Brackish Seahorse

[AMS]

Accoring to fishbase.org the endangered seahorse species Hippocampus capensis is a brackish to marine fish. Here's the link. I wonder if this is actually credible? As most of us know fishbase is an extremely accurate source, and I would not say otherwise against their team of Ichthyologists. (But just for grins they label Pygocentrus nattereri as subtropical which not accurate). Anyone else heard of this? Also according to that link they say this particular seahorse has the smallest known range of any seahorse, so perhaps they do not swim into brackish waters but periodically brackish waters cover their range which is a estuary or close to an estuary? This has got me thinking

 

[nmonks]

Hello AMS --

There are actually quite a few brackish water seahorses, and I've mentioned them in the Brackish FAQ for nigh-on ten years. So this isn't news to me, at least.

I'm sure in a high salinity system with lots of seagrasses they'd be perfectly happy. My experience of seahorses is that they are essentially hardy, just difficult to feed. Solve that problem, and they're largely trouble-free.

None of the seahorses gets into low salinity waters though, so we're talking SG 1.012-1.015 rather than 1.005. For a low salinity system, you'd be better off going with pipefish (which are less widely sold than seahorses and often supplied without a Latin name).

Cheers,

Neale

 

[AMS]

Thanks for that Neale. I was just curious as I own a small herd of dwarf seahorses, and I thought that no seahorse species are brackish, too bad none of the brackish species are traded though

PS- What species did you have? They are one of my favorite fish, so cool.

 

[AMS]

And just for grins, what minimum SG would you suggest for Hippocampus capensis?

 

[nmonks]

No idea. If you figure that freshwater fish "pump out" water, while seawater "pump in" water, my assumption is that marine-in-brackish fish that cannot live in freshwater are species that can reduce the rate at which they pump in water, but cannot actually pump out water. Marine-in-freshwater fish, like scats and monos, can do both, as required.

So, there is some salinity at which water neither goes in nor goes out, i.e., pumping in either direction isn't required (in science speak, the fish's tissues are isotonic to the surrounding medium, as oppoosed to being either hypotonic [freshwater] or hypertonic [seawater]). While I don't know what the exact salinity is, if I recall correctly both freshwater and marine fish have the same salinity inside their bodies, and it's about equal to 2/3rds the salinity of seawater.

Rather than saying I don't know the answer, this is a roundabout way of suggesting that the lower level is probably above 2/3 SG of normal marine, i.e., around 1.012 (where 1.018+ is normal salt water). I would recommend this as the lower level for any "mariney" brackish water fish, whether a seahorse or, for that matter, an adult mono or GSP. Below this SG, the seahorse would need to pump out water, which I predict it cannot do. Above 1.012, all it needs to do is "tweak" the rate at which water is pumped in, a much simpler operation in terms of divergence from the baseline marine fish physiology.

If somebody knows what the salinity of fish tissues are, that would help revise my estimate upwards or downwards.

Cheers,

Neale

And just for grins, what minimum SG would you suggest for Hippocampus capensis?

 

[AMS]

Wow Neale, very scientific and beyond my comprehension I was just curious as I found a source that sells captive bred Hippocampus capensis for $65.

 

[andywg]

If somebody knows what the salinity of fish tissues are, that would help revise my estimate upwards or downwards.

Ok, I am not entirely happy with my own understanding of the figures, or at least how the units used relate in an absolute way, but according to "Biology of Fishes" by Bond (2nd Edition) the osmotic concentrations are as below:

Freshwater fish have an osmotic concentration of about 265 to 325 mOsm kg-1 (milliosmol per kilo) or a Freezing point depression of -0.5 to -0.61 degrees C).


Marine fish have an osmotic concentration of about 380 to 470 mOsm kg-1 (milliosmol per kilo) or a Freezing point depression of -0.7 to -0.87 degrees C) .

The salt content of Elasmobranches (sharks and rays) in marine conditions is slightly higher.

So it appears that marine fish tend to keep a higher salt content in the body (although some of this is due to the retention of nitrogenous substances).

 

[nmonks]

Hello Andywg --

Thanks for the data. Here's some more:

In terms of sodium ions, a typical marine bony fish has a sodium concentration of 180 milliosmoles per litre, while a typical freshwater fish has a sodium concentration of 130 milliosmoles per litre. Normal seawater has a sodium concentration of 439 milliosmoles per litre, and freshwater practically zero. So, there is only slightly less sodium in the tissues of a freshwater fish when kept with a marine one.

This is actually very important, because it supports the story of how bony fish evolved. The question is why marine fish don't have the same salinity as the sea. After all, hagfish do, as do virtually all marine invertebrates, so the assumption is primitive fishes must have done as well. The explanation is that bony fish originally did have the same salinity as their environment: but their environment was not the sea, but brackish water!

As they diversified, and went into the sea and up rivers, they evolved systems that allowed them to compensate for the differences in salinity between their new homes and their "brackish" tissue fluids. At heart, all bony fish are brackish water fish!

Cheers,

Neale

 

[AMS]

]This is actually very important, because it supports the story of how bony fish evolved. The question is why marine fish don't have the same salinity as the sea. After all, hagfish do, as do virtually all marine invertebrates, so the assumption is primitive fishes must have done as well. The explanation is that bony fish originally did have the same salinity as their environment: but their environment was not the sea, but brackish water!

As they diversified, and went into the sea and up rivers, they evolved systems that allowed them to compensate for the differences in salinity between their new homes and their "brackish" tissue fluids. At heart, all bony fish are brackish water fish!

Very interesting point Neale. I remember reading somewhere that in the pre-jurrasic period coral reefs were living in brackish waters. I do not know if they adapted to brackish water or were first found in brackish waters. Who knows, maybe the first corals and fish originated in brackish seas? More scientific explanations please

 

[andywg]

I recall seeing Freshwater Sponges in the BBC series on the Amazon river last year, so it is certainly possible that some inverts have/do survive in lower salinity.

 

[AMS]

I recall seeing Freshwater Sponges in the BBC series on the Amazon river last year, so it is certainly possible that some inverts have/do survive in lower salinity.

Sponges are highly adaptable as far as I recall. There are various freshwater species not just in the Amazon, but also in various other countries. There are also many brackish species as well.

 

[nmonks]

The science behind freshwater invertebrates is very interesting.

There are something like 30-odd phyla of invertebrates (i.e., echinoderms, crustaceans, insects, molluscs, etc.). Of these, most are either entirely or primarily marine in distribution.

Molluscs (snails, clams, etc.): over 100,000 species, ~10,000 in freshwater/on land
Crustaceans (crabs, shrimps, etc.): well over 50,000 species, ~5,000 in freshwater/on land
Annelida (earthworms, leeches, etc.): 15,000 species, ~4000 freshwater/on land
Cnidaria (corals, anemones, etc.): 11,000 species, ~3000 freshwater
Echinoderms (starfish, urchins, etc.): 7,000 species, all marine
Sponges: 5,000 species, only 150 in freshwater
Bryozoans (moss animals): 5,000 species, only 50 in freshwater

Almost all of the "minor phyla" -- chaetognaths, brachiopods, comb jellies, etc. -- are marine. The only two phyla I can think of from the top of my head that are predominantly freshwater are the rotifers and the tardigrades, which are both microscopic. The insects are of course a giant phylum and include many freshwater species, while the arachnids are essentially terrestrial with only a few aquatic or marine species.

Anyway, what restricts 20+ phyla to the sea is the osmoconforming thing: they cannot regulate the movement of salt and water in and out of their bodies. Instead, they simply use sea water in the way we use blood or tissue fluid: as the universal "bath" within which their tissues are surrounded.

The story of why a few phyla adapted to freshwater and then the land is interesting. Only two phyla of invertebrates are fully terrestrial in the sense of being independent of ambient moisture; these are the insects and the arachnids. Vertebrates (our phylum) include a mix: amphibians are not fully terrestrial, but reptiles, birds, and mammals are. Molluscs, worms, and crustaceans are like amphibians, and need damp habitats or they dry out, so they are considered only partially adapted to land.

Why so few inverts bothered adapted to freshwater is also interesting, but it seems that basically there was nothing to entice them there. Compared with the sea, freshwaters lack plankton (mostly), so there's much less food.

Another big question is what routes the different land animals took. Insects seem to have started off as freshwater animals, then lived in marshy bogs, and eventually became fully terrestrial. Crustaceans, on the other hand, seem to have gone from the sea to the land and by-passed freshwaters. Many land crabs, for example, live in forests but breed in the sea. Molluscs seem to have done a bit of both, with some terrestrial snails apparently having marine ancestors while others seem to have had freshwater ancestors.

Anyway, as others have said, "nothing in biology makes sense except in the light of evolution". All good stuff!

Cheers,

Neale