Can fishes build things?
Stéphan G. Reebs
Université de Moncton, Canada
2009-2020

When biologists want to give examples of animals being able to build things, fishes
don’t readily come to mind. It’s understandable. Fishes have a hard time building
things because they are ill-equipped for the job. Unlike land vertebrates, they don’t
have digits with which to hold things. They can only shovel earth with their fins, or
carry earth and small objects in their mouths.
Yet, some fishes do build things. Their methods can be classified into three
categories:
1) Excavations: digging up depressions or burrows in gravel, sand or mud;2) Pile-ups: piling up sand, pebbles, mud, shells or twigs to build mounds orsimple walls;
3) Gluing: sticklebacks use kidney secretions to glue plant bits together and form
nests for their eggs, while gouramis and some armoured catfishes use mucus
to create bubble nests.
Following are some specific examples of building in the fish world.

The burrows, turrets and walls of mudskippers
Let’s start with mudskippers – a group of 25 species from the goby family. Found on
the coastal mudflats of Africa, Southeast Asia and Australasia, they are well adapted
to an amphibious lifestyle. They are also good excavators. Using their mouths tocarry mud, they dig up burrows that can be as much as 2 m deep (about 6 feet),though most reach only half that depth. The burrows are shaped like a J, or, when
they have two entrances, like a Y. (Burrow shape can be determined by pouring resin
into the burrow and making a cast of it, after having flushed out the resident ofcourse.) Mudskippers stay in their burrow at high tide, when the mudflats are underwater, so as to avoid the predatory species that cruise the shallows. At low tide, they
emerge and go about their business on the exposed mud, but they are wary and retreat
to their burrows at the least sign of danger.1
At least two species, the giant mudskipper Periophthalmodon schlosseri and thewalking goby Scartelaos histophorus, build a special chamber at the bottom of theirburrows into which they carry mouthfuls of air. Once released the air accumulates at
the top of the chamber and forms a reserve from which the fish can breathe – like allamphibious fishes, mudskippers are good air breathers. The usefulness of this air

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1reserve must be related to the fact that water in the burrow typically contains little
oxygen.2
Two other species, Periophthalmus argentilineatus and Periophthalmus minutus,
build mud turrets several inches high around the entrance of their burrow. They dothis by depositing mouthfuls of mud. The role of these turrets is unclear. They may
constitute observation posts from which the fish may check for the presence of
predators before “stepping” onto the mudflats, or, given that the water in the turrets iswarmer than in the burrow, they may act as little saunas where the fish can warmthemselves up and activate their metabolism before the day’s activities.3
Mudskippers maintain territories around their burrows. When the population getscrowded, little mud walls a few inches high start to appear at the territorialboundaries. At the highest densities, continuous walls delimit polygon-shaped
territories. The whole mudflat looks like a mosaic of mostly pentagonal andhexagonal-shaped enclosures, each slightly less than 1 m (3 feet) wide. The walls areseveral inches high and made from piled-up mouthfuls of mud. They keep aggressiveinteractions between neighbours to a minimum (“out of sight, out of mind”). Whenmischievous researchers knock down walls, mudskippers assiduously start to rebuild
them, but until the fortifications are completed there is a measurable increase in
hostility between neighbours. Fortunately, in summer the mud walls harden under the
hot baking sun and can last up to several months, even though they are regularly
exposed to the tides.4

The burrows of red band-fish, Cepola rubescens
The technique of pouring resin down a burrow to obtain a cast has also been applied
to the red band-fish off the coast of Lundy, an island in the Bristol Channel, UK. The
red band-fish has a long ribbon-like body, well suited to burrow living. The fishstudied at Lundy were 12 to 74 cm long (about 5 to 29 in) and lived on the sea bottom
at a depth of 10-20 m (about 33-66 ft). There they excavated burrows in mud.
Researchers cast a total of 13 burrows (after chasing the occupant away with a narrowrod) and dipped a ruler into 117 more. 5
The burrows turned out to be single shafts with a funnel-like entrance and an enlarged
section at the bottom. Their mean depth within the sediments was 49 cm (19 in). The
deepest burrows, which belonged to the largest males, were up to 1 m deep (3 ft). The
shafts were mostly vertical but lateral deflections were sometimes forced by thepresence of buried stones or shells. Many of the burrows featured a side branch at
mid-depth, pointing slightly upward. In cross section the burrows were elliptical, withenough room to allow pectoral fin movements and body undulations.
Some fish were captured and taken into captivity where their burrowing behaviour
could be observed. The fish excavated using their mouth, though they also sometimes
pushed the mud aside with their body. Mouthfuls of mud were dumped no more thana body length away from the entrance. Between bouts of mud transport the fish
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2regularly “coughed”: they opened their gill covers and forcefully passed water
through the gills to dislodge any mud particles trapped in them.6 After 6 h of hard
work the burrow was deep enough for the fish to hide completely inside. Further field
and laboratory work showed that even when a burrow is finished, maintenance work
is still necessary to prevent siltation and to repair collapsed areas. The fish do thiswork mostly at dawn and dusk.
It has been calculated that up to 6 liters (about 1.5 gallon US) of sediments can bedisplaced by the construction of one large band-fish burrow. For an average-sizeburrow, the volume is 3 liters.

The roofed burrows of yellowhead jawfish, Opistognathus aurifrons
This small fish (maximum length 10 cm, or 4 in) builds burrows 11-22 cm deep
(about 4.5-9 in) in rubbly sand. The burrow usually takes the shape of a chamber
excavated underneath a rock, or a chamber lined with coral fragments to solidify it in
the absence of a rocky roof. One individual was filmed in the wild building a new
burrow after a large pebble fell into the entrance of its former home (the fish had tried
to dislodge the obstacle, but without success).
Using its mouth the jawfish first excavated a funnel-shaped crater, 10 cm deep (thismeasure included the height of the rim formed by the displaced sand). Then, again
using its mouth, it carried and dropped pebbles into the crater, slowly filling it, all thewhile maintaining a central vertical tunnel that gave access to the bottom of the hole.
It then alternated bouts of pebble carrying with the excavation of a chamber at thebottom end of the tunnel. Eventually the chamber was complete and the crater above
was full of pebbles except for the masonry-lined central tunnel. Together with smallburied rocks, the pebbles formed a solid roof for the chamber. Finally, the fishcovered the pebbles at the surface with sand. The whole operation took 8 h tocomplete.7

The burrow of the convict blenny ( = engineer goby, = convict fish),Pholidichthys leucotaenia
This fish from the southwest Pacific can be up to 50 cm (20 inches) long. It is shaped
like an eel and adults live in male-female pairs in single burrows dug in coral reefs.
The burrow is in fact a labyrinth of tunnels, estimated to be as much as 6 m long (20feet). And it needs to be that big because it also shelters all the young the pair
produces. At dawn, hundreds of young, each 1-10 cm long (0.4 to 4 inches), leave theburrow to feed on plankton in the water column. During that time, the parents clear
the burrow of sand and debris, spitting out as much as 3 kilos (6.6 pounds) of sand ina single day. At dusk, the young stream back inside the burrow through its single
entrance. At night, the young hang from the ceiling of the tunnels, suspended by athread of mucus stuck to their head (this was observed by inserting an endoscope – awww.howfishbehave.ca

3flexible tube of fiber optics equipped with a lens at one end and a camera at the other– inside some of the burrows).
It seems the parents never leave their burrows; so how do they feed? Nobody knowsfor sure, but hypotheses include micro-organisms living inside the sand the fish digout, old mucus threads and faeces produced by the young, or even food regurgitatedby the young to their parents. For more information, see the following web sites:
http://ngm.nationalgeographic.com/ngm/0506/resources_cre.html
http://www.mote.org/index.php?src=directory&view=magazine&refno=56&srctype=magazine_detail
http://www.youtube.com/watch?v=gTlPFG6_Ly8 From episode 4 of the Life series
produced by the BBC.

The spawning chamber of the moga ( = Nicaragua cichlid), Hypsophrys
nicaraguensis ( = Cichlasoma nicaraguense)
At the 2007 Cichlid Classic meeting in Chicago, researcher Ron Coleman reported
the following observation he made in the field. He found moga eggs lying at the endof a foot-long (30 cm) tunnel – such a deep nest may explain why the eggs are nonadhesive, an oddity among New World cichlids. He also observed some of these fishin the act of digging the tunnel: they were spinning inside, like a living drill. The
dorsal fin of the spinning fish raked the side of the tunnel, enlarging it.

Various examples of mounds
Six families of fishes – cyprinids, gobiids, malacanthids, cichlids, tetraodontids, andlabrids – include species that build mounds.
Cyprinids: In North American streams, the male cutlip minnow Exoglossum
maxillingua, 90-115 mm long (3.5-4.5 in), assembles mounds that are 75-150 mm
high (3-6 in), 30-45 cm in diameter (12-18 in), made up of more than 300 pebbles 1319 mm in diameter (a quarter to half an inch). The fish carry these pebbles one by one
in their mouths, sometimes stealing some from the mounds of other males. The
females deposit their eggs on the upstream slope of the mounds, and the males coverthese eggs with more pebbles. Males of the hornyhead chub Nocomis biguttatus, 90
mm long (3.5 in), and of the river chub Nocomis micropogon, 100 mm long (4 in),also build mounds during the reproductive season. They start by clearing a slight
depression in the substrate, which they overfill with up to 10,000 pebbles until the
mounds are 60-90 cm (2-3 ft) long (in the direction of the water current), 30-90 cm
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wide (1-3 ft), and 5-15 cm high (2-6 in).8 Females lay their eggs among those
pebbles. They are presumably impressed by the biggest mounds, as these are built by
the biggest and strongest males. The stone accumulation is free of sand and it exposes
the eggs to a good water current that supplies oxygen.
Gobiids: Gobies of the genus Valenciennea live in male-female pairs that excavateand share burrows. Several burrows can be present within the same territory. They areused as refuge, as a sleeping place, and also as a place to tend eggs during the
reproductive season. Coral rubble is often heaped by the fish over the burrow
entrances. In a Red Sea population of twostripe gobies Valenciennea helsdingenii,
such mounds were found to be as high as 30 cm (about 1 ft), with base diameters of
25-60 cm (about 2-3 ft). Not bad for a pair of fish that was only about 15 cm (6 in) in
length! A team of divers led by ichthyologist Eugenie Clark, of the University of
Maryland, removed approximately half of a medium-size mound and counted no lessthan 4062 individual pieces of coral rubble, shells, and shell fragments (so double that
number for the whole mound). There were also a few odd pieces such as worm tubes,
egg cases, and even one watermelon seed! While building their mounds, goby pairs
could collect and carry as many as 9 pieces of coral rubble per minute.9
In the long-finned goby Valenciennea longipinnis, pairs build a mound over one of
the two entrances of their spawning burrow. This mound, 6-13 cm high (about 2.5-5in), helps ventilate the burrow. The mound deflects water currents, which increases
water speed in that spot, which in turns lowers pressure. Pressure is therefore lower atthe entrance surrounded by a mound than at the other entrance. This pressure
differential generates a water flow within the burrow. This is particularly usefulduring the parental phase, as the water flow can supplement the fanning done to theeggs by the caring male. This functional role of mounds was demonstrated byTakeshi Takegaki and Akinobu Nakazono of Kyushu University. The two scientists
injected ink at the mound-less entrance of burrows and observed that the ink wasquickly sucked into the burrow and soon percolated through the mound at the otherend. When the mound was removed, the ink took longer to come out. Also, theconcentration of oxygen was found to be higher in burrows with mounds than inburrows whose mounds had been removed. Takegaki and Nakazono reported thatover the four days of the parental phase, males stayed inside their burrows to fan theeggs while females stayed outside to maintain the mounds. When females wereexperimentally removed, the mounds soon lost their height, and many males –perhaps discouraged by the impoverished ventilation of their burrow – abandoned
their breeding attempt and ate the eggs they were caring for.10
Malacanthids: At least five species of tilefishes build mounds of coral rubble over theentrance of their burrows: Hoplolatilus fronticinctus, H. geo, H. pohle, Malacanthus
brevirostris, and M. plumieri. The mounds can be as high as 80 cm (31 in) and theiroval-shaped base can cover as much as 2 x 3 m (about 6 x 9 ft). From partial
sampling it has been estimated that the largest mounds contain as many as 200,000
individual pieces of coral rubble. Such mounds are probably the largest built
structures in the fish world, but even for tilefish they represent an extreme. In one
study of 33 mounds built by M. plumieri (the sand tilefish) off the coast of Colombia,
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5the minimum size was 85 x 67 x 15 cm, the maximum was 208 x 198 x 17 cm, andthe average was 133 x 102 x 22 cm (52 x 40 x 9 in). The fish themselves were about
20-50 cm long (8-20 in). The mounds, and of course the burrows associated withthem, may offer protection against predators such as sharks. 11
Cichlids: Males of many mouthbrooding cichlid species in Lake Malawi and Lake
Tanganyika, Africa, build sand cones that are flattened or crater-shaped on top. Thesemounds are sometimes called bowers, because like the elaborate constructions ofbowerbirds they are solely meant to attract and impress females.12 They do notprovide shelter for the fish nor for their eggs. Some of these sand mounds are huge:they can reach 3 m in diameter (10 ft) and 40 cm in height (16 in). The males
themselves are all under 25 cm in length (10 in).13 Some females seem to prefer tomate with males whose sand mounds are bigger and/or with a rim of even height,
suggesting that good building is taken as a sign a good genetic quality or good
health.14 However, in some species there is no preference for bigger sand castles, andfor them it has been argued that the structure allows species recognition (Lake
Malawi and Lake Tanganyika contain complexes of numerous and very similarlooking cichlid species).15
Tetraodontids: Male pufferfishes Torquigener sp. also build sand mounds to attract
females. The mounds are circular, up to 2 m in diameter, and quite beautiful, showing
a radiating pattern of ridges and valleys. You can view the fish building their mounds
at http://www.youtube.com/watch?v=MciAVgTuIGM and read the scientific reportat http://www.nature.com/srep/2013/130701/srep02106/full/srep02106.html. 16
Labrids: Many wrasses sleep buried in sand at night. Around the time of sunset they
dive into loose sand and spend the night there. Two species go the extra mile andmake sleeping mounds (or sleeping nests). The Jordan’s tuskfish Choerodon jordani
uses its head to scrape out sand from under coral rubble, and then it adds coralfragments over the site. It slips into this nest just after sunset. For its part the
rockmover wrasse Novaculichthys taeniourus, 14-26 cm long (5.5-10 in), carries sandin its mouth to make a small mound which it covers with pieces of dead coral. Then it
moves a few pieces of coral aside and dives into the sandy mound, where it spends
the whole night. The coral bits presumably hinder or deter nocturnal predators. A
single fish can make more than one such mound every evening, spending an averageof 34 minutes on the task. Larger individuals have the gall to steal the sleeping
mounds of smaller fish at sunset, and that’s probably why some fish make more than
one nest. 17

Depression nests
Many fish species dig up and clear a depression in the substrate where the female will
lay her eggs and where the young will be defended. Examples include the malebowfin, Amia calva, which cuts or pulls out all reeds in a circular area 38-60 cm(about 15-24 inches) in diameter; and the male smallmouth bass, Micropterus
dolomieui, whose nest can reach 185 cm (6 feet) in diameter.18
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In preparation for laying their eggs (which will not be defended later on), female
Pacific salmon use their fin to dig large bowl-shaped pits in gravel. In the case of avery big Chinook salmon, these depressions can be 0.4 m deep (a foot and a half) andcover an area the size of two parking lot stalls. It has been estimated that salmon
spawning in southwestern Alaska disturb 30 % of the available streambed in thismanner –and this is in populations that are not at their peak numbers, since they are
fished commercially.19

Vegetation nests
Male threespine sticklebacks, Gasterosteus aculeatus, build vegetation nests at the
beginning of the reproductive season. A male will start by digging a shallow pit insand or mud with his mouth. Then he places slender pieces of vegetation in the pit.
This material is glued together with mucus secretion from the male’s kidneys. The
gluing behaviour is easy to recognize, as the male swims over the nest with peculiar
movements of the pectoral and anal fins, bringing his cloacal region in contact withthe vegetation. When enough material has accumulated, the male starts pushing his
snout into the heap. Eventually the male creeps through the heap, forming ahorizontal tunnel. It takes him about five hours to build such a nest. A study in the
U.K. has found that nest characteristics (number of pieces used, area of the nest, totalmass) differ between males but are consistent within males.20
During spawning the female swims through the tunnel and deposits her eggs there.
The male follows to release his milt. The male then guards the nest and fans water
into the entrance to bring oxygen to the eggs. Sometimes he pokes holes into the nest
walls, presumably to allow better water circulation during fanning.
Other stickleback species build nests in a similar fashion. There is a twist in the caseof the ninespine stickleback Pungitius pungitius. Here the male builds his nest off theground, among the branches and leaves of aquatic vegetation. He also displays aparticular gluing behavior in addition to the “standard” one. First he bends his head
and tail towards each other. A drop of glue is extruded from the cloaca, forming athread which the fish grabs in his mouth. He then bores into the mass of vegetation
pieces he has accumulated and deposits the thread there. This probably helps toconsolidate the inside of the nest.
The male sea stickleback, Spinachia spinachia, starts his nest by laying down a
network of glue threads in a dense patch of seaweed. Then he adds pieces ofvegetation to the matrix until a spherical mass of material is obtained. 21
A few other fishes beside sticklebacks make vegetation nests, though glue does notseem to be involved in the building process. For example the aba, Gymnarchus
niloticus, makes a floating nest from weeds that it uproots or nips free. The nest canbe 0.5-1.0 m long (20-40 in). The female lays her eggs inside this mass of plants andthe male subsequently guards it.
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Bubble nests
Male gouramis (anabantoids) are well known for their capacity to produce floating
bubble nests. Some armoured catfishes from the family Callichthyidae can also
produce foam nests. In all cases the bubbles or foam are produced by mixing air withmucus in the fish’s mouth. The mucus makes the bubbles stick together. Bits of plants
are often incorporated into the bubble nest. The eggs released by the female in midwater are picked up by the male, who spits them into the mass of bubbles. The malethen guards the nest. If any egg falls down from the nest, the male picks it up in his
mouth and spits it back into the nest.

1

Clayton, D.A., and Vaughan, T.C., 1986, Territorial acquisition in the mudskipper Boleophthalmus
boddarti (Teleostei, Gobiidae) on the mudflats of Kuwait, Journal of Zoology (London) 209: 501-519.
2Ishimatsu, A., Hishida, Y., Takita, T., Kanda, T., Oikawa, S., Takeda, T., and Khay Huat, K., 1998,
Mudskippers store air in their burrows, Nature 391: 237-238; Lee, H.J., and Graham, J.B., September
2002, Their game is mud, Natural History 111: 42-47.
3

Lee, H.J., and Graham, J.B., September 2002, Their game is mud, Natural History 111: 42-47.

4

Clayton, D.A., 1987, Why mudskippers build walls, Behaviour 102: 185-195; Clayton, D.A., andVaughan, T.C., 1986, Territorial acquisition in the mudskipper Boleophthalmus boddarti (Teleostei,
Gobiidae) on the mudflats of Kuwait, Journal of Zoology (London) 209: 501-519.
5

Atkinson, R.J.A., and Pullin, R.S., 1996, Observations on the burrows and burrowing behaviour of
the red band-fish, Cepola rubescens L., Marine Ecology 17: 23-40.
6Apparently the swordtail jawfish, Lonchopisthus micrognathus, has another means of cleaning its
gills after a bout of digging : it can insert its tail into the gill chamber through the opercula! And
speaking of special adaptations: some digging gobies are capable of ejecting their mouthful of
sediments directly through the opercula; so, while digging headfirst inside a shallow burrow, they candischarge the sediments without having to back out. See: Hansell, M., 2005, Animal architecture,
Oxford University Press, Oxford.
7

Colin, P.L., 1973, Burrowing behavior of the yellowhead jawfish, Opistognathus aurifrons, Copeia
1973: 84-90.
8

Scott, W.B., and Crossman, E.J. 1998. Freshwater fishes of Canada. Galt House Publications Ltd.,Oakville, ON; Wisenden, B.D., Unruh, A., Morantes, A., Bury, S., Curry, B., Driscoll, R., Hussein,M., and Markegard, S., 2009, Functional constraints on nest characteristics of pebble mounds ofbreeding male hornyhead chub Nocomis biguttatus, Journal of Fish Biology 75, 1577-1585; Bshary,R., Wickler, W., and Fricke, H., 2001, Fish cognition: a primate’s eye view, Animal Cognition 5: 1-13,
and references therein.
9

Clark, E., Stoll, M.J., Alburn, T.K., and Petzold, R., 2000, Mound-building and feeding behavior of
the twostripe goby, Valenciennea helsdingenii, in the south Red Sea, Environmental Biology of Fishes
57: 131-141.

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10

Takegaki, T., and Nakazono, A., 2000, The role of mounds in promoting water-exchange in the eggtending burrows of monogamous goby, Valenciennea longipinnis (Lay et Bennet), Journal of
Experimental Marine Biology and Ecology 253: 149-163.
11

Büttner, H., 1996, Rubble mounds of sand tilefish Malacanthus plumieri (Bloch, 1787) andassociated fishes in Colombia, Bulletin of Marine Science 58: 248-260; Clark, E., Pohle, J.F., andHalstead, B., 1998, Ecology and behavior of tilefishes, Hololatilus starcki, H. fronticinctus and related
species (Malacanthidae): non-mound and mound builders, Environmental Biology of Fishes 52: 395417.
This use of the word “bower” for cichlids is contested: Tweddle, D., Eccles, D.H., Frith, C.B., Fryer,G., Jackson, P.B.N., Lewis, D.S.C., and Lowe-McConnell, R.H., 2004, Cichlid spawning structures:bowers or nests? Environmental Biology of Fishes 51: 107-109.
12

13

The following article has a picture of a male Cyrtocara eucinostomus over its sand castle. The
diameter of the mound is at least 7 times the length of the fish: McKaye, K.R., 1983, Ecology andbreeding behavior of a cichlid fish, Cyrtocara eucinostomus, on a large lek in lake Malawi, Africa,
Environmental Biology of Fishes 8: 81-96. The following book also has drawings of sand mounds:
Keenleyside, M.H.A., 1979, Diversity and Adaptation in Fish Behaviour, Springer-Verlag, Berlin. See
also: McKaye, K.R., Louda, S.M., and Stauffer, J.R. Jr, 1990, Bower size and male reproductive
success in a cichlid fish lek, American Naturalist 135: 597-613, and references therein.
14

McKaye, K.R., Louda, S.M., and Stauffer, J.R. Jr, 1990, Bower size and male reproductive successin a cichlid fish lek, American Naturalist 135: 597-613; Taylor, M.I., Turner, G.F., Robinson, R.L.,
and Stauffer, J.R. Jr, 1998, Sexual selection, parasites and bower height skew in a bower-building
cichlid fish, Animal Behaviour 56: 379-384; Stauffer, J.R. Jr., Kellogg, K.A., and McKaye, K.R.,
2005, Experimental evidence of female choice in Lake Malawi cichlids, Copeia 2005, 657-660.
Barlow, G.W., 2000, The cichlid fishes: Nature’s grand experiment in evolution, Perseus Publishing,Cambridge (Massachusetts).
15

16

Kawase, H., Okata, Y., and Ito, K. 2013. Role of huge geometric circular structures in the
reproduction of a marine pufferfish, Scientific Reports 3, article number 2106, doi: 10.1038/srep02106.
See also: Bond, T., Muelle, R.J. and five others, 2020, Mystery pufferfish create elaborate circular
nests at mesophotic depths in Australia, Journal of Fish Biology https://doi.org/10.1111/jfb.14506
17

Takayanagi, S., Sakai, Y., Hashimoto, H., and Gushima, K., 2003, Sleeping mound construction
using coral fragments by the rockmover wrasse, Journal of Fish Biology 63: 1352-1356; Nanami, A.,
and Nishihira, M., 1999, Nest construction by the labrid fish Choerodon jordani (Snyder 1908), Coral
Reefs 18: 292.
18

Scott, W.B., and Crossman, E.J., 1998, Freshwater fishes of Canada, Galt House Publ., Oakville.

19

Moore, J.W., 2006, Animal ecosystem engineers in streams, Bioscience 56, 237-246. See also:
Hassan, M. A., et al., 2008, Salmon-driven bed load transport and bed morphology in mountainstreams, Geophysical Research Letters, 35, L04405.
20

Rushbrook, B.J., Dingemanse, N.J., and Barber, I., 2008, Repeatability in nest construction by malethree-spined sticklebacks, Animal Behaviour 75, 547-553.
21

For more information on sticklebacks, see: Wootton, R.J., 1976, The biology of sticklebacks,
Academic Press, London.

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