Molluscan veliger larvae possess a neuronal system that modulates their locomotive
behaviors. These neurons emanate from the central nervous system and innervate the velum, a
ciliated larval swimming and feeding organ. Velar neurons release two important
neurotransmitters, serotonin (5-HT) and FMRFamide (FMRF), which modulate activity of
ciliated cells. 5-HT abolishes ciliary arrest and increases ciliary beating while FMRF produces
opposite effects. Though both these neuromodulators (NMs) help modulate ciliary activity, it is
unclear how these NMs are organized anatomically in the organs which they regulate. In the
present investigation, we questioned how the NMs distribution in the velum influence larval
swimming characteristics, and specifically, we wanted to know if these two antagonistic NMs
are co-localized in the pre-synaptic terminals or part of an entire separate neural circuit. To
investigate this, pre-competent and competent veliger larvae were fixed, and
immunohistochemistry procedures were performed to identify each neuromodulator. For both
groups of larvae, some pre-oral (locomotive) cells were found to have co-localized NMs, but
most cells did not display colocalization. Additionally, 5-HT and FMRF were colocalized in
some fibers and terminals in the proximal part of the velum, consistent with their roles in
modulating velar musculature. Furthermore, kinematic data of larval swimming were analyzed
via MATLAB. On average, 5-HT treated young larvae swam faster while 5HT-treated old larvae
exhibited higher level of swimming activity compared to age-matched filtered sea water (FSW)
control and FMRF-treated Larvae. FMRF treatment had little to no effect compared to FSW for
both young and old larvae.