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Fishmonger Newsletter

Trout Goodeids & Construction Updates

April, 2022

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Fish Store Construction Progress

We have water in the aquariums! The system is up and running and is being calibrated and stress-tested. We are very close to having everything balanced and finalized, at which point we will be able to get the first fish in the tanks. The finish line approaches!!!

We Have Water in the AQUARIUMS!!! New Fish STORE Build.

Here’s the rip-rap creek we built to return our water to the main river. It is clean, clear, and cool.

Construction of the rip-rap creek.

Putting water in the aquariums for the first time!!!

Closeup pic of a vinyl hose delivering fresh water to the tank.

All tanks are filled, plumbed, and lit!

Air blower has been plumbed in and turned on. The blower itself is very quiet. The bleed valve…not so much. We need to get a diffuser on it to cut the noise down, but it is working very well.

The “big boy” pump that brings the water up to the building from the source. This is a 15’ long beast!

Surface well hardware as the water draw site was getting completed.

Firing up the boilers. These are amazing…almost 100% efficient so far.

The bank of the river getting stabilized as construction on the water draw site nears completion.

The local wildlife is taking up residence on our water draw location.

Featured Article

Trout Goodeids: Keeping & Breeding Ilyodon furcidens

Article by Andrew Fasbender

Edited by Dan Hodnett

If you told me I could only keep five species in my fishroom Ilyodon furcidens would be the first species I would list. Colloquially referred to as the ‘Trout goodeid’ I. furcidens receives that common name from a resemblance to the popular game fish, though this livebearer is much smaller at around 3.5 inches in length. As an aquarium subject this fish hits a sweet spot of interesting behavior, easy maintenance and a size big enough to draw the eye but not too big for moderately sized tanks. Best of all they are constantly moving and interacting with each other, usually placidly rather than in a frenetic or aggressive manner.

Ilyodon belongs to the family Goodeidae, a group endemic to the southwestern United States and Mexico. This group is a member of the order Cyprinodontiformes (colloquially known as toothcarps even though they are not closely related to carp). This order includes killifish along with the more familiar livebearers in the family Poeciliidae (which contains guppies, mollies, platies, swordtails, etc.). However, Goodeidae and Poeciliidae are not particularly closely related within Cyprinodontiformes, with goodeids closely related to Cyprinodontidae (pupfish) and Fundulidae (the topminnows), while Poeciliidae’s closest relations are Anablepidae (the ‘four-eyed fish’) and Rivulidae (South and Central American egg-laying killifish) (Helmstetter et al. 2016). Giving birth to live offspring evolved independently in the goodeids (though not all give live birth) and poeciliids. This is evident in their reproductive physiology: poeciliid embryos, though retained within the mother, still have a yolk sac which fuels their development (though there are a few exceptions like Heterandria that transfer nutrients from mother to offspring, see Pollux et al. 2009). Livebearing goodeid embryos instead have ‘trophotaenia,’ an analog to the umbilical cord seen in placental mammals, which extends out of their bellies and absorbs nutrients directly from their mother.

Goodeidae is split into two subfamilies, the egg-laying Empetrichthyinae (a group of three endangered species found in springs in Nevada, USA; GWG 2022) and the livebearing Goodeinae, consisting of 38 extant species endemic to west-central Mexico, with their epicenter of diversity in the state of Jalisco. Many goodeid species have extremely limited ranges, often only a single watershed and in some instances a single spring, and over half of them are endangered, with some species extinct in the wild (Lyon et al. 2019). While not endangered, I. furcidens is considered ‘Threatened’ by the Mexican government, and Lyons et al. suggest wild populations are currently declining. This is mainly due to the introduction of predators such as cichlids and bass for sport fishing, and the diversion of springs and rivers for agricultural use throughout the region.

The genus Ilyodon is easily distinguished from the other goodeids, which tend to be rather ‘husky,’ by its streamlined shape and long caudal peduncle (the base of the tail). Currently the genus contains two species: Ilyodon whitei and I. furcidens. Both are extremely similar in appearance and are difficult to differentiate without dissection, as the main character distinguishing the two is number of teeth in their jaw, with I. whitei having 40+ while I. furcidens has around 25. There have been over a half dozen other species names proposed in the genus, but these are treated as synonyms of the two accepted species. These other names were based primarily off of superficial and variable morphological features such as body shape and the width of the mouth, which have not been supported by genetic studies (Beltran-Lopez et al. 2017). Male and female I. furcidens show some sexual dimorphism, males developing yellow caudal and dorsal fins speckled with back as they mature as well as a gold color on the chest under the pectoral fins, but most distinctively possessing an andropodium (i.e. the anal fin being shaped like a mitten). Older males develop a convex back, somewhat resembling male salmon in breeding phase. Mature females grow longer than the males by about half an inch.

Ilyodon furcidens is found in six river systems flowing into the Pacific Ocean west of Guadalajara. The climate in the region is moderate, temperatures rarely dropping below 40F in the winter, but not becoming overly warm in the summer due to the altitude of much of its range and because most of the streams it resides in are groundwater fed. Small to medium rivers and creeks with moderate current are the preferred habitat, typically on rocky bottoms where the fish form large shoals (GWG 2022). Primarily a grazer, I. furcidens also feeds on aquatic invertebrates. Natural populations show some phenotypic plasticity, with individuals grazing off rocks developing a wider mouth than those with a higher proportion of insects in their diet (this morphological variation contributing to the description of synonymous species noted above) (GWG 2022). They are seasonal breeders, with young offspring recorded from February through July (GWG 2022).

That’s all well and good, I hear you say, but what about keeping them in aquariums? Greg Sage, the guru of rare livebearers, has been breeding this species for years and developed his own high pattern strain, where the black marking on the fins are much bolder in the males and even a present in the females (which have clear unpatterned fins in the wild type). I acquired my initial breeding stock from him several years ago and have been refining my keeping of the species ever since, so can provide some observations on their husbandry and breeding.

First, for tank size I recommend a 20 gallon long as a minimum for this species, since they get nearly four inches long and are quite active. A tank of that size can easily house a half dozen adults, possibly a few more if you are willing to keep up with maintenance – this species eats a lot and thus produces a proportionate amount of waste. While I keep my breeding colonies in 20 longs, my display tank is a 55 gallon; a group of a dozen or more adults in such a set up is an entertaining display, always in motion. My water runs a pH of about 7.8, and varies in TDS from 150-220 ppm depending on seasonal conditions, and I’ve seen several people keeping the species in extra hard, high TDS ‘liquid rock’ type water without issue.

Ilyodon do not require a heater for indoor conditions, and should not be kept at 80F or above since the water temperatures in their natural habitat never exceed the mid-upper 70s (and even drop into the 50s during the winter months). My tanks run between the low-mid 60s for 7-8 months of the year, and hit the low-mid 70s in the peak of summer. Most keepers recommend giving your Ilyodon a cool period for a couple months during the winter where the tanks temps stay under 68F, as this will extend their lifespan by giving them a break from the stress of breeding. That said, may people keep them in fishrooms that stay in the 70s year round with success. With proper care I. furcidens will live 3-5 years, and I still have some of the original individuals I bought from Greg Sage which are over four years old.

Despite being a rheophilic species, I have found that Ilyodon do perfectly fine without any significant current in the aquarium. For filtration I typically run a sponge filter, sometimes just an airstone in my more heavily planted tanks.

I. furcidens plays well with most plants. I have kept them with hornwort (Ceratophyllum demersum), Cryptocoryne sp., sword plants (Echinodorus sp.), frogbit (Limnobium laevigatum) Ludwigia sp., guppy grass (Najas guadalupensis), and Vallisneria sp. The one plant I have found they will eat is duckweed (Lemna sp.), especially if they are hungry – which should be a selling point for most of us. They will also pick at hair algae and seem to prevent it from establishing in their tanks, though from personal experience they will not clear out a full carpet of the stuff! One note on tank decoration: while Ilyodon won’t pace back and forth in the tank like some cyprinids or characins, they are an active fish that should have a lane of clear swimming space for most of the tank’s length.

As a shoaling fish Ilyodon like to be kept in a group, and I would suggest keeping groups of at least six. While they are generally bold, I’ve found that they do like to have some cover such as plants or rockwork for security, and any major upsets such as moving them between tanks or rescaping can make them shy for a period of time. That said, my established groups will come right up to the glass when I’m standing by the tank to beg for food and will engage in their full suite of behaviors even as I am sitting only two feet away. The most exciting of these are the sparring bouts between males, when they approach each other broadside and waggle their fins, sometimes escalating to short gyrations around each other. Despite their evident fervor, I’ve never seen the males contact each other in these contests. Males also engage in less intense displays towards females, apparently to impress potential mates.

Though I haven’t experimented much mixing them with other fish, Ilyodon seem relatively peaceful, and in one of my tanks I have a singleton Norman’s Lampeye (Poropanchax normani) that hangs out with a group of mature 3 inch+ I. furcidens adults. That said, if you put them with other fish I would keep a close eye for aggression, and also to ensure that the other species are getting access to food since Ilyodon can be somewhat frenzied in their feeding response. On the topic of food, Ilyodon are not picky. They will eat any quality flake or pellet of appropriate size, though I suggest providing at least some vegetable based foods since that is a major component of their diet in nature. Any frozen or live foods you offer will be taken with enthusiasm.

As for breeding, I’ve found Ilyodon to be some of the easiest in my fishroom to breed so long as a couple details are understood. Myself and several other people who work this fish have found they breed seasonally, which is likely controlled by water temperatures and day length. When the water temperatures start rising and the days are longer than the nights the males start actively courting females. Gestation is around 60 days, and I typically get two broods per year, one in July and a second in September (though I did get a surprise drop in January this year!). Females don’t look as obviously gravid as most poeciliids or some other goodeids, though they will get a bit more “square” when they are nearing their drop date. Litters are typically between six and ten fry, though larger drops from older females can happen. While not voracious fry eaters, I like to keep a corner of the tank heavily planted for the females to retreat to and drop their fry out of view of the other fish. For up to a couple hours after birth the neonates are rather helpless and lay on the substrate, making them vulnerable to other fish, though they quickly get control of themselves and hide towards the surface of the tank. For this purpose, I like to keep plants like Vallisneria, hornwort or guppy grass on the surface for them to hang out in and keep away from the larger fish, though I’ve never seen adults bother free swimming fry. If you look closely at new born fry you can see their trophotaenia, small tentacles that emerge from their belly-button region; these fall off after a day or so. Ilyodon fry are nearly a half inch at birth, and are much more developed than poeciliid fry – and weeks ahead of egglayers of equivalent age. They will readily accept crushed flake food, though offering larger powder foods like golden pearls certainly won’t hurt them. Once they are swimming and eating fry care is just standard grow out maintenance: feeding and water changes. I find that fry reach an inch within a couple months, sex out around 1.5 inches, and are ready to breed by nine months to a year of age.

As noted above, Greg Sage has selectively bred his line of I. furcidens for increased fin pattern, especially in the females, but there remains a lot of morphological variability in the population that could be selectively bred into additional strains by an interested hobbyist. One phenotype that deserves exploration are orange fins: some of my mature males have vibrant orange patches in the corners of their caudal fin and apex of their dorsal fin. It would be interesting to try and determine if this is a genetic trait, and if so can a fish with wholly orange fins be produced. Another highly variable trait is the black patterns on the flanks of the fish. Most photos I’ve seen of wild Ilyodon furcidens have a single continuous stripe along the lateral line, while most of the fish from the Greg Sage line have a constellation of black spots across their sides. Some fish from this line, particularly females, still have the stripe but with large diamond shaped splotches along its length. With careful selection it may be possible to create a new “diamond line” strain.

Overall, Ilyodon furcidens is an undemanding yet fascinating fish with something to offer in many facets of fish keeping, whether as the centerpiece of a planted show tank or a breeding project to fix a new strain. If you are interested in goodeids it is one of the best species to start with, and even if you aren’t drawn to that group the ‘trout goodeid’ is attractive solely on its own merits. That said, many have found Ilyodon to be a gateway to a goodeid addiction that leads to the pursuit of such fish as Xenotoca, Skiffia, and even the fabled ‘Black Prince’ Characodon…

References (multiple authors truncated)
Beltran-Lopez R.G. et al. 2017. Phylogeny and taxonomy of the genus Ilyodon Eigenmann, 1907 (Teleostei: Goodeidae), based on mitochondrial and nuclear DNA sequences. Journal of Zoological Systematics and Evolutionary Research 55: 1-16.

Goodeid Working Group (GWG). 2022. http://www.goodeidworkinggroup.com/ Accessed 27.iii.2022.

Helmstetter A.J. et al. 2016. Viviparity stimulates diversification in an order of fish. Nature Communications 7:11271: 1-7.

Lyons J. et al. 2019. Distribution and current conservation status of the Mexican Goodeidae (Actinopterygii, Cyprinodontiformes). Zookeys 885: 115-158.


Pollux B.J.A. et al. 2009. Evolution of Placentas in the Fish Family Poeciliidae: An Empirical Study of Macroevolution. Annual Review of Ecology, Evolution and Systematics 40: 271-289.