Bearded horse mussel and Noah's ark shell are sometimes observed to live together in groups. Oxygen consumption, ammonia excretion and clearance rate were investigated on these species under laboratory conditions considering different size of individuals divided into three size groups (less than 25 mm; 25 to 50 mm and larger than 50 mm). Body size of both species significantly influenced oxygen consumption, ammonia excretion and clearance rate of the Noah's ark shell. In both species, respiration and clearance rate decreased with body size while excretion rate increased with body size. For Noah's ark shell groups of 25 to 50 mm and smaller than 25 mm, the atomic ratio between oxygen consumption and ammonium excretion i.e. O:N ratio was detected to be greater than 30. The same applies to a group of the bearded horse mussels smaller than 25 mm. Animals of both species larger than 50 mm have O:N ratio smaller than 30 indicating use of proteins as metabolic substrate, compared to use of lipids and carbohydrates in younger animals.

Vlašić M., Glavić N., Bolotin J., Hrustić E., Dupčić Radić I.: Influence of salinity on physiological response of the bearded horse mussel Modiolus barbatus and Noah , s ark shell Arca noae. Ekológia (Bratislava), Vol. 37, No. 4, p. 345-357, 2018. Bearded horse mussel Modiolus barbatus and Noah's ark shell Arca noae are a species of interest for the diversifying shellfish aquaculture on the south-eastern coast of the Adriatic. In this study, oxygen consumption (OC), total ammonia excretion (TAM) and clearance rate (CR) responses to the changes in seawater salinity (37, 30, 25 and 20) were investigated in the laboratory. There is a statistically significant influence of salinity on oxygen consumption and TAM excretion of Noah's ark shell, while the time of exposure to different salinities is significantly correlated to TAM excretion by the bearded horse mussel. Mean OC of Noah's ark shell ranged from 0.14 ± 0.06 to 0.54 ± 0.27 mg O 2 g -1 h -1 and that of bearded horse mussel from 0.18 ± 0.17 to 0.26 ± 0.14 mg O 2 g -1 h -1 . Mean values of TAM excretion of Noah's ark shell ranged from 2.14 ± 1.52 to 7.22 ± 6.04 μmol g -1 h -1 and for bearded horse mussel from 0.98 ± 0.53 to 2.78 ± 2.96 μmol g -1 h -1 . Salinity and exposure time have a significant influence on the CR of Noah's ark shell, whilst salinity has been found to be the determining factor for the bearded horse mussels' CR. Mean values of Noah's ark shell CR ranged from 0.96 ± 0.54 to 4.18 ± 1.15 l h -1 g -1 and for bearded horse mussel from 2.43 ± 0.99 to 4.23 ± 0.84 l h -1 g -1 . Higher oxygen consumption to total ammonia excretion (O:N) ratios at lower salinities indicated the use of proteins as a metabolic substrate for both species. Noah's ark shell has greater energy expenditure related to respiration and TAM excretion than the bearded horse mussel.

Bearded horse mussel Modiolusbarbatus(Linnaeus, 1758)and the Noah's Ark shell ArcanoaeLinnaeus, 1758have been an important fishery product in Mediterranean coastal areas since ancient Greek times (Voultsiadou et al., 2010). Recently these species were considered as aquaculture candidates on the eastern coast of the Adriatic Sea, so an aquaculture assessment was performed for both of them (Peharda et al., 2013; Županet al., 2014). Among other information, data on the impact of food quantity, especially the impact of starvation, on the physiology of shellfish is important for the selection of the farming site. The influence of food quantity on oxygen consumption and the ammonia excretion was investigated on clam Ruditapesdecussatus(Khalil, 1994), banded carpet shell Polititapesrhomboides and dog cockle Glycymerisglycymeris (Savina and Pouvreau, 2004), clams Ruditapesdecussatusand the pullet carpet shellVenerupiscorrugata (Albentosaet al., 2007). There are numerous studies about the influence of algae

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