UNWALLED UNIVERSITY

Fishes Worth Their Salt

UNWALLED UNIVERSITY

DEPT: Marine Sciences: Ichthyology: EvG

Fishes Worth Their Salt

Aquarium keeping is one of the biggest leisure-time activities in the country today, and yet even the amateur aquarist of many years standing has seldom kept a brackish water tank.   Some of our most popular specimens have been considered as delicate, short lived, breedable only by specialists or by sheer luck or not at all, only because the brackish water tank was lacking.  In a day when Saltwater tanks are proliferating, it seems strange that the missing link between the two is neglected.    I think that this because most aquarium keepers do not quite know what fish are available for this environment, and what the rewards can be.  Some of the most exotic fish available like a brackish tank.

The nearly invisible glass fish (CHANDA BURENSIS); the Mud –skipper who looks like an amphibian and climbs trees like a lizard; the Bumblebee goby, who presents us with little gobies; the Halfbeak, a live bearer who looks like a miniature marlin; the Mono; the Scat; and the ever-gorgeous Sailfin Mollie (latipina Molliensia) are just for starters.  All of these fish and many more need some salt in their habitat to be healthy, live long, and breed.

Most texts and hobby magazines mention water conditions for each fish but, the aquarist who buys an exotic specimen and watches it slowly fade away without knowing why, is not apt to try again.

Lets look at a little basic fish biology so that we understand what happens with this Salt/no salt /some salt situation.

We have all heard of Salmon who are born in FRESH water and migrate to the OCEAN to live returning to the FRESH water to spawn and die.  Fish that must return to fresh water to spawn are called ANADROMOUS, while EELS who return to the salt water of the Sargasso to breed are called CATADROMOUS.  Both types of migration are lumped together under the term DIADROMOUS. These fish DO adapt to the change of salinity but only very slowly.  And usually only once in their lives.

The changes might be considered as unrepeatable as adolescence and menopause.

The SHARK   had a special problem.  It is so old as a species that it evolved BEFORE the ocean was salt.   It has adapted to the excess salinity by growing a thick skin; by having no wide flapping gills, and little mouth contact with the water.  It has also developed a system of building up inner pressure by recirculating certain body fluids resulting in a high urea content in all its tissues.  Freshwater fish and the diadromous fishes are called Stenohaline because they do not tolerate salinity changes.  Euryhaline fishes, however, can go from one kind of water to another over and over again.

Most euryhaline fishes have evolved around river mouths that empty into the sea and that have tides running in and out of the fresh water.  The advantage to living in a delta where animal foods of both fresh and salt water types may be had at the turning of a tide, pulls an abundance of algae produced by sunlight in shallow mineral rich water, is an important one.  The question is not why they do it but how?

We do not need to get very technical here one or two simple analogies will give us the general picture of the mechanism.  Basically it is the kidney that makes the difference in adaptability.  The whole process works on a molecular level and it has to do with osmosis.  Considering the water in the lake and liquid in the cells of the living fish as being separated only by membranes that are subject to osmosis, we see that whichever liquid has the highest pressure will force itself through the membranes into the other in an attempt to equalize that pressure.  In fresh water the fish with its mineral concentration, hormones, and the like will have the higher density just as we do.  To avoid loosing all its water to the lake the fish must evolve a system of absorbing and holding salt in its cells so that the salt will help retain water.  If it seems silly to think of fish drying out while under water think of how your hands look after a long emersion.  An ocean fish has just the opposite problem.  The water of the ocean is denser than his cellular fluid.  He must constantly get rid of any excess salt acquired through osmosis or drinking or breathing (gills) before he retains so much salt and water that he swells up and ruptures his cells.  This is what would happen to the normal fresh-water fish with a salt storage mechanism when put into salt-water.  The ocean fish place in a lake would continue to throw away his salt and water, and would dehydrate to death.  The Euryhalines, however, seem to have developed a switch that allows them to change from one environment to the other, and back again.  How it is done is only generally known even now.

Because of the sexual implication of the salmon and the lampreys scientists assume that the switch, at least in some cases, is triggered with hormones.  Those of us who have raised mollies are apt to agree.   Any healthy Mollie spends 98% of his time chasing (if males) or being chased (females).  It would be no surprise to the Mollie fan to hear that the frisky little creatures have four more chromosomes than we have.  It would be unfair though to assume that they use them all on sex in view of their tremendous range of adaptation to salinity.  The range in mollies can vary because of the ancestry of the Mollie.  Some years ago they were fairly clearly divided into two main categories.  The sphenops Mollie who was mostly silver and had a dorsal fin that an only about one sixth of the length of the back and was merely an ordinary fin; and the Sailfin (Latapinna) whose fin at the root was full third of the length of the back, and stood up grandly like a sail as high as the depth of the fish itself.  There was also a Velifers or veilfin whose dorsal was even higher than the latapinna but slightly shorter at the root giving it a pinched in look.  The first to be bred in the black color was the sphenops and today the speckedled and black are the normal colors.  The Sphenops tolerated salt water but could not be adapted to full marine life.  The first lyretail was developed from this or a cross.  You can see the spenhops ancestry in the narrow base of the dorsal in spite of its length.  All the breeds have been so mixed today that it takes something of an expert to identify the ancestry.  Because of the mixed ancestry occasional difficulty is met in adapting a Mollie to salt water.  Generally though ordinary pet store mollies can be adapted with very little trouble to full ocean water, and once adapted can go from that to brackish or fresh almost immediately in an emergency.  [It is best to ease the transition, when possible, over an hour or two].  They grow much bigger in Marine environments, and never seem to get any fungal diseases.  I have kept salt-water mollies (albino, silver, gold, black, lyretail, and guppy-tail) with success for years.  I originally adapted them so that my precious marine specimens would have fresh supply of live food, (since the female mollies produced their young every month,) and became so entranced with the mollies that I phased out most of the other marines.

If the mollies have varying degrees of adaptation so do other members of the poecilidae.  The guppies are able to live well in brackish water, but not full salt, the swords and platies can tolerate brackish but do not need it, and the former marine animals like the Monos and Scats love it.  Tidal flat fishes like the mud-skippers like a fluctuating amount of salt and are not bothered by evaporation and refilling.  If breeding them you must remember that some of these animals only like added salt when they reach puberty, (if you’ll permit that phrase in connection with fish).  Not all of the species that inhabit salt and brackish waters use the same mechanism to adapt.  In most species the gills aid the kidneys and the many forms of adaptations are still being studied to determine what else is involved.  What is known is enough to feel confident in setting aside one tank for a brackish community.

One very attractive way to do this is to keep the brackish tank (around .10 on a Hygrometer assuming that .25 is sea water,)  just half filled and construct arches and rocks sticking up out of it for climbers like the mud-skippers bumblebee gobies will live in the crevices, half-beaks skim the surface, although if they are too small they may be mistaken for insects by the mud-skippers, and if too big may bother others a bit.  Glassfish and monos inhabit the middle regions and scats police the bottoms of the tanks.  Make sure with a good text or with advice from an expert that you get the right kind of glassfish there is one kind that does not like brackish water.  Mollies, of course will be all over.

Plantings will enhance the setting too, and may even grow right out of the surface.  Both Val and Sag will stand brackish water but sag stands the higher salinity.  Water sprite will also survive and duckweed thrives.  Lots of light is needed for luxurious plants.  If you prefer, a terrium with a few amphibians can be arranged in the top half of the tank.  If you use non-brackish toads or amphibians though, remember to set a dish of fresh water in the jungle so they can get wet.  There are many sedges and swamp plants that will grow in and around your brackish tank But I haven’t room here to tell you all of them.  I suggest that you browse through some aquarium books you’ll find them worthwhile.  When you compile a list of all the fish you’ve never had, or never had successfully without a brackish tank I think you’ll agree that those fishes are worth their salt.

Tags: short story

This entry was posted on Friday, April 17th, 2009 at 8:09 pm and is filed under Short Stories. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.