Sea urchins, as echinoderms, occupy an interesting position in animal phylogeny in that they are genetically closer to vertebrates than the vast majority of all other invertebrates but have a nervous system that lacks a brain or brain-like structure. Despite this, very little is known about neurobiology of the adult sea urchin, and how the nervous system, is utilized to produced behavior. Here we investigate effects on the righting response of antagonists of ionotropic receptors for the neurotransmitters acetylcholine, GABA, and glycine, and antagonists of metabotropic receptors for the amines dopamine and norepinephrine. Antagonists slowed the righting response in a dose-dependent manner, with a rank order of potency of strychnine>haloperidol>propranolol>bicuculline>hexamethonium, with RT50s (concentrations that slowed righting time by 50%) ranging from 4.3 µM for strychnine to 7.8 mM for hexamethonium. It is also shown that both glycine and adrenergic receptors are needed for actual tube foot movement, and this may explain the slowed righting seen when these receptors are inhibited. Conversely, inhibition of dopamine receptors slowed the righting response but had no effect on tube foot motility, indicating that these receptors play roles more in the neural processing involved in the righting behavior, rather than the actual physical righting. Our results identity the first effects of inhibiting the glycinergic, dopaminergic, and adrenergic neurotransmitter systems in adult sea urchins and distinguish between the ability of sea urchins to right themselves, and the ability of sea urchins to move their tube feet.

Despite lacking a brain and having an apparent symmetrically pentaradial nervous system, echinoderms are capable of complex, coordinated directional behavioral responses to different sensory stimuli. However very little is known about the molecular and cellular mechanisms underlying these behaviors. In many animals, dopaminergic systems play key roles in motivating and coordinating behavior and although the dopamine receptor antagonist haloperidol has been shown to inhibit the righting response of the sea urchin Strongylocentrotus purpuratus, it is not known if this is specific to this behavior, in this species, or whether dopaminergic systems are needed in general for echinoderm behaviors. We found that the dopamine receptor antagonist haloperidol inhibited multiple different behavioral responses in three different echinoderm species. Haloperidol inhibited the righting response of the sea urchin Lytechinus variegatus and of the sea star Luidia clathrata. It additionally inhibited the lantern reflex of S. purpuratus, the shell covering response of L. variegatus, and the immersion response of L. variegatus, but not S. purpuratus or L. clathrata. Our results suggest that dopamine is needed for the neural processing and coordination of multiple different behavioral responses in a variety of different echinoderm species.