165
which these glands facilitate the net secretion of sodium (or potassium) chloride, has
been studied in great detail during last 60 years (Peaker and Linzell 1975 ; Gerstberger
and Gray 1993 ; Shuttleworth and Hildebrandt 1999 ; Hildebrandt 2001 ; Dantzler
and Bradshaw 2009 ; Holmgren and Olsson 2011 ; Babonis and Brischoux ( 2012 ).
Interesting results has been reported in comparative studies between marine and
freshwater species (Babonis and Evans 2011 ). Studies include work looking at the role
of water-regulatory proteins in modulating the secretory output of the salt glands, and
researching the diversity in the composition of the secreted fl uids. In addition, such
topics as function of salt glands under various environmental conditions, bacterial
infections of salt glands, their phenotypic plasticity as well as the regulatory mechanisms of secretion by various neurological and endocrine agents, have been studied.
Additional work has also been done to investigate the combined “osmoregulatory
function of salt glands and other organs,” (Babonis and Brischoux 2012 ; see also
Babonis et al. 2011 ). Interestingly, although the anatomy, structure and function of
these glands has been quite well studied, there have been no reports on NaCl as
form of a biomineral that is formed because of activity of the corresponding cells.
Here, I would like to introduce the term “ biohalite ” for the NaCl-based biomineral
that originates from the salt glands.
3.6.1 Diversity and Origin of Salt Glands
in Marine Vertebrates
Fish Observations of marine shark’s species in rivers suggest their specifi c ability
for acclimatization in aquatic niches with different levels of salinity. Also experiments under laboratory conditions has demonstrated that marine elasmobranchs “do
have the capacity to acclimate to changes in salinity through independent regulation
of Na
+
, Cl
−
, and urea levels,” (Hazon et al. 2003 ). Thus, some stenohaline marine
elasmobranchs can be accepted as partially euryhaline. However, “once optimally
adapted to fresh water, recolonization of sea water by elasmobranchs is problematic
due to the loss of urea synthetic capacity and renal structures for urea retention,”
(Ballantyne and Robinson 2010 ).
Sharks like Squalus acanthius can effectively maintain osmotic homeostasis on the
level of urea concentrations ranging between 300- and 400-mM, and corresponding
marine osmolalities of 900–1,000 mosmol/kg H 2 O (Zeidel et al. 2005 ). Also the rectal
gland that is isotonic with the plasma in this fi sh species can maintain salt balance
without losing urea by secreting a NaCl-rich (500 mM) and urea-poor (18 mM) fl uid.
An investigation into structural features of apical and basolateral membranes from
shark rectal glands showed that their epithelial cells are permeable to water and not to
urea (Zeidel et al. 2005 ). For example, it was reported that “the basolateral membrane
urea permeability is fi vefold lower than would be anticipated for its water permeability,” (Zeidel et al. 2005 ). These results show the important role of basolateral membranes as selective barrier within the rectal gland of sharks (see also Silva et al. 1990 ).
3.6 Biohalite
which these glands facilitate the net secretion of sodium (or potassium) chloride, has
been studied in great detail during last 60 years (Peaker and Linzell 1975 ; Gerstberger
and Gray 1993 ; Shuttleworth and Hildebrandt 1999 ; Hildebrandt 2001 ; Dantzler
and Bradshaw 2009 ; Holmgren and Olsson 2011 ; Babonis and Brischoux ( 2012 ).
Interesting results has been reported in comparative studies between marine and
freshwater species (Babonis and Evans 2011 ). Studies include work looking at the role
of water-regulatory proteins in modulating the secretory output of the salt glands, and
researching the diversity in the composition of the secreted fl uids. In addition, such
topics as function of salt glands under various environmental conditions, bacterial
infections of salt glands, their phenotypic plasticity as well as the regulatory mechanisms of secretion by various neurological and endocrine agents, have been studied.
Additional work has also been done to investigate the combined “osmoregulatory
function of salt glands and other organs,” (Babonis and Brischoux 2012 ; see also
Babonis et al. 2011 ). Interestingly, although the anatomy, structure and function of
these glands has been quite well studied, there have been no reports on NaCl as
form of a biomineral that is formed because of activity of the corresponding cells.
Here, I would like to introduce the term “ biohalite ” for the NaCl-based biomineral
that originates from the salt glands.
3.6.1 Diversity and Origin of Salt Glands
in Marine Vertebrates
Fish Observations of marine shark’s species in rivers suggest their specifi c ability
for acclimatization in aquatic niches with different levels of salinity. Also experiments under laboratory conditions has demonstrated that marine elasmobranchs “do
have the capacity to acclimate to changes in salinity through independent regulation
of Na
+
, Cl
−
, and urea levels,” (Hazon et al. 2003 ). Thus, some stenohaline marine
elasmobranchs can be accepted as partially euryhaline. However, “once optimally
adapted to fresh water, recolonization of sea water by elasmobranchs is problematic
due to the loss of urea synthetic capacity and renal structures for urea retention,”
(Ballantyne and Robinson 2010 ).
Sharks like Squalus acanthius can effectively maintain osmotic homeostasis on the
level of urea concentrations ranging between 300- and 400-mM, and corresponding
marine osmolalities of 900–1,000 mosmol/kg H 2 O (Zeidel et al. 2005 ). Also the rectal
gland that is isotonic with the plasma in this fi sh species can maintain salt balance
without losing urea by secreting a NaCl-rich (500 mM) and urea-poor (18 mM) fl uid.
An investigation into structural features of apical and basolateral membranes from
shark rectal glands showed that their epithelial cells are permeable to water and not to
urea (Zeidel et al. 2005 ). For example, it was reported that “the basolateral membrane
urea permeability is fi vefold lower than would be anticipated for its water permeability,” (Zeidel et al. 2005 ). These results show the important role of basolateral membranes as selective barrier within the rectal gland of sharks (see also Silva et al. 1990 ).
3.6 Biohalite