Amphipod With Red Spot

[Whys]

My tank has an abundance of gammarid amphipods (Gammarus roeselii). They are mostly white, partly translucent, with a dark gray core. Some are as large as a 1/4", and as I have no predators at the moment, they feel free to swim about the tank.

I saw one today with a bright red spot below the head but above the mid-section. Bright blood red and not like anything I've seen on any of the others.

Any thoughts on what this might be? Where do they store their eggs?

[Crustman]

All amphipods are members of the Superorder Peracarida that also includes mysids, cumaceans, leptostracans and tanaidaceans and a few obscure orders. These are all crustaceans with brood pouches. Amphipods have separate sexes but the eggs are carried on inner projections of the anterior legs called oostegites so this is likely what you are seeing and yes the eggs can be bright orange or red in color. The amphipod will brood these and hatch out little versions of their kind that are called manca. The manca look just like the adults but lack the 7th limb. They will moult frequently and will soon attain the adult characteristics.

[Whys]

Very cool. Thank you for the details.