Better late than never, today I am going to introduce the organism I am working with: Parhyale hawaiensis!
These “little shrimps” as my mom called them, are small (less than 2 cm) amphipod crustaceans. Well, we all know what a crustacean is but what does mean amphipod? The name Amphipoda comes, from the Greek roots ἀμφί (“different”) and πούς (“foot”) in reference to the two different orientations of the thoracic appendages. They are also commonly referred to as scuds, side swimmers, or beach hoppers and is one of the most ecologically successful group among crustaceans matching with a high level of diversity.
Parhyale are detritivores (they consume decomposing plant and animal parts) with a circumtropical, intertidal, and shallow-water marine distribution (not bad huh?). Adults are sexually dimorphic: females are smaller with prominent ovaries (located in the dorsal part of the thorax) while males are larger and have a pair of larger chelipeds on the T3( third thoracic segment). One of the things that caught my attention is that animals ready to mate can found in pairs (called amplexus); the male grasp and hold the female until deposits the sperm into the female oviducts and then the male releases the female which will moult. I was not expecting anything romantic, but this doesn’t really seems like a consensus.
The body plan of Parhyale contains 20 segments, 19 of which have appendages. There are six head segments: an ocular segment (which does not contain an appendage), two antennal segments (An1, An2), and three segments with feeding appendages: mandible (Mn), maxillule (Mx1), and maxilla (Mx2). The thorax consists of eight segments, beginning with a segment bearing a feeding-like appendage called a maxilliped (T1/Mxp). T2 and T3 segments have chelipeds, or claws; in sexually mature males, the T3 cheliped is enlarged and used to grasp onto females before copulation. T4–T8 segments have pereopods, or locomotary legs; T4–T5 legs face forward and are used for walking, while T6–T8 legs are oriented in opposition to T4–T5, and are used when the animal backs up or “jumps” rapidly. Finally, the abdomen consists of six segments with biramous appendages, with A1–A3 segments bearing pleopods, or swimming appendages, and A4–A6 bearing uropods, or anchoring appendages.
Before the female’s new cuticle hardens she sheds her eggs into a ventral brood pouch through two bilaterally symmetric oviducts, fertilising them in the process. The females brood their young in this ventral pouch and can produce embryos every 2 weeks, all year round (yes! You can do experiments the whole year!). The embryos are large and easy to collect; the yolk is opaque, but the superficially positioned embryo and surrounding chorion are transparent, which make them suitable to microinject and generate transgenic lines. Parhyale hatchling emerge as miniature versions of the adults (in contrast to other crustaceans) and about 2 months after, animals become sexually mature.
But don’t be fooled by their size. I don’t know if this also happens in their natural habitat, but they can be very aggressive and not nice at all. In dense population you can observe Parhyale fighting and… also devouring each other! They don’t matter if the other is the lifelong neighbour or school friends. Oh! And they love start from the head.
Why I am working with Parhyale? It is clear that their life history makes them amenable to culture and genetic manipulations allowing broader comparative studies of arthropod development. However there is another very interesting feature that I didn’t mention before; these colleagues are also able to regenerate their appendages!. During my PhD I will do my best to unravel which cell types are involved, which one are the progenitors and what they are doing during this process. So if you want to learn more about regeneration and Parhyale, stay tuned!