The objective of this study was to examine whether 12 h of light exposure would lead to an increase in the pH of and a decrease in the concentration of total ammonia in the extrapallial fluid of the giant clam Tridacna squamosa. We also aimed to elucidate indirectly whether movements of ammonia and/or protons (H(+)) occurred between the extrapallial fluid and the outer mantle epithelium. The pH of the extrapallial fluid of T. squamosa exposed to 12 h of light was significantly higher than that of clams exposed to 12 h of darkness. Conversely, the total ammonia concentration in the extrapallial fluid of the former was significantly lower than that of the latter. In addition, the glutamine content in the mantle adjacent to the extrapallial fluid of clams exposed to 12 h of light was significantly greater than that of clams exposed to 12 h of darkness. These results suggest that in the extrapallial fluid of T. squamosa exposed to light, NH(3) combined with H(+) as NH(+)(4) and that NH(+)(4) was transported into the mantle and used as a substrate for glutamine formation. Injection of NH(4)Cl into the extrapallial fluid led to an instantaneous increase in the total ammonia concentration therein, but the total ammonia concentration decreased subsequently and returned to the control value within 1 h. This is in support of the proposition that NH(+)(4) could be transported from the extrapallial fluid to the mantle. Injection of HCl into the extrapallial fluid led to an instantaneous decrease in the pH of the extrapallial fluid. However, there was a significant increase in pH within 1 h in light or darkness, achieving a partial recovery toward the control pH value. The increase in pH within this 1-h period in light or darkness was accompanied by a significant decrease in the total ammonia concentration in the extrapallial fluid, which supports the proposition that H(+) could be transported in combination with NH(3) as NH(+)(4). Therefore, our results prompt a reexamination of the previous proposition that the removal of H(+) by NH(3) can facilitate calcification in molluscs in general and an investigation of the relationship between H(+) removal through NH(+)(4) transport and light-enhanced calcification in T. squamosa.

A significantly higher concentration of testicular spermatozoa was obtained from freshwater Oreochromis mossambicus (9·9 10 9 spermatozoa ml 1 ) than seawater O. mossambicus (4·6 10 9 spermatozoa ml 1 ). The mean osmolality of the urine of freshwater fish (78·5 mOsmol kg 1 ) was significantly different from that of seawater fish (304·8 mOsmol kg 1 ). The mean length of the mid-piece of the spermatozoa together with the tail was more variable in freshwater O. mossambicus (8·80 0·23 m) than in seawater specimens (8·27 0·18 m). Stripped sperm of freshwater O. mossambicus was highly contaminated by urine which was a good activator of sperm motility in O. mossambicus held in both fresh and sea water. The osmolality for initiation of motility in freshwater O. mossambicus spermatozoa was from 0 to 333 mOsmol kg 1 while for seawater O. mossambicus spermatozoa it was from 0 to 1022 mOsmol kg 1 . The optimum osmolality for motility was from 70 to 333 mOsmol kg 1 for freshwater O. mossambicus spermatozoa and from 333 to 645 mOsmol kg 1 for seawater fish. In freshwater O. mossambicus spermatozoa, the presence of 20 m CaCl 2 increased the permissive osmolality of NaCl from 184 to 645 mOsmol kg 1 . For seawater O. mossambicus spermatozoa, solutions of NaCl devoid of CaCl 2 were unable initiate motility, but the addition of 1·5 to 30 m CaCl 2 to the NaCl solution (0-934 mOsmol kg 1 ) had a full motility initiating effect. 1999 The Fisheries Society of the British Isles

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The enzymatic activities of alpha-amylase and its corresponding messenger RNA levels in developing sea bass (Lates calcarifer) larvae were studied from hatching until 27 days post hatching (dph). An increasing activity of amylase enzyme was measured until 5 dph, and the activity gradually decreased thereafter and reached a constant level by 12 dph. To achieve a better understanding of the molecular mechanisms underlying amylase expression, we have cloned and sequenced a 318-bp fragment of alpha-amylase complementary DNA. Based on this sequence, a real-time reverse transcriptase polymerase chain reaction technique to monitor the changes in the mRNA levels in the larvae was developed. A correlation between enzymatic activity and mRNA level of alpha-amylase could be demonstrated during the early development of sea bass larvae. This suggests that the changes in alpha-amylase are controlled at least at the transcriptional level during early larval development of sea bass.

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