p1: After studying the larval development of brine shrimp, we will now focus on a few
p1: other species. I propose four examples: the Malocastracan penaeid, the Copepod tigoripus
p1: the Malocastracan shore crab and the Cirriped barnacle. You will have to compare
p1: each of these examples with the development of brine shrimp and deduce
p1: the similarities and differences.
p1: In each of the examples, you will find information about the environment and
p1: sometimes the modality of life, as well as the systematic position and of course,
p1: the larval development. All conclusions on development variations will be presented
p1: at the end. Green crabs are a characteristic element of the tidal zone.
p1: The crab is very resistant to desiccation, salinity, and temperature variation,
p1: and therefore it can be the most common crab locally. The larvae appear with a very
p1: different morphology from adults, including a dorsal spine and a well-developed
p1: rostrum to make it more difficult to swallow. However, the number of segments is almost
p1: the same as that of the adult, which indicates that the posterior elongation
p1: mainly precedes the hatching. After four zoe and one megalopic stages, the animal
p1: becomes a juvenile. As in many malacostra -cans, a stage equivalent to the naupliar
p1: stage has been observed during embryonic development.
p1: Onto the penaeid shrimp: they are shrimp or prawns that are frequently consumed by the
p1: human population. Several thousand tons are produced per year on aquatic farms,
p1: particularly in the tropics.
p1: This breeding is made possible by the massive availability of brine shrimp.
p1: The particularity of this group of shrimp is to have a very abundant progeny composed
p1: of small larvae. This makes it possible to optimize livestock production.
p1: At hatching the larva is a typical nauplius which will be followed by five other stages
p1: also called nauplius, where the segments are gradually added.
p1: The animal continues to grow in three protozoan stages and then mysis stages.
p1: Next we will look at copepods. These copepods live in the rock pools of high
p1: tide areas all over the world. It can be locally abundant when conditions are suitable.
p1: In this video you can see Tigoripus brevicornis which is very common in Western Europe.
p1: Sometimes we see the male clinging to the female and swimming in tandem,
p1: sometimes even before the female is mature, to make sure he will fertilise her.
p1: The embryos resulting from fertilization are carried by the female in a posterior bag.
p1: When embryonic development is complete, the swimming larva are released into the
p1: environment. Adults can live with high densities, but in this case adults eat larvae
p1: allowing the population to be regulated in a context where predation is often limited.
p1: Larval development is composed of six naupliar and five copepodite stages
p1: and is relatively rapid in this species. After storms or big waves during high tide,
p1: populations can reform since the full life cycle can be completed in only one week.
p1: Last but not least: barnacles. Barnacles live fixed on hard substrates such as rocks or shells.
p1: The body is surrounded by tests made of plates that form a wall and an operculum.
p1: The latter allows the thoracic appendages, called cirri, to pass through, which have
p1: been used to catch prey mostly composed of zooplankton. As you can see in this movie
p1: this hermaphroditic animal has a long tube that bring sperms to receptive neighbouring
p1: animals. Darwin had already said these animals had giant penises.
p1: Incubation is done under the protection of the tests until hatching.
p1: These organisms have a fixed, immobile life while their ancestor probably had a mobile
p1: life at all stages of development. The only phase of mobility of this species
p1: are the larval stages which are therefore essential for the dispersal of the species.
p1: It is composed of six nauplius stages sharing very specific frontal horns.
p1: These nauplian stages are externally very similar to the early stages of copepods
p1: or penaeid shrimp. However, the construction of a thorax is internal and synchronous
p1: without clearly establishing a morphological gradient and also without the abdomen.
p1: The last naupliar stage suddenly gives after a moult the typical cypris stage which
p1: is a synapomorphy of the cirripeds. It is at this stage that the animal chooses its
p1: definitive fixation site according to the biofilm and the presence of adults of
p1: the same species. This choice is strategic for the individual's survival and reproductive
p1: capacity. As you have seen, there are many differences between these lineages.
p1: It's already clear that adults diverge considerably between them, both in terms
p1: of lifestyle and morphology. But there are also significant differences in larval
p1: development. Here are the most significant elements related to our examples:
p1: Penaeid shrimp have a fairly similar developmental sequence with brine shrimp
p1: although the anatomical differences are relatively marked between the two lineages.
p1: The crab has an original larval sequence since there is no nauplius stage. It seems
p1: that in this lineage the larva nauplius is actually retained in the egg.
p1: As a result, this stage is not free and part of the larval development has been
p1: transformed into embryonic development.
p1: We cannot therefore speak of anamorphic development, but despite everything, the
p1: conditions of body lengthening are quite similar to what is known in many
p1: pancrustaceans, including short germ band insects or brine shrimp.
p1: For Tigoripus the developmental sequence is comparable to brine shrimp because the
p1: the larval development is anamorphic, but with more benthic forms. However, the
p1: number of segments is reduced and the animal's body looks a lot like a nauplius.
p1: Finally the adult's eye is unique and corresponds to the naupliar eye.
p1: Adult animals retain marked larval characteristics.
p1: This phenomenon of developmental heterochrony is a form of pedomorphosis
p1: For barnacles, the larvae have original frontal horns that are synapomorphic
p1: because they are not found in the other groups.
p1: There is also a development that is not clearly anamorphic, as in the other species
p1: studied. Finally, the metamorphosis is particularly severe and brutal, unlike the
p1: other lineages. The cirripeds present important developmental changes probably
p1: related to their fixed life. Despite all this, the larva is an easily indentifiable nauplius.