tolerance, but greater tolerance to low salinity than developing embryos. 

 Growth of larvae over the 10 days was affected most by TXS , T, t2, and S. 

 Maximum growth (80%) occurred between 22.5° and 36.5°C and salinities 

 between 21.5 and 30°/°°. Combined 10 day survival and growth was affected 

 by all variables in the equation, but more by salinity than temperature. 

 Optimum temperature and salinity to maximize larval growth and survival to 

 12 days was 21.5°-33°C and 22-31°/°°. Embryos and larvae should be reared 

 under two different sets of conditions. Juvenile clams may have yet 

 another set of optimum conditions than late larval clams. - D.L. 



1158 



Loveland, R. E. 1962. 



Further evidence for a transmitter action of 5-hydroxytryptamine in the 

 heart of Mercenaria (Venus) mercenaria. 



Cited as ms in preparation. This is interpreted to be a paper published by 

 Robert E. Loveland (1963), under a somewhat different title, abstracted 

 elsewhere in this bibliography. - J.L.M. 



1159 



Loveland, Robert E. 1963. 



5-Hydroxytryptamine, the probable mediator of excitation in the heart of 

 Mercenaria (Venus) mercenaria. Comp. Biochem. Physiol. 9(2) : 95-104. 



The cardio-regulator nerves of hard clam are thought to be composed of 

 inhibitory and excitatory fibers. -Excitatory fibers are probably less 

 important because inhibition is more readily evoked by stimulating cardio- 

 regulatory nerves in an untreated or normal heart. No anatomical evidence 

 is available to support the hypothesis that the 2 kinds of fiber actually 

 exist. 5-Hydroxytryptamine is the most probable mediator of excitation in 

 heart of M. mercenaria. It has been found in nerve and heart tissue in 

 relatively high concentrations. Its synthesis from its immediate precursor, 

 5-hydroxytryptophan , occurs in presence of nerve extracts of Mercenaria. 

 The pattern of excitatory response to 5-HT is similar to that caused by 

 stimulating cardio-regulator nerves. Also, 5-HT is among the most reactive 

 tryptamine derivatives on heart. Iproniazid, an inhibitor of monoamine 

 oxidase, multiplied the action of 5-HT and nervous excitation. Rate of 

 destruction and multiplication of 5-HT was low. Substances which blocked 

 applied 5-HT, such as methysergide and 2-bromo-D-lysergic acid diethylamide, 

 also decreased or blocked nervous excitation. Decreased concentration of 

 endogenous 5-HT in nerve tissue of Mercenaria produced a decrease or loss of 

 evoked excitatory response, and when, from tachyphylaxis, heart lost its 

 sensitivity to 5-HT, there was concomitant loss of ability of heart to 

 respond to action of excitor fibers. 5-HT has never been collected and 

 chemically identified in perfusate from stimulated heart-nerve preparation 

 of Mercenaria. This is an important criterion for determining that a 

 substance is a neurotransmitter. From the results of these studies, however, 

 it can be concluded that 5-HT, or a similar compound, is the probable 

 augmentor substance in hard clam heart. - J.L.M. 



1160 



Loveland, Robert E. 1963. 



Some aspects of the cardioregulation system in Mercenaria (Venus) mercenaria. 

 Proc. 16th Internatl. Congr. Zool . 2: 106 (abstract). 



The hard clam heart is innervated symmetrically by 2 lateral nerves, entering 

 the auricle on either side. These nerves presumably have their origin in the 

 visceral ganglion. After passing through the inside of the auricle, branches 

 of the nerve end abruptly on the surface of the auriculo-ventricular valve. 

 Nerve fibers are most concentrated in this region, and few fibers can be seen 

 in the ventricle. Neurons in cardioregulator nerves are of 2 distinct types. 

 One contains agranular vesicles and resembles a typical unmyelinated motor 

 neuron. The other is filled with granular vesicles and appears to be a 

 neurosecretory cell. Glial cells are associated with both types. Heart 



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