| Metamorphose is an important developmental stage during Meretrix meretix ontogeny.Manypharmacological experiments have been conducted to determine the effects of adrenergic agonistand antagonist of catecholamine receptors on Meretrix meretrix metamorphosis. Results showedthat adrenaline (AD) and noradrenaline (NA) had substantial effects (p<0.05) on larvalmetamorphosis at concentrations ranging from 10μM to 100μM. 10μM β-adrenergic receptor (AR)agonist isoproterenol showed the same inducement effect as that of NA and AD on metamorphosis,whereas the α-AR agonist phenylephrine had no significant effect at concentrations between0.1μM and 100μM concentrations (p>0.05). Furthermore, 1 μM β-AR antagonist propanolol, butnot α-AR antagonist prazosin, depressed the larval metamorphosis induced by NA or AD. Theresults of pharmacology suggest that β-adrenergic-like receptor might play a considerable role inthe larval metamorphosis of M. meretrix by AD or NA.Moreover, we investigated the development of beta adrenergic receptor and nervous system bywhole mount immunocytochemistry technique. The results showed larval nervous system firstappeared during late trochophore larva stage (18 hours post fertilization), and beta adrenergicreceptor was not detected. At 1 day post fertilization, there appeared apical ganglion, cerebralganglion, visceral ganglion and some neurons near the mouth. And at the same time, betaadrenergic receptor appeared in the apical ganglion and cerebral ganglion, named AR (apicalreceptor) and CR1 (cerebral receptor 1) respectively.At 5dpf, it was difficult to detect the immunoreactive signal of apical ganglion. The cerebralganglion and visceral ganglion leaved from the velum to the dorsal. There appeared twoimmunoreactive cell bodies in ventral and dorsal to the mouth. And there were someimmunoreactive cells around the visceral ganglion. AR was not detected and CR2 appeared in theposterior of CR1 at 3dpf. And CR2 developed quickly. At 5dpf, the size of CR2 was similar to theCR1. Moreover, many little immunoreative dots were around CR1 and CR2.After metamorphosis (7dpf), the apical ganglion disappeared and there appeared pedalganglion in the foot. Moreover, there appeared some peripheral neurons in the mantle. Exceptjuvenile cerebral ganglion and visceral ganglion, beta adrenergic receptor was detected in thejuvenile pedal, mantle and siphon. The result of western blot showed that beta adrenergic receptorwas detected in the adult gill, pedal, heart, siphon, lip and mantle.After the metamorphosis signal reached the target organs, the larval morphology andbehaviour changed. Whole mount technique using fluorescent-labelled phalloidin for actinstaining was applied to investigate the ontogeny of the various muscular systems during larvaldevelopment in the clam Meretrix meretrix. Larval shell retractor muscles (LR) and anterioradductor (A-AD) were the first detectable muscle structure in the later trochophore larva (18 hourspost fertilization). Soon after, trochophore larva developed into veliger larva. And the larvalmuscular system developed fully at 22 hpf. There were 5 pairs of LR, named LR1-LR5 fromanterior to posterior respectively. The LR1-4 had a dendritic progress the venter, and the dendritesprojected into the velum region, thus formed velum muscle ring (VR). The anterior adductorbecame two muscular fibers, and the ventral part was bigger. The overall morphology of muscularsystems during the whole veliger stage (1-5 dpf) was same, except became stronger day by day.During metamorphosis (6-7 dpf), the VR collapses, and then disappears completely. The LRatrophied and contracted to the ventral gradually and also disappeared. And there appeared threemuscular structures, pedal retractor (PR), posterior adductor (P-AD), and mantle muscle (MM).The PR was composed of two muscular fibers, whose fine dendrites formed reseau muscularstructure in the foot. The P-AD was composed two fibers and similar to the A-AD. The MMextended from the A-AD to the P-AD, which comprised 2 long anterior-posterior muscular fibersand many short left-right muscular fibers.After metamorphosis, the postlarval muscle system was simpler because of thedisappearance of the larval muscle system. There were only adductors, pedal rectractors andmantle muscle, which compose the adult muscle system.
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