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I was simply wondering whether the Tyranids were known to have any units that can 'swim' in lakes and ocean.

There's an aquatic version the the ripper that is meant to devour stuff in oceans

Trygon could be adapted to swim like a sea snake fairly easily I think

Ok, and in comparison to a water-based navy, such as that found on say Armaggedon, how would they fair in naval combat?

The Krakens of Fenris are Tyranids.
The first time they entered the Milky Way in force they assaulted Tyran which was a water world. So their first major engagement against the imperium was under water.

The Tyranid first attack was on the planet tyrant. They first made planetfall in its oceans before coming into land. I would say water own a big obstacles for them.

CommanderRednaxela wrote:
Ok, and in comparison to a water-based navy, such as that found on say Armaggedon, how would they fair in naval combat?

I would imagine it depends on the relative quality of tyranid and imperial sonar. What with the effects of explosive overpressure in water, imperial explosives would be quite effective, but equally if tyranid beasties just needed to swim deep and release swarms of hull munching critters that float up under ships then it could be quite hard to combat.

CommanderRednaxela wrote:
I was simply wondering whether the Tyranids were known to have any units that can 'swim' in lakes and ocean.

They do.

From the novel "Cassius":

"Once the xenos have a complete grip on this continent they’ll use aquatic bioforms to cross the ocean and take the rest of the planet."

epronovost wrote:
The Tyranid first attack was on the planet tyrant. They first made planetfall in its oceans before coming into land. I would say water own a big obstacles for them.

No, it was called Tyran. And water is no problem for them.

 DeffDred wrote:
The Krakens of Fenris are Tyranids.

We don't know that for sure.

The first time they entered the Milky Way in force they assaulted Tyran which was a water world. So their first major engagement against the imperium was under water.

The fighting took place in space and on volcanic islands. Not in/on the ocean.

This is always has preyed on my mind about the Tyranids, or even the Xenomorphs from Alien.

They would stop at nothing. No-where would ultimately be safe (unless you kill them all of course). They could command the skies and the land, but the seas would be worst!

I would argue that the sea would be the last place we could wipe them from. They would hide deep and we'd never see them. They'd just keep coming, munching on all that biomass, taking energy from geothermal vents. Plankton and algaes would be in huge supply for them, and all that organic matter that falls to the bottom of the ocean will supply them too.
They could take the gene material and traits from creatures that live down there too, and either grow to massive proportions or develop new adaptations. Think of the electric eels, or new poisons, or even just develop mutations to deal with the severe cold or crushing depths, lack of light etc!!!

We'd have to scour all of the ocean, either draining them (where would put the water) which would cause arguably irreparable damage to the biosphere and planet, or we'd have to field ships and submarines in such huge numbers it'd be more costly than a land or sky extermination.

The only other things would be a virus or disease that only affected Tyranids or some kind of nanomachinery that floated around the oceans. Both are probably beyond the Imperium or the Tyranids would adapt.

Terrifying - once an infestation has gone beyond a certain stage, there's no real stopping it.

TarkinLarson wrote:


We'd have to scour all of the ocean, either draining them (where would put the water) which would cause arguably irreparable damage to the biosphere and planet, or we'd have to field ships and submarines in such huge numbers it'd be more costly than a land or sky extermination.

Since when has the Imperium cared much about the biosphere If the alternative is exterminatus, they are likely to just wipe out the ocean to eradicate the infestation. The planet will recover eventually. It won't if the surviving 'nids attract another Hive tendril.

The only other things would be a virus or disease that only affected Tyranids or some kind of nanomachinery that floated around the oceans. Both are probably beyond the Imperium or the Tyranids would adapt.

The Imperium has achieved this multiple times, but it is always presented as a great feat of bioengineering that required very specific samples of "primitive" (old) strains, rare experts in genetics, a healthy dollop of luck, and a suicide mission to accurately deliver the toxins. However, that is to wipe out an entire Hive fleet at full synaptic strength, not some leftovers sheltering in an ocean.

Disconnected 'nids trapped in the ocean without a Hive fleet above will be feral, so it should be much easier to generate a toxin for them. This is a plausible method of killing them, although it will require repeated attempts and sampling I am sure. Probably still just easier to extract the water, send it off the feed a hive world (after extensive purification), and kill anything still moving at the bottom.

 Haighus wrote:

Disconnected 'nids trapped in the ocean without a Hive fleet above will be feral

Not nesseceraily. Just take the the ghorala swarm as an example.

It is speculated that the aquatic wildlife on Fenris might be Tyranid bioforms.

It has never been confirmed, AFAIK.

Anphellion project too, leave the feral nids long enough and they will form a new hive mind

One of the Cain novels has him on an ice covered world from memory where some nids are frozen under the ice, which awaken when the hive fleet turns up. I seem to recall these might be aquatic versions of the serpentine nids, but I've not read it in a while.