extremely careful to respect this illumination and moonlight regime, as
p1: errors have been shown to have long- term negative effects on the culture,
p1: so the worm room schedule has to be followed meticulously.
p1: Maturation of worms occurs over two weeks. During this period, the members of the team
p1: are in charge of collecting animals that are in the course of maturation by checking
p1: all the worm boxes three times a week. We can easily recognise which adults
p1: are mature as they dramatically change their morphology, notably their colors.
p1: Males turn white (full of sperm in their anterior part) and red in their posterior
p1: part, while females become yellow (full of eggs).
p1: We can also find worms that are more or less orange, and at that time we are not
p1: sure if they will become male or female. Morphologically, it's quite unclear, but
p1: it's of course genetically determined.
p1: When we find mature or maturing worms, we collect them from their boxes using
p1: a pipette and place them in specific, separate boxes that either contain
p1: females or males. This way, we can choose the exact moment we will allow them to
p1: reproduce by putting a male and a female together.
p1: When males and females are ready to reproduce, they start to become quite
p1: active and they leave their tubes. When we want to induce a fertilization, we put
p1: a male and a female in a beaker together where they will start to swim elegantly
p1: around each other. They will soon release gametes almost simultaneously in the sea
p1: water, so the fertilization is external. Several thousand eggs are released by
p1: a big female, by spontaneous rupture of its body wall.
p1: Consequently, the female will have emptied itself in a couple of minutes and
p1: will soon die. The male can release its sperm thanks to a modified terminal part of
p1: its body and is able to release sperm a couple of times, but it too will die in a
p1: few hours. When the fertilization is done, we then put the beaker in a thermostatic
p1: room at 18°C to develop.
p1: Around 15 minutes after fertilization, a substantial egg jelly will be produced and
p1: will cover the whole developing egg. This jelly blocks the fusion of supernumerary
p1: sperm with the egg and protects the egg. It is also a clear indicator of successful
p1: fertilization. Indeed, when the fertilization has worked properly, the jelly is produced
p1: and the eggs are no longer in close contact. The jelly creates a regular spacing between
p1: the eggs and gives the whole a geometric shape.
p1: In contrast, if fertilization fails, the eggs will be stuck together at the bottom
p1: of the beaker.
p1: In the wild, it will be progressively removed by natural elements
p1: such as the movement of the sea, but in the lab, in order to allow a proper
p1: embryonic development, we have to remove it manually at around 24 hours post-
p1: fertilization. To do that, we use a net that retains the embryos and we rinse
p1: them with a lot of sea water to remove the jelly. Free of their jelly,
p1: they can now swim and can continue their development.
p1: Platynereis larvae will not need to eat for a couple of days as they will live on their
p1: reserves, but after around five days, the small larvae will become benthic and will
p1: thus start to feed on the bottom. At this stage, they are too small to eat fish food.
p1: So, in the lab, we specifically developed a culture of algae named Tetraselmis
p1: marinus to feed the young worms from five days to two months.
p1: In between, at around 10 days, we remove the larvae from the beaker with a pencil and
p1: place them in boxes filled with seawater and algae. At this stage, they are ready
p1: to join the rest of the collection in the worm room.
p1: Young worms, from 10 days to two months old, will be placed in a specific
p1: area of the worm room -a kind of kindergarten- and at that age, their
p1: seawater is not changed. They are too small for that and we may lose them
p1: during this procedure. After two months, they are big and strong enough to allow
p1: the seawater replacement and to feed them with fish food. They have grown a lot
p1: and they're thus very cramped in their small boxes.
p1: As we want them to continue to grow to become big mature worms, also named
p1: atoke worms, we will need to separate them. This "transplanting" procedure allows us
p1: in a matter of a couple of weeks to obtain much bigger of juveniles, also named
p1: epitoke worms. Epitoke worms will continuously grow throughout their lives
p1: until they reach maturity and the life cycle is fully completed.