Ionoregulatory changes during metamorphosis and salinity exposure of juvenile sea lamprey (Petromyzon marinusL.)

Abstract

SUMMARY Ammocoetes of the anadromous sea lamprey Petromyzon marinus L. spend many years in freshwater before metamorphosing and migrating to sea. Metamorphosis involves the radical transformation from a substrate-dwelling,filter feeder into a free-swimming, parasitic feeder. In the present work we examined osmoregulatory differences between ammocoetes and transformers(metamorphic juveniles), and the effects of salinity acclimation. We measured the expression of key ion-transporting proteins[Na+/K+-ATPase, vacuolar (V)-type H+-ATPase and carbonic anhydrase (CA)] as well as a number of relevant blood parameters(hematocrit, [Na+] and [Cl–]). In addition,immunofluorescence microscopy was used to identify and characterize the distributions of Na+/K+-ATPase, V-type H+-ATPase and CA immunoreactive cells in the gill. Ammocoetes did not survive in the experiments with salinities greater than 10‰,whereas survival in high salinity (⩾25–35‰) increased with increased degree of metamorphosis in transformers. Plasma [Na+] and[Cl–] of ammocoetes in freshwater was lower than transformers and increased markedly at 10‰. In transformers, plasma ions increased only at high salinity (>25‰). Branchial Na+/K+-ATPase levels were ∼ tenfold higher in transformers compared to ammocoetes and salinity did not affect expression in either group. However, branchial H+-ATPase expression showed a negative correlation with salinity in both groups. Na+/K+-ATPase immunoreactivity was strongest in transformers and associated with clusters of cells in the interlamellar spaces. H+-ATPase (B subunit) immunoreactivity was localized to epithelial cells not expressing high Na+/K+-ATPase immunoreactivity and having a similar tissue distribution as carbonic anhydrase. The results indicate that branchial Na+/K+-ATPase and salinity tolerance increase in metamorphosing lampreys, and that branchial H+-ATPase is downregulated by salinity.

Exposure of the gill epithelial cells of larval lampreys to an ion-deficient environment: a stereological study

“…They thus die when exposed to osmolarities >350 mosmol l −1 , a value that corresponds to approximately one-third of full-strength sea water [9]. This is consistent with the fact that ammocoetes of P. marinus can survive direct transfer to a salinity of 10‰ but not 20‰ [7].…”

“…This conclusion is supported by a recent immunohistochemical study on the ammocoete gill epithelium, which used an antiserum for labelling carbonic anhydrase IIb [7].…”

“…Since Na + and Cl − are taken up in exchange for H + or HCO 3 − , respectively [28,29], which are generated by the activity of carbonic anhydrase, the absence of this enzyme in AMRCs [7] implies that these cells do not contain a basic requirement for the uptake of Na + and Cl − and thus cannot be involved in this process.…”

“…In contrast to fully metamorphosed individuals of the anadromous species of lampreys, which can be readily acclimated to full-strength sea water [4][5][6][7], the larvae (ammocoetes) of the lamprey Petromyzon marinus, whose serum osmolarity is ∼225 mosmol l −1 , are unable to osmoregulate in hypertonic environments [7,8]. They thus die when exposed to osmolarities >350 mosmol l −1 , a value that corresponds to approximately one-third of full-strength sea water [9].…”

“…One of the MR cell types resembles, both ultrastructurally and through its possession of carbonic anhydrase and vacuolar (V)-type H + ATPase, the intercalated MR cell (IMRC) that is found in other ion-transporting epithelia, such as the toad and turtle urinary bladders, renal collecting duct and amphibian epidermis. In those ion-transporting epithelia, the IMRC actively secretes protons and plays a role in acidbase regulation and/or ion uptake [7,[11][12][13]. The other type of MR cell in the ammocoete gill, the mitochondria of which are characterized by a distinct electron-dense matrix, has no morphological counterpart in the ion-transporting epithelia of other vertebrates.…”