Is there any scientific evidence that marijuana has any effect on lobsters?
Yes.
According to Changes in neurotransmitter release at a neuromuscular junction of the lobster caused by cannabinoids Neuropharmacology Volume 27, July 1988, pages 737-742:
Delta-9-tetrahydrocannabinol and 11-hydroxy-THC, in relatively small concentrations, increased the amplitude of the excitatory junctional potential and the mean quantum content of a muscle fiber, whereas larger concentrations produced depression.
The later study Cannabinoid receptors in invertebrates J. Evol. Biol. 19 (2006) 366–373 says, in reference to the 1988 study:
In vitro assays offer more precise levels of evidence, such
as the Level-II study of THC’s effects at the neuromuscular
junction of the lobster Homarus americanus (Turkanis &
Karler, 1988). This study provided indirect proof of
cannabinoid receptors, because THC could have affected
the neuromuscular junction via other lobster protein
targets or via membrane disrupting effects.
...
THC suppressed neuromuscular
junction activity in the lobster H. americanus (Level-II
evidence, Turkanis & Karler, 1988). This positive study
was supported by our tritiated ligand binding study with the rock lobster J. edwardi.