Cichlids, cichlidae are no doubt the classic model group for the study of evolution, they display a wide diversity of morphology in both the America’s and Africa (Arbour & López‐Fernández, 2024; Santos et al., 2023). There are 1727 species in the group cichlidae making this family currently the largest family of fishes. (Fricke et al., 2024). Cichlids both sides of the Atlantic are popular in the aquarium hobby, partially due to their diversity (no doubt colouration) but also their comparatively heightened aggression compared to other clades. No doubt aggression is where many people empathize with with these fishes.

What do cichlids specialize in?

Not unexpected, many species lack a complete understanding of their diet to say what they are actually feeding on. This is a vast topic given the number of species, there are algivores (algae specialists) such as reported in Tropheus (Wagner et al., 2009) to piscivory in peacock bass, Cichla (Aguiar‐Santos et al., 2018). No doubt given I said that Tropheus are reported algivores means I can’t find any solid evidence that they are, this is certainly a topic that needs the right wording and ambiguity included where it might be unsure.

Morphology

Much of the research into cichlid morphological disparity has long focused on the jaws. Cichlids like many fishes have two pairs of jaws. The oral jaws evolved for prey capture, the pharyngeal jaws evolved for prey processing (Fraser et al., 2009). To some extent the lips might also provide a purpose in some fishes in some fishes to move a surface to extract food further as seen in other fishes (Krings et al., 2023; Cohen et al., 2023).

These two pairs of jaws can be diverse in shape and structure but the teeth further vary between individuals and species.

Cichlids display a very traditional head shape that reflects most other fishes. The oral jaw is often gape limited and evolved to reach forwards to remove or capture a prey item. Realistically no cichlids lack this.

Earlier I mentioned Tropheus being algivores yet it doesn’t seem there is any solid evidence that they exclusively are but we can look at the fishes morphology. They are extremely similar to a fish we know likely consumes a lot of bacteria, algae and similar microbes, discus, Symphysodon (Crampton, 2008). These fishes are not related and this similar morphology is likely due to convergent evolution. Both display a shorter blunter head with strong lips, similar to silver dollars and pacu, Serrasalmidae who some force to break apart at that ‘herbivorous matter’ (Cohen et al., 2023), but they do take it to a more extreme level given they are feeding on plant and fruit matter not algaes and bacteria. Tropheus display some densely packed oral teeth (Richardson-Coy, 2017). This differential oral, mouth morphology could really be due to the differences in what is required to feed on different algaes and in fact, Tropheus are confirmed to specialize in diatoms (Richardson-Coy, 2017), diatoms cling to a surface so much more then the more loose algae/biofilm based diet of Symphysodon (Crampton, 2008) so the more numerous teeth would be more effective. Tropheus oral teeth remind me much more of the jaws of Baryancistrus and similar Loricariids who are scraping algaes of rocks. These teeth might be much more useful for feeding on those Discus, Symphysodon display reduced pharyngeal jaws (Burress, 2016) as this diet might not require the same level of processing as plant matter (hence Serrasalmidae have large robust oral teeth). It seems unclear as to the morphology At the end of the day algivores are diverse, no more needs said but just look at the diversity of algivores in Loricariidae, the queens of algivory/detritivory.

In comparison to Tropheus and Symphysodon would be those that specialize in fishes, Cichla as mentioned before has that large and explosive jaw to reach and consume fish. Fishes are a comparatively more difficult food item to obtain then anything herbivorous so quickly grabbing food much more ahead is beneficial.

The position of jaws is a big clue as to what fishes feed on, terminal mouth’s point forward inferring feeding in front of the fish and is usually associated with carnivory. An inferior mouth points downwards so feeding from the bottom, benthic usually associated with invertebrates and herbivory. Superior mouth’s point upwards and therefore specialize in feeding from the surface. We can clearly see while not extreme Tropheus particularly has an inferior mouth whereas Cichla, being a piscivore is terminal.

While these jaw shapes I mention cover the more traditional cichlids we cannot forget the earth eaters, those fishes who find their food in the substrate, shifting and moving around the sand or silt. While this is generally associated with many Geophagini such as Geophagus, Satanoperca but this clade does include species who are limited in this ability to move substrate e.g. Apistogramma, Mikrogeophagus. Distantly related to these other South American genera is Retroculus, another ‘earth-eater’ (Lopez-Fernandez et al., 2012). The substrate feeding trophic niche seems mostly associated with South American cichlids but it is found in the Rift Valley, although much of this behaviour might be more about breeding sites. Interestingly, it seems these substrate interacting cichlids are more then often mouth-brooders. Substrate digging seems much common in many more Rift Valley clades we keep.

The interesting thing some what I can infer from Burress (2016) is that while jaw shapes might be similar tooth shapes seem divergent between neotropical (fishes from the America’s) and old world (Africa, Asia etc.) regardless of a shared niche. This seems to move away from convergent evolution and multiple solutions to the same problem, the same problem as I noticed being a similar diet.

Liem’s Paradox

This is the biggest part of the cichlid story. To put simply Liem’s paradox is the fact that while fishes might display specialist diets and morphology, they still are capable of generalization. This theory is based on the behaviour and morphology of Rift Valley cichlids (Liem, 1980). This I can assume is some factor limiting morphology for further specialization in morphology, and we see these extreme specializations in other clades e.g. Loricariidae, Gymnotiformes and Moryridae. This plays out in the wild where algivores, Lepidophages (scale specialists) and other niches are shown to feed on fishes when given the chance (Golcher-Benavides & Wagner, 2019). Perhaps there is a nutritional reason, but just because a fish will eat something it doesn’t mean they eat it frequently or it is good for them. This plays out in our aquariums, discus Symphysodon will eat smaller tetra yet they are detritivores and Rift Valley algivorous cichlids (Hata et al., 2014).

It is well accepted that fishes feed and act very differently in the aquarium to how they would in the wild but it does occur in the fishes natural habitats (Golcher-Benavides & Wagner, 2019). This theory in fishes is largely based on Rift Valley cichlids, it’s quite clear that Tropheus are much more capable of generalism then Symphysodon. This opportunistic generalism would be limited by certain specialist morphology e.g. the body shape of Symphysodon or mouth shape of angelfish (Pterophyllum). The Symphysodon is limited in it’s ability to feed on fishes and invertebrates but the Pterophyllum is limited in it’s ability to eat algae’s.

This might not be so much of a simple topic with so many taxa. With the focus on Africa, there is so much we seem not to know about the South American clades despite their diversity. Regardless many cichlids while specialist do not seem to take it to the extreme, this might be behind their diversity but also why unlike clades like Siluriforme, they are limited in that morphological disparity.

References:

Aguiar‐Santos, J., deHart, P. A., Pouilly, M., Freitas, C. E., & Siqueira‐Souza, F. K. (2018). Trophic ecology of speckled peacock bass Cichla temensis Humboldt 1821 in the middle Negro River, Amazon, Brazil. Ecology of Freshwater Fish, 27(4), 1076-1086.

Arbour, J. H., & López‐Fernández, H. (2014). Adaptive landscape and functional diversity of Neotropical cichlids: implications for the ecology and evolution of Cichlinae (Cichlidae; Cichliformes). Journal of evolutionary biology, 27(11), 2431-2442.

Burress, E. D. (2016). Ecological diversification associated with the pharyngeal jaw diversity of Neotropical cichlid fishes. Journal of Animal Ecology, 85(1), 302-313.

Cohen, K. E., Lucanus, O., Summers, A. P., & Kolmann, M. A. (2023). Lip service: Histological phenotypes correlate with diet and feeding ecology in herbivorous pacus. The Anatomical Record, 306(2), 326-342.

Crampton, W. G. (2008). Ecology and life history of an Amazon floodplain cichlid: the discus fish Symphysodon (Perciformes: Cichlidae). Neotropical Ichthyology, 6, 599-612.

Fraser, G. J., Hulsey, C. D., Bloomquist, R. F., Uyesugi, K., Manley, N. R., & Streelman, J. T. (2009). An ancient gene network is co-opted for teeth on old and new jaws. PLoS biology, 7(2), e1000031.

Fricke, R., Eschmeyer, W. N. & Van der Laan, R. (eds) 2024.  ESCHMEYER’S CATALOG OF FISHES: GENERA, SPECIES, REFERENCES. (http://researcharchive.calacademy.org/research/ichthyology/catalog/fishcatmain.asp). Electronic version accessed 04 August 2024.

Golcher-Benavides, J., & Wagner, C. E. (2019). Playing out Liem’s paradox: opportunistic piscivory across Lake Tanganyikan cichlids. The American Naturalist, 194(2), 260-267.

Hata, H., Tanabe, A. S., Yamamoto, S., Toju, H., Kohda, M., & Hori, M. (2014). Diet disparity among sympatric herbivorous cichlids in the same ecomorphs in Lake Tanganyika: amplicon pyrosequences on algal farms and stomach contents. Bmc Biology, 12, 1-14.

Krings, W., Konn-Vetterlein, D., Hausdorf, B., & Gorb, S. N. (2023). Holding in the stream: convergent evolution of suckermouth structures in Loricariidae (Siluriformes). Frontiers in Zoology, 20(1), 37.

Liem, K. F. (1980). Adaptive significance of intra-and interspecific differences in the feeding repertoires of cichlid fishes. American zoologist, 20(1), 295-314.

Lopez-Fernandez, H., Winemiller, K. O., Montana, C., & Honeycutt, R. L. (2012). Diet-morphology correlations in the radiation of South American geophagine cichlids (Perciformes: Cichlidae: Cichlinae). PLoS One, 7(4), e33997.

Richardson-Coy, R. (2017). Feeding Selectivity of an Algivore (Tropheus brichardi) in Lake Tanganyika. PhD Thesis

Santos, M. E., Lopes, J. F., & Kratochwil, C. F. (2023). East African cichlid fishes. EvoDevo, 14(1), 1.

Wagner, C. E., McIntyre, P. B., Buels, K. S., Gilbert, D. M., & Michel, E. (2009). Diet predicts intestine length in Lake Tanganyika’s cichlid fishes. Functional Ecology, 23(6), 1122-1131.

While my fascination is largely with Loricariids, rasping species are interesting. Unlike the majority of fishes who are limited in what they can eat by the size of their mouth rasping fishes are not and yet ecologically seem very misunderstood. It’s also no lie that I have a soft spot for loaches and have kept quite the diversity of different species.

Balitoridae are those dorsio-ventrally flattened loaches from the group Cypriniformes, often referred to the superfamily Cobitoidea. So while Balitoridae are commonly confused with pleco’s, Loricariids they are actually more closely related to carp, barbs and minnows.

There are many genera we keep under this group in the trade, the most common being Sewellia lineaolata but otherwise followed by a variety of species from the genera Pseudogastromyzon and Gastromyzon. Occasionally other genera appear but as by catch many other Sewellia come into the trade such as SEW001 and S. breviventralis. If you want a rare fish and can do some research, it’s not difficult to find something rare or unusual in this group.

Not all have this extreme suction cup-like morphology, some are much more elongate.

The Habitats

As the name suggests these fishes seem to be highly riverine, rocks and high velocity water. There is little to no botanicals, wood and certainly no plants but this clear water (Randall et al., 2023) is where algae can thrive where plants cannot compete. These fishes thrive in a habitat more similar to some Ancistrus, Chaetostoma or Astroblepus would in South America.

Balitoridae Diets

Balitoridae no doubt shows likely a wide range of diets, there doesn’t seem to be the research on them. While few studies exist of these fishes diets the evidence suggests the genus Pseudogastromyzon feeds largely on algaes, both cyanobacteria and traditional Chlorophytic algaes (Fig 1; Yang & Dudgeon, 2010).

Although a later study inferred Pseudogastromyzon myersi feeds on 60-100% of their diet is algaes, with a lot of diversity. In comparison another genus with a similar body morphology, Liniparhomaloptera while feeding on largely algae’s displayed a diet of largely 6-20% invertebrates (Mantel et al., 2004). This is also displayed in Homaloptera sp. which feeds on around 13-14% insects, although the majority of their diet is detritus (Fuadi et al., 2016). As discussed before detritus is a vague classification and could realistically mean anything, it is most likely ‘bacteria’ over waste. A large amount of invertebrates consumed by Homaloptera is ostracods and aquatic larvae, it seems that they feed on minimal algae’s (Nithirojpakdee et al., 2014).

The most unusual thing seems to be this is the only study on the diet of Balitoridae, the majority of studies as with other groups has focused in their phylogenetics and taxonomy (Yang, 2008).

The Dietary Morphology

Surprisingly here we have a really good comparison of morphology just externally, compare Gastromyzon, Sewellia etc. with Liniparhomaloptera and immediately their head is much wider. We can some what assume a wider head correlates with a wider jaw and wider jaws, more numerous teeth correlates with a more algivorous diet (Lujan & Armbruster et al., 2012). If we look at the ventral morphology of these fishes one has much wider jaws then the other, Pseudogastromyzon has comparatively wider jaws and general head size then Liniparhomaloptera but is still wider then Homaloptera. The two former species, similar to Sewellia display jaws more similar to Loricariids, plate like jaws with numerous teeth.

There is some obvious jaw diversity in Balitoridae, the mouth of Gastromyzon is much wider then the curved mouth of Sewellia (Willis et al., 2019). I can assume that Gastromyzon is much more similar to Pseudogastromyzon ecologically and regarding diet although they are not closely related (Shao et al., 2020). There is no doubt either species probably feeds on a large amount of algae and other microbial films, but without any ecological records we can only assume.

The shape of the jaw isn’t just about taking up the food but can also be related to being able to extract that food item, while wide, those curved jaws could infer a niche involving extracting microbes from the cracks in rocks and between rocks. Although these cracks, fishers etc. are unlikely to have as many of these microbial films compared to where invertebrates might find refuge. There could be a rugosity aspect as if that habitat has rocks that are naturally bumpy it makes sense to have those more curved jaws then a long jaw which could be unable to access some of these films. A bit like a hoover, you have one part for large flat areas and another to deal with areas that aren’t so flat.

Unlike Loricariids I do not feel looking at their jaws they quite share the exact same niche as they seem to lack the same morphology. Perhaps this is exclusive to siluriformes and there are rasping catfishes in Africa and Asia.

Similar to Loricariids (Krings et al., 2023) it’s is likely not just about the skeletal anatomy that infers diet. The soft tissues at the mouth could play a role as displayed in Chen et al. (2023), there are many ridges on the mouth of the Pseudogastromyzon included these might play a role in sticking to surfaces in their high velocity environment but perhaps when feeding aid in the removal of algaes from a surface.

What should you feed hillstream loaches?

Regardless of the diversity of Balitoridae diets, the majority of their diet is still either detritus and/or algae’s. So I’d build up from that then maybe including a small range of frozen foods for them to forage for.

A brand like Repashy soilent green would be ideal, it is largely algaes with some invertebrates. Alternatively if you can find the algae based New Life Spectrum, AlgaeMax but a product under the same name is higher in fish meals. In the Bag Tropical Fishkeeping UK’s pleco pops would also be great.

It is really tricky to find diets that contain a reasonable volume of algae’s, even if they are listed as algae wafers. Vegetables and cereals do not make up nutritionally for algaes either. At the end of the day if you can’t get these diets then these fishes are not tricky to feed and making your own gel diet is possible, I will maybe write an article about that in future.

References:

Chen, J., Chen, Y., Tang, W., Lei, H., Yang, J., & Song, X. (2023). Resolving phylogenetic relationships and taxonomic revision in the Pseudogastromyzon (Cypriniformes, Gastromyzonidae) genus: molecular and morphological evidence for a new genus, Labigastromyzon. Integrative Zoology.

Fuadi, Z., Naira, K. B., & Hasri, I. 2016. HABITS EATING FISH OF ILI (Homaloptera Sp.) PESTAK RIVER DISTRICT IN CENTRAL ACEH INDONESIA.

Krings, W., Konn-Vetterlein, D., Hausdorf, B., & Gorb, S. N. (2023). Holding in the stream: convergent evolution of suckermouth structures in Loricariidae (Siluriformes). Frontiers in Zoology, 20(1), 37.

Lujan, N. K., & Armbruster, J. W. (2012). Morphological and functional diversity of the mandible in suckermouth armored catfishes (Siluriformes: Loricariidae). Journal of Morphology, 273(1), 24-39.

Mantel, S. K., Salas, M., & Dudgeon, D. (2004). Foodweb structure in a tropical Asian forest stream. Journal of the North American Benthological Society, 23(4), 728-755.

Nithirojpakdee, P., Beamish, F. W. H., & Boonphakdee, T. (2014). Diet diversity among five co-existing fish species in a tropical river: integration of dietary and stable isotope data. Limnology, 15, 99-107.

Randall, Z. S., Somarriba, G. A., Tongnunui, S., & Page, L. M. (2023). Review of the spotted lizard loaches, Pseudohomaloptera (Cypriniformes: Balitoridae) with a re‐description of Pseudohomaloptera sexmaculata and description of a new species from Sumatra. Journal of Fish Biology, 102(1), 225-240.

Shao, L., Lin, Y., Kuang, T., & Zhou, L. (2020). Characterization of the complete mitochondrial genome of Balitora ludongensis (Teleost: Balitoridae) and its phylogenetic analysis. Mitochondrial DNA Part B, 5(3), 2308-2309.

Wang, Y., Shen, Y., Feng, C., Zhao, K., Song, Z., Zhang, Y., … & He, S. (2016). Mitogenomic perspectives on the origin of Tibetan loaches and their adaptation to high altitude. Scientific reports, 6(1), 29690.

Willis, J., Burt de Perera, T., Newport, C., Poncelet, G., Sturrock, C. J., & Thomas, A. (2019). The structure and function of the sucker systems of hill stream loaches. bioRxiv, 851592.

Yang, Y. (2008). The ecology of a herbivorous fish (Pseudogastromyzon myersi: balitoridae) and its influence on benthic algal dynamics in four HongKong streams. HKU Theses Online (HKUTO).

Yang, G. Y., & Dudgeon, D. (2010). Dietary variation and food selection by an algivorous loach (Pseudogastromyzon myersi: Balitoridae) in Hong Kong streams. Marine and Freshwater Research, 61(1), 49-56.

Bloodworm has been a food that has been discouraged against for around 10 or more years. There has never been any doubt that many fishes feed on Chironomatid larvae in the wild (Delariva & Agostinho, 2001; Valtierra-Vega & Schmitter-Soto, 2000), even discus feed on them (Crampton, 2008). Chironomatid larvae being the more scientific name for bloodworm. You might have even seen these larvae in your pond, buckets or similar.

So why are bloodworms so feared?

I don’t think there is a true origin to this, it is still one of the most common live and frozen foods used, it’s cheap and easy to find. Maybe there are still some aspects of when tubifex was feared due to sewage contamination.

Bloodworm being the larvae of an insect does have a casing, known as an exoskeleton formed of chitin. This can be difficult to digest and for some fishes impossible, although many species are possibly sold as bloodworm it’s likely there is some diversity. There are reports of constipation and similar caused by bloodworm, although bloating is tricky and can have many causes from bacterial infections to a food item not being processed.

Bloodworm is not the entire diet of any fishes I’ve researched and when fed as an entire diet it results in liver enlargement and and poorer body condition (Žák et al., 2022), these results are similar to Hao et al. (2021) where removing to a formulated diet increases general health of the fish.

I chose this topic to find reasons that I am wrong, that bloodworm is much better then I thought it was. It doesn’t seem bad for fattening up fishes but does seem to come with a variety of problems, as a result I think I will continue to recommend Tubifex.

The real worm to try

Tubifex is a relatively common live, frozen and freeze dried food available. Unlike bloodworm it’s an annelid, true worm and is fully aquatic. It lacks the thick chitin casing of bloodworms, making them much more easy to digest. To cultivate they are less tricky given their lifestyle but do need a constant flow of freshwater.

Unlike bloodworm, Tubifex was vilified for years, this amazing genus is extremely adaptable. This adaptability has meant that Tubifex can be found in the most hostile and polluted habitats and at one time this is where they were collected for the aquarium trade. Due to being collected from very polluted habitats it was not the most ideal live food to avoid pollution. Frozen will be another story as most are gamma irradiated, killing most bacteria on them. But for many years Tubifex is farmed in a clean and sustainable matter.

Unlike bloodworm Tubifex is very high in nutrition (Herawati et al., 2016) and has shown to increase growth when mixed with a prepared diet (Alam et al., 2021) even when compared to other live foods (Mellisa et al., 2018).

Human health

Bloodworm is a known allergen leading to asthma (Wu et al., 2005; Nandi et al., 2014). As someone who is not a medical practitioner I cannot offer much advice, allergies can occur at any time and it is good to be cautious. When dealing with bloodworm, handling particularly wearing disposable gloves can be a good idea, using a tub to defrost it in. It does beg the question given it is quite a common allergen whether to use it at all? In my experience I have met very few people who are allergic to bloodworm.

References:

Alam, M. A., Khan, M. A., Sarower-e-Mahfuj, M. D., Ara, Y., Parvez, I., & Amin, M. N. (2021). A model for tubificid worm (Tubifex tubifex) production and its effect on growth of three selected ornamental fish. Bangladesh Journal of Fisheries, 33(2), 205-214.

Crampton, W. G. (2008). Ecology and life history of an Amazon floodplain cichlid: the discus fish Symphysodon (Perciformes: Cichlidae). Neotropical Ichthyology, 6, 599-612.

Delariva, R. L., & Agostinho, A. A. (2001). Relationship between morphology and diets of six neotropical loricariids. Journal of Fish biology, 58(3), 832-847.

Hao, Q., Teame, T., Wu, X., Ding, Q., Ran, C., Yang, Y., … & Zhou, Z. (2021). Influence of diet shift from bloodworm to formulated feed on growth performance, gut microbiota structure and function in early juvenile stages of hybrid sturgeon (Acipenser baerii× Acipenser schrenckii). Aquaculture, 533, 736165.

Herawati, V. E., Nugroho, R. A., Hutabarat, J., & Karnaradjasa, O. (2016). Profile of amino acids, fatty acids, proximate composition and growth performance of Tubifex tubifex culture with different animal wastes and probiotic bacteria. Aquaculture, Aquarium, Conservation & Legislation, 9(3), 614-622.

Mellisa, S., Rahimi, S. A. E., & Umiati, U. (2018). The effect of different live feeds on the growth and survival of comet goldfish Carrasius auratus auratu larvae. In IOP Conference Series: Earth and Environmental Science (Vol. 216, No. 1, p. 012025). IOP Publishing.

Nandi, S., Aditya, G., Chowdhury, I., Das, A., & Saha, G. K. (2014). Chironomid midges as allergens: evidence from two species from West Bengal, Kolkata, India. Indian Journal of Medical Research, 139(6), 921-926.

Valtierra-Vega, M. T., & Schmitter-Soto, J. J. (2000). Feeding habits of cichlid species (Perciformes: Cichlidae) in Caobas lake, Quintana Roo, Mexico. Revista de Biologia Tropical, 48(2-3), 503-508.

Wu, K. C., Räsänen, K., & Hudson, T. J. (2005). Fishing for allergens: bloodworm-induced asthma. Allergy, Asthma & Clinical Immunology, 1, 1-2.

Žák, J., Roy, K., Dyková, I., Mráz, J., & Reichard, M. (2022). Starter feed for carnivorous species as a practical replacement of bloodworms for a vertebrate model organism in ageing, the turquoise killifish Nothobranchius furzeri. Journal of Fish Biology, 100(4), 894-908.

Little seems known about the diet of Corydoradinae, it might be due to a generalization of the genus but also a lack of understanding of their ecology. For scientists it is a relatively small genus from a small family, Callichthyidae which also contains genera such as Megalechis, Hoplosternum and Aspidoras.

There is no doubt that Corydoras feeds largely as a carnivore but as a term that is very vague. These fishes are hardly hunting down capybara that enter the water or swarming round the carcasses of fishes. Carnivory purely just refers to the fact an animal eats an animal, and what is defined as an animal is just Animalia which is a gigantic category of organisms. Animalia covers from the simple sponges and corals all the way to molluscs, mammals and fishes, it includes some strange organisms as well like bryozoa and jellyfish. Each of these animals will have different nutritional compositions, some toxins but also accessibility. It is well known that insectivores (carnivores that specialize on insects) are not able to access nutrition from fishes, mammals etc. efficiently (Žák et al., 2022), this also increases nitrogenous waste. There might be other aspects of nutrition commonly missed such as perhaps the importance of chitin? It’s very difficult to digest, too much and the food item wont be processed, too little might result in blockages. It reminds me very much of the bloodworm used in the aquarium trade having a strong chitin casing compared to the chromatid larvae the fishes are likely eating in the wild. I have seen bloodworm pass out the fish as if it hadn’t been eaten at all, even carnivorous fishes. Not just does carnivory cover a wide range of different nutritional profiles and species but also modes of feeding. It is very different to catch and feed on a whole fish as it is to maybe feed on scales (lepidophagy), break down snails (durophagy), maybe extract a snail from it’s shell etc. Carnivory is so obviously diverse compared to herbivory but it still is best visualized like a field of grass, all grass specialists but put sheep, cattle and horses out and they will all feed on very different parts of the grass.

What I am emphasizing is not to generalize any dietary category and just fall to the general diets for fishes. Carnivory is just a man made category regardless and doesn’t reflect realistically aquatic dietary niches.

The Wild Corydoras Diet

Corydoradinae has previously been identified as an omnivore by Nijssen (1970) although only used aquarium fishes as evidence. This paper records them feeding on fallen leaves of which I find particularly strange. Although a particular fondness for invertebrates is noted, particularly tubifex and daphina, identifying the worms using their highly evolved sense of smell/taste. This record is later referenced in Alexandrou et al. (2011) but also noted that they feed on algae, insects, zooplankton and annelids. Algae I would not be surprised that is consumed but I do not believe they are targeting it, much like invertebrates are found in small numbers the guts of grazing Loricariids.

Isotope analysis was used to compare different lineages of Corydoras and identify any partitioning in where and what they feed on. Different lineages display divergent nutritional profiles between different genera of Corydoradinae based on head shape, eye placement and body depth. There is a clear difference between the diet of the longer snouted and shorter snouted Hoplisoma and Gastrodermus, I can assume due to the depths of substrate that can be exploited by either. These longer snouted, Corydoras are referenced as feeding on a lower trophic level (Alexandrou et al., 2011). This could infer on feeding on more algaes but maybe those lower trophic level invertebrates such as worms who would be lower down in the substrate then predatory invertebrates.

The shorter to medium snouted Hoplisoma paleatum, the peppered ‘Corydoras‘ is recorded as feeding largely on fly larvae such as chromatid’s with a small addition of nematode’s. Algae and plant fragments are recorded in the gut but near minimal volumes, less then substrate ingested (Bertora et al., 2021).

So the picture of what Corydoradinae eat in the wild is unclear.

The Dietary Morphology of Corydoras

Again, an unclear topic. We know Corydoradinae have oral jaws (Fig 1) at the front of the head humongous with our the general vertebrate jaw. These oral jaws contain teeth (Huysentruyt et al., 2011) but there seems to have been no exploration of the diversity of these teeth. These oral jaws are very similar to other invertivores (feeds on invertebrates), being elongate to extract food items out of crevices or the substrate.

What is not researched is the secondary pair of jaws found in most fishes, the pharyngeal jaws. While the oral jaws in fishes are often involved in prey capture, the pharyngeal jaws are involved in prey processing, so the grinding and breaking down. These are at the back of the mouth so aren’t obvious but when you see a fishes head move after feeding it’s likely those jaws are moving. Corydoradinae do have pharyngeal jaws, they contain teeth (Huysentruyt et al., 2011) but we have no idea how this morphology differs across Corydoradinae. In Osteogaster aenea Huysentruyt et al. (2011) identified elongate pharyngeal teeth which would confirm that at least O. aeneus is not evolved to feed on snails but the jaws do seem some what robust. It contrasts from those species that feed on algae to any extent who seem to have much more simplistic and often bladed pharyngeal anatomy. Most research into pharyngeal anatomy focuses on cichlids of which might not be the best reference given differential feeding behaviour.

Head shape can tell a lot about the fish, these fishes have such inferiorly facing mouths will be feeding around the substrate. What is more interesting is the shape of the snout mentioned earlier but we know so little about it. Those elongate snouts certainly allow the fish to dig deeper for food but how it effects them we only have clues.

What should I feed my Corydoras?

These are certainly not feeding on fishes, there is also the misconception that because fishes die that all fishes have access to them. Generally weaker individuals would be picked up by predators before they die and any dead fish would be more quickly exploited by species evolved to detect and quickly feed on carcasses. So fish meal is logically best avoided. These fishes also do not feed on plants so cereals are certainly of little use. This means those general diets are not great for them, most containing fish meal, cereals, vegetables etc. usually in that order.

Luckily for carnivores there is a few options for insect based foods. Fish Science is generally good and they offer some diversity. Fluval bug bites can be okay but still has quite a lot of fish meal and cereals in it. Repashy bottom scratcher is certainly worth looking at, being a gel diet you can add additional ingredients to it. I wouldn’t be afraid to use either Fish Science or Repashy bottom scratcher as the basis of the diet then frozen and freeze dried foods to build on it creating a more well rounded diet. Certainly live foods are worth looking at and would offer a lot of enrichment.

Corydoradinae are very forgiving regarding diets so experimenting is certainly possible.

What about frozen foods?

Frozen foods are great but often will not contain all of the nutrition a species might require. In the wild most fishes will feed on hundreds of species and the narrow range of frozen foods available likely doesn’t compare nutritionally. These are great as enrichment or an addition to a fishes diet but not as a complete diet.

Protein Blisters

This is something that will always need mentioning regarding diets. I am not convinced it is caused by too much protein, even protein that isn’t absorbed/taken up by the fish e.g. excreted as nitrogenous waste. There is no evidence either way, it is clear bacteria can cause blisters and cysts but it can’t be said every cyst is caused by them without exploring further. Some frozen/live foods such as bloodworm can harbor Aeromonas (Senderovich et al., 2008) so it is difficult to make assumptions but there is little research on that.

A suggestion has been a form of gas bubble disease, true gas bubble disease as I call it though is caused from supersaturation of gases in the water, or some change in pressure resulting in bubbles forming in tissues very rapidly like the bends. It kills extremely rapidly as those bubbles form and burst blood vessels. There are other similar diseases like I have seen extreme algae growth associated, not proven though, to cause bubbles in some fishes but doesn’t kill and they are very localized.

We see similar in Loricariids, also armored and personally I would associate it with an infection of some kind. Blisters are tricky though as it’s such a general pathological symptom it could mean anything.

References:

Alexandrou, M. A., Oliveira, C., Maillard, M., McGill, R. A., Newton, J., Creer, S., & Taylor, M. I. (2011). Competition and phylogeny determine community structure in Müllerian co-mimics. Nature, 469(7328), 84-88.

Bertora, A., Fontanarrosa, M. S., Grosman, F., Sanzano, P., & Rosso, J. J. (2021). Trophic ecology of the Neotropical tolerant fish Corydoras paleatus under the influence of contrasting environmental conditions in a prairie stream. Anais da Academia Brasileira de Ciências, 93, e20200981.

Huysentruyt, F., Geerinckx, T., Brunain, M., & Adriaens, D. (2011). Development of the osteocranium in Corydoras aeneus (Gill, 1858) Callichthyidae, Siluriformes. Journal of Morphology, 272(5), 573-582.

Lowe, A., Summers, A. P., Walter, R. P., Walker, S., & Paig-Tran, E. M. (2021). Scale performance and composition in a small Amazonian armored catfish, Corydoras trilineatus. Acta Biomaterialia, 121, 359-370.

Nijssen, H. (1970). Revision of the Surinam catfishes of the genus Corydoras Lacépède, 1803 (Pisces, Siluriformes, Callichthyidae). Beaufortia, 18(230), 1-75.

Senderovich, Y., Gershtein, Y., Halewa, E., & Halpern, M. (2008). Vibrio cholerae and Aeromonas: do they share a mutual host?. The ISME journal, 2(3), 276-283.

Žák, J., Roy, K., Dyková, I., Mráz, J., & Reichard, M. (2022). Starter feed for carnivorous species as a practical replacement of bloodworms for a vertebrate model organism in ageing, the turquoise killifish Nothobranchius furzeri. Journal of Fish Biology, 100(4), 894-908.