Here are some important parts from a review piece on the effects of anti-depressants on crustaceans and other anthropods:

Consequently, any chemicals in the environment that have the ability to modulate these hormones conceivably have the ability to disrupt the normal endocrine and biological function in exposed organisms in a vast number of ways. Pasztor and MacMillan (1990) reported in a study of the crayfish, Cherax destructor and the lobster, Homarus americanus that some instances neurohormones that have excitatory responses whereas in other species may be ineffective or depressive.

So it sometimes has excitatory effects and sometimes it dampens the effects, depending on the species, again because it varies greatly between species.

Then they say,

Therefore, those wishing to extrapolate the roles and effects of exogenous neurological modulators in one species may find it more difficult to discern those occurring in other species.

Which is deliciously ironic because Peterson wants to extrapolate from lobsters and co to humans.

Let's continue,

The roles of serotonin, dopamine and octopamine have attracted a lot of attention within the aquaculture sciences for their potential to speed up growth, maturation and spawning (Diwan, 2005; Wongprasert et al., 2006).

So it does in fact increase reproduction in some species (opposite to humans as I understand it). More of that:

In addition, the authors reported that the hatching rate and the amount of nauplii produced per spawner were also significantly higher in 5-HT injected shrimp. Similar results have been observed for the crayfish Procambarus clarkia, the white Pacific shrimp Litopanaeus vannamei, the freshwater giant prawn, Macrobrachium rosenbergii, the fiddler crab Uca pugilator (Vaca and Alfaro, 2000; Cheng et al., 2005; Sarojini et al., 1995a,b,c).

Now, on aggression (again, as I understand it, opposite humans):

In decapod crustaceans, serotonin has been suggested to serve important roles in mediating aggressive behaviours (Huber et al., 1997; Huber and Delago, 1998; Sneddon et al., 2000). Doernberg et al. (2001) investigated the role(s) of serotonin in the fighting behaviour of lobsters, Homarus americanus. In some individuals, 5,7-dihydroxytryptamine (a neurotoxin) was injected in order to deplete the animals of serotonin in their nervous tissue. They found that the treated animals showed an increased tendency to engage in agonistic encounters.

Another case of just how different their response is to these hormones compared to us:

Serotonin has been shown to influence red-pigment dispersing hormone whilst dopamine influences red- and black-pigment concentrating hormone. Fingerman (1997a,b and papers within) report norepinephrine triggers release of black-pigment dispersing hormone but not RGDH.

This is the one used by PZ Myers, which is not my ideal source generally speaking, but he is an evolutionary developmental biologist so this is close to his area of expertise. I thought it might help to have a few annotated bits.