Copepod bioenergetics exercise, check my math

[dansyr]

I'm asking the internet, and a hobbyist corner of it, to point out errors so we're off to a good start :)

Background - I've been growing Tisbe copepods lately for fun (and corals feeding), and I enjoy thinking somewhat quantitively about what's going on in terms of inputs/standing stock / output. And been seeing lots of posts lately about copepods and dragonettes in new or very small tanks so that got me thinking:

Order of magnitude estimation - Tisbe pods (smaller end but maybe typical of what they eat) are ~0.5mm and if you estimate wet weight off that length you get about 0.01-0.001 mg (wet weight) per 0.5mm copepod.

If a dragonette eats 1,000-10,000 tisbe pods in a day (peck every 4-40s over 12h day), that's only 0.01 * 10,000 10-100mg of food.
* sanity check - seems a challenge for one to peck every 0.4 seconds so order of magnitude seems right here for number of strikes, miss rate is probably factor of 2, not another order of magnitude 10.
* in nature, they're probably eating a more diverse range, so average might be 1-2mm, but that's 1 order magnitude in mass/copepod, so 100-1000mg (0.1-1g) wet weight.

Intuitively seems like the legitimate ratio for a small, efficient fish like a dragonette, close to the range if you have a 3g cube of frozen food (1.5g after draining?) split across a couple fish.

Bioenergetics people please correct my math here. Am I missing something in the orders of magnitude here?

If that holds, then applications - I looked up what's in commercial pods (as I had no idea about what those numbers would be), and the first one I found for Tisbe list ~3000 per 16oz bottle, which means one of those bottles is ≤30 mg wet weight = the mass of 1/100th a frozen cube = one day's meal.

Please note I'm not asking "how can I keep a mandarin/dragonette in a nano / brand new tank / etc" or trying to spark discussion that way - I am well aware of that and personally support captive bred all the way, both for ethics and for ease of feeding them. I have a synchiropus ocellatus that accepts frozen and pellets. I have just seen a lot of posts recently, was curious about the numbers of 'how many grams of copepods are in a bottle a random person could buy' and 'how many mg is typical a copepod?' and didn't see that anywhere, so I tried to work it out over coffee this morning. But because it's worked out over coffee, wanted to see if the collective could catch errors I made here. Thoughts?

 

[KrisReef]

If the fish fed constantly for 12 hours pecking (eating) every 4 seconds it would devour 10,800 prey items each day. That would be 3 bottles of (3000 each) pods a day. With a strainer and a scale it would be easier to obtain a wet weight estimate?

I also think these fish do more than just eat pods every 4 seconds, all day. I think if all they did was eat pods they would get a stiff neck.

I'm not sure I have ever seen a Mandarin defecating? Perhaps they have really efficient digestive tracts? I remember a feeding study where a guppy was given very small rations and it reportedly assimilated everything.

 

[dansyr]

If the fish fed constantly for 12 hours pecking (eating) every 4 seconds it would devour 10,800 prey items each day. That would be 3 bottles of (3000 each) pods a day. With a strainer and a scale it would be easier to obtain a wet weight estimate?

I also think these fish do more than just eat pods every 4 seconds, all day. I think if all they did was eat pods they would get a stiff neck.

I'm not sure I have ever seen a Mandarin defecating? Perhaps they have really efficient digestive tracts? I remember a feeding study where a guppy was given very small rations and it reportedly assimilated everything.

Agreed, from what little i've seen I would believe upper range of prey chomps is the most realistic. I picked the other end of the range (40s / peck, they do need that other 3-39 seconds to do stuff like be cute or hide or other fish behaviour) just to be in the middle of the range. if they truly peck only ~40s then 1000 pods = 10mg food = 1/3 a bottle of pods a day, or if peck 4s then 10800 = 100mg food = 3 bottles of pods, agreed on the math there.

I do wonder about assimilation efficiency, you're probably right that it's high. I mean, if they are truly eating 100mg food a day, that's... efficient. They do have efficient locomotion strategies so that helps.

I tried finding some papers on their efficiency or feed conversion ratios but seem to have a hard time finding that for adults or wild studies, would love any pointers. or if you have any anecdotes that would be awesome for my curiosity. I've seen numbers put out on forums about mandarins eating 1000s of pods a day (which fits rough estimates) but having a hard time finding actual measurements or observations.

 

[KrisReef]

Agreed, from what little i've seen I would believe upper range of prey chomps is the most realistic. I picked the other end of the range (40s / peck, they do need that other 3-39 seconds to do stuff like be cute or hide or other fish behaviour) just to be in the middle of the range. if they truly peck only ~40s then 1000 pods = 10mg food = 1/3 a bottle of pods a day, or if peck 4s then 10800 = 100mg food = 3 bottles of pods, agreed on the math there.

I do wonder about assimilation efficiency, you're probably right that it's high. I mean, if they are truly eating 100mg food a day, that's... efficient. They do have efficient locomotion strategies so that helps.

I tried finding some papers on their efficiency or feed conversion ratios but seem to have a hard time finding that for adults or wild studies, would love any pointers. or if you have any anecdotes that would be awesome for my curiosity. I've seen numbers put out on forums about mandarins eating 1000s of pods a day (which fits rough estimates) but having a hard time finding actual measurements or observations.

All that stuff may be hiding behind a paywall?
I miss going into a library with books to wheel around the card catalogue and find dusty old books with descriptions and newer volumes with white paper discussions about studies folks have done.

What is interesting to me is that folks have gotten them eating dry and captive breeding so that they live fat and long in captivity.

 

[afboundguy]

I'm not sure I have ever seen a Mandarin defecating? Perhaps they have really efficient digestive tracts? I remember a feeding study where a guppy was given very small rations and it reportedly assimilated everything.

Mandarins don't have the "best" digestive tract and it's basically a straight shot from mouth to rear end which is why they have to constantly eat all the time. They don't have a typical digestive tract where food sits in a stomach to be digested and absorbed.

What is interesting to me is that folks have gotten them eating dry and captive breeding so that they live fat and long in captivity.

I have a captive bred male and female mandarin who from @Biota_Marine that was eating frozen BBS and TDO pellets when I got him. The female must have died/jumped as she didn't last more than a week but she was eating pellet and frozen as well. He quadrupled in size and still eats frozen and pellet food. After I feed the tank I use a long bulb syringe to spray pellet food in the back corner and he is trained and swims as fast as he can and pigs out.

About a month ago I picked up a wild caught female who was skinny and I probably shouldn't have bought her but it was a reputable LFS. I put her in my display refugium (about 8 gallons) instead of my in tank acclimation box.

She pecked at TDO pellets but there was tons of pods in my system (I have 3 different types of copepods cultures and dose tank 1-2 times a week). After a few weeks I put her in the 35 cube DT and she quickly learned from the male where to go for the pellet feast. She now eats frozen and pellet food just as good as the captive bred male and has fattened up significantly.

The two of them constantly peck and I still have pods all over the glass. I am still a big believer in captive bred mandarins and I only jumped on the wild caught female as it was from a reputable LFS who had had her for a few weeks and said she had been fattening up since they got her.

 

[DaJMasta]

It's an interesting exercise, but it's important to note that copepod wet weight is not a linear relationship with length (theoretically, it should be about cubic, right?), so why you may need many thousands of 0.5mm copepods, you need way fewer than half that worth of 1mm copepods (which tisbe still do get to), and larger organisms are all the more calories still. Amphipods seem to be on the order of 10-20mg wet weight each ( https://pmc.ncbi.nlm.nih.gov/articles/PMC7881717/ ), so a handful of them would count for thousands of adult copepods, and mandarins will go after small worms and things too.

If you want to do some optimization or theoretical experimentation, this paper gives an example wet weight formula https://www.sciencedirect.com/science/article/pii/S1470160X20312498

WW = 0.0299 * Length ^ 2.8348

While not really the point of the exercise, I think this underscores the value of training them onto frozen. A single mysid shrimp or bloodworm is upwards of a centimeter long, which could represent similar nutrition to most of a day worth of forraging for just copepods (though I do think they find many more things than that.)

As for the gut efficiency, assuming quite low is right. Like other constant-feeding fish, they do it because they have to, and they have to because they don't really have a stomach. Fishes, on the whole, are known for having pretty inefficient guts, and fish that feed the most frequently are probably at the bottom of those efficiency charts.

 

[KrisReef]

I understand that fishes have "primitive digestive systems," but if they are able to thrive in the wild with what we describe as primitive, I think that perhaps our understanding of the efficiency of the fish gut may be what should be described as primative?

I kept a red scooter blenny that I also target fed frozen pods to using a similar strategy as @afboundguy (who is using pellets) but target feeding and training the fish to come to dinner! :beaming-face-with-smiling-eyes: I kept him and watched him grow just to see how that might progress. I traded him back in when I upgraded that system.

My contention is, that by definition A fish that hunt and pecks to eat must be able to assimilate energy very well compared to other fishes and animals with a stomach.

Behind the paywall
https://www.sciencedirect.com/science/article/abs/pii/S1546509808600278

So frustrating to see the library get commercialized.

 

[dansyr]

It's an interesting exercise, but it's important to note that copepod wet weight is not a linear relationship with length (theoretically, it should be about cubic, right?), so why you may need many thousands of 0.5mm copepods, you need way fewer than half that worth of 1mm copepods (which tisbe still do get to), and larger organisms are all the more calories still. Amphipods seem to be on the order of 10-20mg wet weight each ( https://pmc.ncbi.nlm.nih.gov/articles/PMC7881717/ ), so a handful of them would count for thousands of adult copepods, and mandarins will go after small worms and things too.

If you want to do some optimization or theoretical experimentation, this paper gives an example wet weight formula https://www.sciencedirect.com/science/article/pii/S1470160X20312498

WW = 0.0299 * Length ^ 2.8348

While not really the point of the exercise, I think this underscores the value of training them onto frozen. A single mysid shrimp or bloodworm is upwards of a centimeter long, which could represent similar nutrition to most of a day worth of forraging for just copepods (though I do think they find many more things than that.)

As for the gut efficiency, assuming quite low is right. Like other constant-feeding fish, they do it because they have to, and they have to because they don't really have a stomach. Fishes, on the whole, are known for having pretty inefficient guts, and fish that feed the most frequently are probably at the bottom of those efficiency charts.

Agreed on the cubic, - that 2nd paper's Fig 4 was what I was basing estimates off of :)

 

[afboundguy]

I understand that fishes have "primitive digestive systems," but if they are able to thrive in the wild with what we describe as primitive, I think that perhaps our understanding of the efficiency of the fish gut may be what should be described as primative?

I kept a red scooter blenny that I also target fed frozen pods to using a similar strategy as @afboundguy (who is using pellets) but target feeding and training the fish to come to dinner! :beaming-face-with-smiling-eyes: I kept him and watched him grow just to see how that might progress. I traded him back in when I upgraded that system.

Primitive works in the wild as there's a tad bit more available food in the ocean than our tanks. Plenty of things for mandarins and other high energy fish!

It is hilarious that I have trained my mandarins to eat pellet food in the corner! The unfortunate part is I have also now trained all the other fish to eat there too so I end up feeding more and more pellets! It's still really cool to see the mandarin pair swim as fast as they can to the spot!

 

[dansyr]

Amphipods seem to be on the order of 10-20mg wet weight each ( https://pmc.ncbi.nlm.nih.gov/articles/PMC7881717/ ), so a handful of them would count for thousands of adult copepods, and mandarins will go after small worms and things too.

Sorry for splitting up your post into a couple replies bad at the quote thing. Mathing the copepod : mg : number consumed per day per mg was the main question. But I guess on while posing this in terms of mandarin diets, this is the interesting thing to me from the mandarin/dragonette physiology - are they snacking on the 0.5mm fraction to hold them over while they are foraging for the payoff 3mm polychaetes/amiphods, or are they truly generalist and can scale. I found a cool paper where they tracked them over pretty high-resolution time courses for determining spawning locations, but didn't see much on the foraging side.

Anyway, it's reassuring to hear that i'm not off by several orders of magnitude, thank you for the detailed response!

 

[DaJMasta]

It may be worth mentioning that they will also try to eat almost anything they could possibly fit in their mouths. I've trained a few dragonets onto frozen and one of the first things I saw an adult mandarin eat was a whole krill head, which it spent about a minute thrashing around to try and get down its stomach in a satisfactory way. Judging from their gape, I wouldn't have expected it to even try, but if they constantly pick and have this preference for the largest thing they can get in there, it may be that a daily hunt's worth 0.5mm or so copepods is sort of subsistence, but when they can land a few amphipods, worms, or chunks of larger food, that's what gives them the surplus to grow, make gametes, or what have you.