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Danionella priapus – males quarreling or spawning behaviour?

Home Forums Fresh and Brackish Water Fishes Danionella priapus – males quarreling or spawning behaviour?

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    A few months ago I bought a group of 10 Danionella priapus. Or supposedly Danionella priapus, anyway. Either way, I haven’t seen them do much other than hang out together near the front of the tank and the number of specimens is now down to 7. Today I noticed a bunch of them were swimming at the back of the tank all of a sudden which caught my attention. When I looked closer I noticed two fish swimming closely together at the front of the tank. One of the fish has a darker and more greyish colour while the other fish is more translucent with a fluorescent green stripe along the middle of its body. The green colour isn’t very visible but in certain angles it lights up. 

    Anyway, at first I thought it might be two males fighting, but I noticed one of the fish (the lighter and green one) is following the other one (the grey one) around, always swimming underneath it and sometimes sort of pushing itself up against the other fishes body. They’ve been doing this for at least an hour now, and when any other of the fish comes close the lighter one will quite quickly chase them away and they return to the back of the tank.


    Below are some awful photos of the event, which won’t really show you much but it might make this post more interesting. I had to take these without a flash and with manual focus so it wasn’t easy getting pictures worth showing and the pictures have a lot of noise because of the high ISO.


    Here you can easily see the difference between the two fish and you also get an idea of the stalking movements and how it swam up from underneath against the other one. I did make a short (and very low quality) video with my mobile phone so I will upload that later.




    Very interesting behavior.

    I don’t think your pictures all that bad for pictures in a tank. These are difficult fish to photograph.

    I have some Danionella which have bred, but I have never seem it.

    In Pete Cottle’s Danios and Devarios book there is an discussion of Danionella breeding. The person who wrote it thinks they like to dive into a gravel (not sand) substrate to lay their eggs. I had been keeping them for a while in a bare bottom tank with a java moss and a lot of acrylic yarn mops with no breeding success. After reading the above account, I added some trays with various sizes of gravel and some marbles at different levels int he tank (30 G). After a while I got about 100 offspring over a period of a few weeks.


    These look like two males to me or a male and a female maybe but the female does not look like it is carrying eggs.

    When my Danionella are carrying eggs it is really obvious. The females are about 1.5 times the top to bottom thickness of the males. I use an intense little flashlight to look at them and sometimes the light glints off the eggs in the females body.


    I am not sure what species they are but I don’t think they are priapus since the males should have a small projection projection going down in front of the anal fin. The pictures seem good enough to show this and I can’t see it.

    I doubt they are translucida because they have a shorter body axis. I had these in a lab I worked in once.

    If they are dracula they should have two little downward pointing teeth on the upper jaw. I can’t see these, but I don’t know if they would be visible in this kind of picture.

    Among described Danionella species I am aware of, that leaves mirifica. They are supposed to have a line of pigments cells on the ventral midline. Not easy to see. I did this once by knocking a fish out, but it did not wake up. There cold however be other not yet described species. I think it is rare for any Danionella species sold in the US to be correctly identified by the people selling them.


    The green line is probably birefringence due to light reflecting off of muscle fibers similar to the light reflecting off of layers of structural pigments that give some fish an iridescent sheen. You can get this in really clear small fish with the light at the right angle. I have never seen as darkly pigmented Danionella as your dark one.




    @coelacanth and @pablito were breeding Danionella a few years back (maybe still are?) so might be able to help?



    Thanks for the reply, Bill. That’s what I figured regarding priapus, the male should have the modified pelvic fins as shown here: http://www.nhm.ac.uk/nature-online/species-of-the-day/collections/collecting/danionella-priapus/taxonomy/index.html 

    Either way these came straight from the Swedish wholesaler Imazo where they were labeled as D. priapus and they don’t usually change the names of the imported fish in my experience so somebody somewhere obviously thought it was a good idea to lable them as D. priapus. I’ve kept Danionella dracula before and they don’t really look like them, at least not the headshape that I’ve seen in mature D. dracula. 


    I have never seen any of the fishes carrying eggs, and like you say it would most likely be pretty obvious if they were. This behaviour stopped after a while though and the fish then returned to normal and swim around together at the front of the tank like they usually do. 


    I also read the article in Pete’s book and found it very interesting. These fish are already in a tank with gravel although not coarse enough for them to really dive into it.




    Oaken, the more I get dainonins and look at them closely, the more I am convinced of the truth of your words about naming actions of people in the supply chain between the fish in the wild and the people keeping them in their tanks.


    By the way, I looked at the pictures in the topic: Danionella dracula Britz, Conway Rüber, 2009 – from Myitkyina, Myanmar

    based upon Matt’s post about other threads on Danionellas. Although I have never had the priapus species, the second picture in that post indicates to me that it is a priapus not a dracula species fish. You can see a little thing projecting downward in front of the anal fin. Based on what I have read, this is what I would expect to see in this species.


    It would be nice if there more detailed and easily comparable pictures of these fish available. I would like to make an easily useable picture guide to ID’ing these fish. I guess good cartoons could suffice as an alternative. However, to see a lot of these details in fish that I have, I find a decent picture is a great help. Many small details can not be easily viewed in a small, moving, unmagnified fish.


    Below is a picture I took many years ago (in the 1990’s I think) of a Danionella translucida that we got in a lab I was in. Its not a great image, partly because it is a scan of a picture on a photographic projection slide (no good digital cameras back then) and the whites are now kind of blown out. But it was taken through a compound microscope at low mag and shows the short body axis of this species. This is a female that was knocked out and viewed on a microscope slide with a cover slip held up at the sides to keep from crushing the fish. You can see a series of clear blobs below clear air bladder. These are the eggs. At high mag you can see the nuclei in the oocytes (egg cells). I never got these fish to breed, but I did not know what I know know.


    These fish are great potentially research animals because you can see so much inside the bodies of live adults. However, the difficulty in breeding them makes it problematic. One of my “missions from god”.




    I kept Danionella dracula in 2011 and saw similar behaviour which was indeed quarreling. Aside from that, Ralf Britz told me the ones with a red spot (often visible) are females.


    @ BillT: Britz also identified my fish (from preserved specimens and photos) as D. dracula (bought them as ‘sp.’), if that helps?



    That is interesting Stefan. I suppose he should know the species if any one does. Do you know the specific features he used to ID them?

    I suppose a good picture of the front of the head could show the “teeth” but that might be a difficult shot to get but simpler in a dead specimen.

    I have over the years knocked out these fish for various purposes, but they have shown a disturbing tendency to not wake up after that. Since I don’t usually have a lot of them I try to take pictures of swimming fish in a photo box.


    I feed these fish heavily on brine shrimp nauplii, Water worms (like microworms), Moina, and a commerical baby fish powder (100 µm particle size). The females get surprisingly and obviously fat. This is the main way I tell the sexes apart.

    I got a bunch of Danionella in once that had some disease I think was velvet but I do not feel sure about that identification. The fish got less transparent and more of an opaque white in areas. They looked thin in the abdominal area, about the same thickness as the head and tail. Eventually many started dying so I nuked the tank with bleach.




    Identification is what’s stopped me doing additional profiles for these species so far. I do have a fair few good quality photos but am unsure if they’re correctly-identified or not.

    D. dracula appears to have a greensih lateral stripe but don’t know if that’s also present in congeners?



    If the greenish lateral stripe is iridescent (which is what it looks like in some pictures I have seen) I would bet that it has something to do with birefringence of light interacting with the muscle fibers. If it is due to birefringence, it may not be a reliable defining trait unless there is some difference between these species that only allows it to happen in one of them.


    Birefringence a process similar to the iridescent color of structural pigments but it can occur with light passing through the fish. The muscle actin or myosin fibers are periodically spaced at a molecular level (like guanine crystals). Each could reflect of refract the light resulting in a variety of light paths varying by certain wavelengths or fractions of wavelengths. This could then result through constructive or destructive interference in light from the different lights paths summing up to produce colors when looking at a particular place in the fish. Different angles can produce different colors in some cases because the length of the light path differences due to the trigonometry of the situation. This is seen a lot through microscopes in larval zebrafish.

    A shiny oil slick on water that shows different colors in different places or with different angles of light is a good example of this phenomenon. Light reflects off of the top and bottom of the oil layer, producing two different length light paths which looking at the same place that interfere to make a color. The light path differences, the nature of the interference, and the colors produced would vary with the angles of illumination, viewing, and local thickness of the layer of oil.



    That makes sense to me Bill, don’t know what others think?

    Re-reading the descriptions, colour pattern isn’t really mentioned but some potentially-useful differentiating characters between the four species include:

    Anal-fin rays: D. dracula 12-14, D. translucida 12-16, 17-20 in D. mirifica and 20-21 in D. priapus.

    Rows of pigment cells on dorsal surface of body: usually 1 short row, sometimes many scattered, in D. dracula, 3 rows in D. priapus, absent in D. translucida and D. mirifica.

    Pelvic fins: funnel-shaped in D. priapus.

    Based on the above, this is what I’d managed to tentatively id the pics we have as:

    Fish #1 D. dracula (12 anal-fin rays, scattered pigment cells on dorsal surface of body). Pic Peter Macguire.





    Fish #2 D.mirifica (18 anal-fin rays, no rows of pigment on dorsal surface, collected in West Bengal, India). Pic Beta Maharatvaraj.




    Fish #3 D. translucida (12 anal-fin rays, no rows of pigment on dorsal surface). Pic HW Choy. 




    Fish #4 Possibly D. priapus (at least 20 anal-fin rays, odd-shaped pelvic fins). Pic CK Yeo.




    This thread turned out quite interesting. Lots of knowledge being shared here which is what I like about this place.


    Regarding the green colour it would make sense what Bill says. I have two specimens that are greyish in colour and I suspect that they might be sick, although they have had this colouration for quite some time now and appear to be eating well like the rest. When I kept Danionella dracula for the first time I actually had a similar experience to what Bill described with the fish turning white and then dying. 


    And in my experience (and I’m pretty sure Mike Noren had this experience as well) was that mature Danionella dracula males would develop quite an “underbite” as can be seen in the photo in this link http://www.nhm.ac.uk/nature-online/species-of-the-day/collections/collecting/danionella-dracula/index.html

    I actually noticed now that it’s even mentioned on that site under “Taxonomy”:

    “Upper and lower jaws massive in large males, resulting in longer preorbital distance”



    This one appears to be developing it?


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