Better late than never, today I am going to introduce the organism I am working with: Parhyale hawaiensis!
I have recorded some fellas to show you. We can observe some different life stages: an adult male, a juvenile, a couple and a pregnant female (the darker things in the ventral side are the embryos in the brood pouch).
These “little shrimps”, as my mom calls them, are small (less than 2 cm) amphipod crustaceans. We all know what a crustacean is but what does amphipod mean? 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 plants 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 third thoracic segment (T3). Animals ready to mate can be found in amplexus (pre-mating pairs): the male first grasps and holds the female until he deposits the sperm into her oviducts and then, the female is released (not romantic at all).
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 “jumps”. Finally, the abdomen consists of six segments with double-branched 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 broods their young progeny in this ventral pouch being able to 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. Embryonic development takes around 10 days and the hatchling emerge as miniature versions of the adults (in contrast to other crustaceans) and about 2 months later they become sexually mature.
But do not be fooled by their size, since they can be very aggressive and not nice at all. I am not sure if this also happens in their natural habitat, but in dense population you can easily observe Parhyale fighting and even devouring each other, not really caring even if they are lifelong neighbours or school friends. Oh, and they love starting from the head!
Why am I 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 have not mention yet: these little animals are also able to regenerate their appendages! During my PhD I will do my best to unravel the diversity of cell types in Parhyale limbs, which cells are the progenitors, and what are they doing during this process. So, stay tuned if you want to learn more about regeneration and Parhyale!
If you want to read more about these guys, there are very nice reviews from people with true expertise with this new model organism. One of the latest is:
Sun DA, Patel NH. The amphipod crustacean Parhyale hawaiensis: An emerging comparative model of arthropod development, evolution, and regeneration. WIREs Dev Biol. 2019;e355. https://doi.org/10.1002/wdev.355