Spinyhead
Acanthemblemaria spinosa
▶ Watch the guided demo: how to play
The spinyhead blenny is a Caribbean fish about 2.5 cm long that spends its life tail-first in a hole in coral, usually the empty tube of a worm or snail, with only its spiny, tufted head showing. It snaps zooplankton out of the passing current and rarely leaves home completely.
Good holes are scarce and fiercely contested. Larger blennies win, and the larger fish end up in the higher holes, where the current delivers the most food. A resident has the advantage of leverage: an intruder must be bigger to pull it out. A vacant hole is reoccupied within minutes.
Field studies of where male and female spinyheads live found both sexes mostly on dead elkhorn coral, with males twice as likely as females to hold the long tubes left by worms and snails, which give more surface for eggs. Bigger fish sit in higher shelters, and the number of eggs a male guards rises with both his size and his shelter's height. Females lay their eggs in the male's hole, and he guards them alone until they hatch.
This game grew out of field research on exactly that contest: Buchheim and Hixon (1992) , below. Later studies found the same pattern: bigger fish in higher holes, and males in higher holes guarding more eggs.
How this study began
I did this study while working as a field researcher for Dr. Mark Hixon and his graduate student Jeff Harding, building artificial reefs to study the effects of predation on reef fishes.
That field season Dr. Peter Sale came to visit. Riding in the back seat on the drives out to the study sites, I listened to him and Dr. Hixon argue, sometimes heatedly, about what really sets how many fishes live on a reef and how many kinds: competition among the fish already there, or recruitment limitation. Reef fishes are broadcast spawners, and the second idea holds that their numbers are capped by how few larvae happen to settle out of the plankton onto the reef.
I was spending hours staring into coral heads, helping Jeff find pairs of sharknose gobies for his study of their mate choice and fidelity (we believed they mate for life). On those same coral heads I kept watching spinyhead blennies fighting over holes, and it looked like competition happening right in front of me. With Jeff's help I designed a way to test experimentally whether the blennies really were competing for shelter holes, and I was given the time and the means to carry the study out. It became the 1992 paper below, and many years later, this game.
Dr. Seuss summed up the spinyhead's problem in On Beyond Zebra! (1955) with his Nutches: small creatures living in little caves called Niches, where there are always more Nutches than Niches, so every Nutch must keep watch over its Niche, or a Nutch without one will move in.
The research behind the game
Buchheim, J. R. and M. A. Hixon (1992). Competition for shelter holes in the coral-reef fish Acanthemblemaria spinosa Metzelaar . Journal of Experimental Marine Biology and Ecology 154: 45–54. Competition
Greenfield, D. W. and T. A. Greenfield (1982). Habitat and resource partitioning between two species of Acanthemblemaria (Pisces: Chaenopsidae), with comments on the chaos hypothesis . In The Atlantic Barrier Reef Ecosystem at Carrie Bow Cay, Belize , Smithsonian Contributions to the Marine Sciences 12. Ecology
Clarke, R. D. and J. C. Tyler (2003). Differential space utilization by male and female spinyhead blennies, Acanthemblemaria spinosa (Teleostei: Chaenopsidae) . Copeia 2003: 241–247. Habitat use and reproduction
Clarke, R. D., E. J. Buskey and K. C. Marsden (2005). Effects of water motion and prey behavior on zooplankton capture by two coral reef fishes . Marine Biology 146: 1145–1155. Feeding
Waggett, R. J. and E. J. Buskey (2007). Calanoid copepod escape behavior in response to a visual predator . Marine Biology 150: 599–607. Predation on copepods
Böhm, T. and B. W. Hoeksema (2017). Habitat selection of the coral-dwelling spinyhead blenny, Acanthemblemaria spinosa , at Curaçao, Dutch Caribbean . Marine Biodiversity 47: 17–25. Habitat use
Harding, J. A., G. R. Almany, L. D. Houck and M. A. Hixon (2003). Experimental analysis of monogamy in the Caribbean cleaner goby, Gobiosoma evelynae . Animal Behaviour 65: 865–874.
Harding, J. A. (1993). The roles of environmental constraints and aggression on male-female pairing in the coral-reef fish Gobiosoma evelynae . M.S. thesis, Oregon State University.
Meadows, D. W. (1993). Morphological variation in eyespots of the foureye butterflyfish (Chaetodon capistratus ): implications for eyespot function . Copeia 1993(1): 235–240.
Meadows, D. W. and C. M. Meadows (2003). Behavioral and ecological correlates of foureye butterflyfish, Chaetodon capistratus , infected with Anilocra chaetodontis (Isopoda: Cymothoidae). Revista de Biología Tropical 51 (Suppl. 4): 77–81.
Aronson, R. B. (1983). Foraging behavior of the west Atlantic trumpetfish, Aulostomus maculatus : use of large, herbivorous reef fishes as camouflage. Bulletin of Marine Science 33: 166–171.
Matchette, S. R., C. Drerup, I. K. Davison, S. D. Simpson, A. N. Radford and J. E. Herbert-Read (2023). Predatory trumpetfish conceal themselves from their prey by swimming alongside other fish . Current Biology 33: R801–R802.
Matchette, S. R., N. Hamilton, J. Schneider, C. Drerup, S. Winters, A. N. Radford and J. E. Herbert-Read (2025). Predators hide against similarly coloured species to camouflage on the move . Proceedings of the Royal Society B .
Hamner, W. M., M. S. Jones, J. H. Carleton, I. R. Hauri and D. McB. Williams (1988). Zooplankton, planktivorous fish, and water currents on a windward reef face: Great Barrier Reef, Australia. Bulletin of Marine Science 42: 459–479.
Ogden, J. C. and P. R. Ehrlich (1977). The behavior of heterotypic resting schools of juvenile grunts (Pomadasyidae). Marine Biology 42: 273–280.
Foster, S. J. and A. C. J. Vincent (2004). Life history and ecology of seahorses: implications for conservation and management. Journal of Fish Biology 65: 1–61.
Warner, R. R. (1988). Traditional use of mating sites by a coral reef fish. Nature 335: 719–721.
Robertson, D. R. and S. G. Hoffman (1977). The roles of female mate choice and predation in the mating systems of some tropical labroid fishes. Zeitschrift für Tierpsychologie 45: 298–320.
Albrecht, H. (1969). Behaviour of four species of Atlantic damselfishes from Colombia, South America (Abudefduf saxatilis , A. taurus , Chromis multilineata , C. cyanea ). Zeitschrift für Tierpsychologie 26: 662–676.
Mather, J. A. (1992). Interactions of juvenile Octopus vulgaris with scavenging and territorial fishes. Marine Behaviour and Physiology 19: 175–182.
Sazima, C., J. P. Krajewski, R. M. Bonaldo and I. Sazima (2007). Nuclear–follower foraging associations of reef fishes and other animals at an oceanic archipelago. Environmental Biology of Fishes 80: 351–361.
Krajewski, J. P., R. M. Bonaldo, C. Sazima and I. Sazima (2009). Octopus mimicking its follower reef fish. Journal of Natural History 43: 185–190.
Myrberg, A. A. (2001). The acoustical biology of elasmobranchs. Environmental Biology of Fishes 60: 31–45.
Kelley, J. L. and A. E. Magurran (2003). Learned predator recognition and antipredator responses in fishes. Fish and Fisheries 4: 216–226.
Hsu, Y. and L. L. Wolf (1999). The winner and loser effect: integrating multiple experiences. Animal Behaviour 57: 903–910.
Meyer, J. L., E. T. Schultz and G. S. Helfman (1983). Fish schools: an asset to corals. Science 220: 1047–1049.
Grobecker, D. B. and T. W. Pietsch (1979). High-speed cinematographic evidence for ultrafast feeding in antennariid anglerfishes. Science 205: 1161–1162.
Hernández-Urcera, J., M. Cabanellas-Reboredo, M. E. Garci, J. Buchheim, S. Gross, A. Guerra and D. Scheel (2019). Cannibalistic attack by Octopus vulgaris in the wild: behaviour of predator and prey . Journal of Molluscan Studies 85: 354–357.
The neighbors
Bluehead wrasse (Thalassoma bifasciatum ): the blue-headed, banded males and the yellow juveniles patrol the reef and hunt exposed blennies, hardest at dawn and dusk.Christmas tree worm (Spirobranchus giganteus ): filters the same plankton and vanishes into its tube when disturbed, though it gets used to repeated touches.Foureye butterflyfish (Chaetodon capistratus ): browses the worms' plumes in pairs. Its huge false eyespot, round in adults but a tall oval in juveniles, can make a predator misjudge its strike, and about a third carry an Anilocra isopod under the eye, as Dwayne Meadows found.Sharknose goby (Elacatinus evelynae ): a cleaner that picks parasites off bigger fish, which come to its station to be cleaned.Graysby and schoolmaster snapper : predators drawn in by the cleaners. They'll ambush a wrasse that's busy chasing a blenny.Blue chromis (Chromis cyanea ): a cloud of plankton-pickers hovering upcurrent over the coral head. They eat the same food the blennies do before it reaches the holes, most of all the high ones. When a grouper, snapper or triggerfish comes, or at night, they dive into the coral and more food gets through.Juvenile French grunts (Haemulon flavolineatum ): a little yellow-striped school resting by the coral's foot by day, picking plankton near the low holes. At dusk they file off over the sand to feed in the seagrass, and come back at dawn.Coney (Cephalopholis fulva ) and trumpetfish (Aulostomus maculatus ): a golden coney cruises the reef with a golden trumpetfish lying along its back. Hidden there, the trumpetfish slips off to suck in a grunt, a wrasse, or a blenny caught out on a long dart. Then it leaves the coney and, rising toward the blue chromis, turns silvery with a blue snout to hunt them. Once it hunts in the open, blennies see it and wrasses keep clear.How they connect : at midday the bluehead wrasses spawn at their traditional site and stop hunting; the chromis rush the egg clouds. Wrasses also pick eggs from any blenny nest left unguarded. Around the full moon chromis males nest on the rubble, wrasses raid the nests in gangs, and the hatching larvae drift past the holes at dusk. A hunting octopus draws a following of graysby, coney and wrasses that snap up whatever it flushes, sometimes a blenny. A brown trumpetfish sometimes hangs head-down in the sea rods. And the grunts' droppings fertilize the coral head, so new Christmas tree worms settle sooner (and their dead tubes become holes).Longsnout seahorse (Hippocampus reidi ): tails wrapped round the sea rods. The male carries the eggs in his pouch and, toward dawn, pumps out dozens of young, which swim up and drift across the coral head: a rich meal for the blennies downstream. (In the game a brood takes under three days; in the wild, about two weeks.)Longlure frogfish (Antennarius multiocellatus ): a lump of "coral" that isn't. It wiggles a lure in front of its mouth, and a blenny that darts out for it, or strays close, is swallowed in a few thousandths of a second. It moves in at night, stays a few days, and never announces itself.
Thanks
To Jeff Harding, for his dedicated work studying the gobies and for introducing me to coral reef fish ecology. To Dr. Raymond Clarke, whose studies of blenny ecology inspired me. To Dr. Dwayne Meadows, who shared an office with Jeff as a fellow graduate student of Mark Hixon's and worked with him on these same St. Thomas reefs, and whose studies of the foureye butterflyfish (what its false eyespot is for, and how the Anilocra isopod riding under its eye changes its behaviour) shaped the butterflyfish on this reef.
Saving solo games
A solo game saves itself as you play, and Continue on the start screen picks up where you left off. To keep a particular moment, open the menu and choose Save as… and give it a name. Saved games on the start screen, or Restart from a saved game after your fish is eaten, takes you back to any of them. Saves stay in this browser (the 25 newest are kept).
Something not working?
Press Save problem report in the menu right after it happens. It saves a log of your last few minutes of play (and, in a solo game, a snapshot of the coral head) to your downloads, which you can send to the developer.
Credits
While Spinyhead is being tested, it records how it's played (visits, the guided demo, games, errors, and your connection's IP address) to find problems and improve the game.
Made by Jason Buchheim of Odyssey Expeditions , marine biologist and longtime watcher of very small fish.
Playing on a phone
Slide your thumb on the dashed pad to move the crosshair over the reef; the food or hole nearest it lights up. Tap the pad, or press Strike , to go for it. In a fight the button becomes Bite! or Brace!: tap it fast. You can also tap things on the reef directly.
Put the pad on the left
How to play
Eat
Plankton drifts past on a current that swirls and reverses. Tap a morsel to strike. Three rings around your hole show how far you can go:
Green, close (about one body length): a lunge with your tail tip still in the hole. Always safe, and copepods rarely sense it coming, so let food drift close.Amber, middle (about two body lengths): a short dart right out of your hole. A little risky: you're close enough to duck back in, so a predator seldom catches you, and your hole still counts as yours.Orange, far (up to about four body lengths): a long dart for food nobody else can reach. Much riskier: wrasses and groupers usually get you if they're about, the fast dash spooks copepods, and while you're that far away a neighbor can slip into your empty hole.
Size and energy
Bigger blennies store more energy, but burn it faster too, so on balance a big fish lasts longer between meals: it can fan a clutch through lean spells and ride out a fight or a night. It also takes far more food to fill. Every copepod gives every fish the same energy, so a small fish fills up from a few morsels and empties just as fast. The Energy box shows how much you can hold compared with a typical 25 mm fish (×0.6 for a new recruit, about ×2 for a big male).
Graysbys lie in wait
The graysby (a small grouper) is a sit-and-wait predator. It visits now and then (never two at a time) and for only a minute or so, hanging almost motionless at one spot after another around the coral head (at its foot, low on a flank, sometimes up over the crown), moving on every 10 to 20 seconds. Its main prey is wrasses: when one comes down into its strike zone, it gulps, its jointed jaws shooting open and its throat ballooning to suck the prey in. If the gulp misses it gives a short, fast chase, then settles back to wait. At dawn and dusk it patrols low along the coral head instead, running down wrasses as they head for the sand to sleep. A blenny is only in danger if it strays well away from its hole right in front of a graysby. You'll know a graysby by its dense red-brown spots and the three dark spots along its back below the dorsal fin.
Parasites and cleaning
Groupers, snappers and butterflyfish often arrive carrying parasites you can see: pale isopods with dark eyes, and copepods trailing egg sacs. A fish with several parasites heads for the sharknose gobies' cleaning station (or, at night, the cleaner shrimp) before it bothers hunting, and holds still while the cleaners pick them off one by one. A grouper or snapper then opens its mouth wide, and a goby swims in head first to clean inside. While it's being cleaned, a predator doesn't strike: it holds still with its mouth open and cleaners inside, and in experiments predators being cleaned chased prey less. A little after cleaning it stays calmer, then hunts as usual. A fish crawling with parasites is itchy and distracted. Other predators can still be about near a busy cleaning station.
The moray and the shark
A moray eel (spotted or goldentail) lives in a crevice at the foot of the coral head. By day you'll mostly see its head, mouth slowly opening and closing as it breathes. It strikes from its den at prey that comes close, such as a wrasse. A blenny feeding from its hole is always safe from it. The danger is darting out right in front of the eel's face while it rests in its den: it strikes, and a blenny that keeps coming back there is struck more surely each time. A little farther out in front of the den the risk is slight, and the rest of a blenny's feeding range is safe. It never eats its cleaners: now and then by day it slides out to the gobies' station and lies still with its mouth open while they pick off its parasites and swim in and out of its mouth. At dusk and through the night it slides out and noses through the cracks of the coral, flushing out small cardinalfish, shrimp and crabs (and anything else hiding there), catching some itself. A blenny in its hole is always safe: the tubes are too narrow for an eel. Snappers and graysbys follow a hunting eel and snap up much of what bolts. Now and then a Caribbean reef shark cruises past far off in the blue. It never bothers the blennies, but the groupers and snappers leave while it's about, and the wrasses turn cautious.
Christmas tree worms and butterflyfish
The spiral plumes of the Christmas tree worms lean and sway with the current; in a strong current a worm may pull in until it eases. By day a foureye, spotfin or banded butterflyfish sometimes browses the coral head, often in pairs, sculling slowly on its pectoral fins just off the coral, hovering head-down over a worm and then darting in to clip its plume; it also picks at things tucked into the coral and in the sand, and visits the cleaner gobies, holding still with its belly to the coral while they clean it. Usually the worm sees it coming and snaps shut. A bitten plume grows back, but a worm bitten too often dies, and its empty tube becomes a new hole that a blenny can move into. When the coral head has lost worms, new worm larvae settle on clear patches of coral from time to time and grow to full size over about ten minutes. Blennies duck back into their holes when a butterflyfish passes close, and the spotfin picks drifting plankton too.
No two blennies alike
Every blenny is a little different, within what real spinyheads look like: a slightly different hue, a lighter or darker head, more or fewer pale speckles, redder or browner cheek blotches, and longer or shorter head cirri and spines. Some leanings in behaviour go with them: males with long cirri court more vigorously and females slightly prefer them; darker-headed fish forage more boldly; some are feistier and pick fights more often. Your own fish's traits are shown in the top bar and every fish's in the Fish & players table.
Guarding a nest
A male must fan his eggs to keep them supplied with oxygen, and the bigger the clutch the more energy that takes. If he runs out of energy, or is away from the tube, for more than a tenth of the incubation time in all, the eggs suffocate. So build up a good store of energy before you court, and while you have eggs feed close to home.
If a guarding male gets badly hungry, he can eat a couple of his own eggs at a time to keep going (the Eat some eggs button appears when you're guarding and under 40% energy). Every egg eaten is one that won't hatch, but it can save the rest from suffocating.
If a male is driven out, his eggs stay behind in the tube. The next male to move in eats them straight away (he gets their energy, as with any fish eggs, and he can't make milt while a rival's eggs are in his tube). If a female moves in instead, or nobody does, the eggs die unfanned. Females never fight a male for his tube: they visit him to spawn, take a female's hole, or move into an empty one.
Lost your hole?
A fish that loses its hole dives for the nearest crack in the coral. The button at the bottom always does something: it swims to the nearest open hole that fits you, and if there is none, it tucks you into a crevice, where wrasses can hardly see you (though one swimming right over may flush you out). From a crevice you can snap up food that drifts close (a short snap keeps you in cover, however tired you are); going farther means leaving cover. A crevice isn't a home: you can't breed there. To win a hole back, tap it; if you're very tired you'll be warned that you'll probably lose, but the choice is yours.
Night and sleep
The reef keeps a 24-hour clock: sunrise at 6:00, sunset at 18:00, with the light fading slowly through dusk and returning slowly at dawn. Deep night passes quickly, up to ten times faster than the day. You can barely see food or predators, and strikes often miss. The moon behaves as it does on Earth: over a 29.5-day lunar month it rises about 50 minutes later each day. A new moon rises and sets with the sun, a first-quarter moon rises at noon, a full moon rises at sunset and lights the whole night nearly well enough to hunt by and even to spawn, and a last-quarter moon rises at midnight. Wrasses sleep buried in the sand, but groupers and snappers prowl. Rest quietly in your hole and you sleep, burning energy far more slowly. A fish that sleeps less than 70% of the night twice in a row becomes exhausted, with clumsy strikes and a weak grip, until it sleeps through a night.
Lasting coral heads
A shared coral head keeps running while anyone plays, and is saved when the last player leaves, to resume exactly where it was. Your record there (lives, time played, eggs hatched, eggs per hour) follows you through every life. Leaving and coming back always means arriving as a new recruit. Shared coral heads can't be paused; solo games can (P).
Copepods jump
Copepods sense the water moving ahead of a strike with the hairs on their antennae and can explode away in a few thousandths of a second, faster for their size than almost any animal. In calm or slack water most of them escape; in a strong, swirling current the turbulence hides your approach and they're much easier to catch. The closer and slower your strike, the smaller your bow wave: a short lunge rarely spooks a copepod, a long dash usually does. The Current chip shows whether the water around you is calm or rough. Fish larvae sometimes dart away too; eggs never do, but in a swirling current a tumbling egg or larva is easier to miss.
Survive
You're only safe tail-down in your hole. Wrasses hunt hardest at dawn and dusk. When a graysby or snapper is around, wrasses get wary and are easier to dodge.
Climb
Tap another hole to move in, or to bite its owner and lever them out. Mash the button during a fight, or break it off: tap any other hole to bolt for it (a defender who bolts leaves its hole to the attacker). You must be at least about as big as the resident to win, and residents have the leverage. Green numbers above a hole mean you could take it; red means they're too big.
Energy
Everything active costs energy. Waiting in your hole is cheapest, and a lunge with your tail anchored costs little. Darting all the way out costs far more, and so do crossing the coral head, fighting and holding on against an attacker. A tired fish can't dart, can't start a fight, and loses grip. Eat to keep your energy up.
Corals
Brain coral is covered in winding ridges and valleys; great star coral in big cup-shaped corallites; starlet coral in tiny sunken cups. Elkhorn coral is a thicket of branches: wrasses and groupers can swim in front of or behind it, but they have to dive in among the branches to catch anything, and they have to swim around the branches once they're there. Pick one on the start screen for solo play and for any coral head you start.
Holes
Holes come in different widths, and the label on an empty hole says the biggest fish it fits. You can squeeze into a hole a little narrower than you (up to about a tenth), but no tighter. A fish that nearly fills its hole grips well and is hard to pull out; a small fish rattling around in a wide hole is easy to evict. Once you fill your hole you stop growing until you move to a wider one. Holes ringed with a pale lip are old worm or snail tubes, with more room for eggs. If you lose your hole , press Home : you'll head for the nearest open hole that fits, or tuck into a crack in the coral where wrasses can hardly see you. Left alone, a homeless fish tucks into a crack by itself. Even an exhausted fish can creep to a hole, just slowly.
Spawning
You're male or female for life, and males grow somewhat larger. Courtship happens in full daylight.
As a male , eat to build milt, then press Court females : you bob, flare your jaws, raise your dorsal fin and jump. It costs energy. A female who chooses you squeezes into your hole and lays her eggs, and you fertilize them. Then guard them until they hatch. Lunging for food is fine, but if you dart fully out and anyone takes your hole, the whole brood is lost.
As a female , eat to fill your egg supply. When you're ripe, displaying males' holes are ringed in pink: tap one to swim over and lay your eggs in his nest. Your own hole is unguarded while you're away. Females favour big males with high holes and roomy tubes.
Score
Your score is how many of your eggs hatch, counted for both the mother and the father. Fish never shrink, but a starving fish stops growing.
With other players
Each coral head is shared by up to nine players, plus wild blennies filling the empty holes. You join as a small recruit. If you're eaten, settle a new recruit and keep going; the reef carries on without you. The same rules apply to every fish, human or wild.
Your field notebook. When something worth recording happens where you can see it (a copepod jumping clear, a grouper posing for the cleaners, a moray hunting at night), a ✎ marker appears beside it for a few seconds. Click it to note it in your waterproof field notebook, under Field notes in this menu, with a snapshot and a page of natural history. Some behaviours are rare: keep watching. The cleaning gobies have a social life of their own (pairs greeting each other, males tending nests, residents seeing off rival gobies), as studied on these reefs by Jeff Harding. And a few nights after each full moon, a couple of hours after dark, the coral spawns: you'll get a message at dusk, and the night doesn't fast-forward while it happens.
Weather and visitors. Every few days a storm swell surges over the reef, clouding the water and bringing in extra food; rarely a hurricane hits, sweeping away fish caught in the open and breaking coral, which leaves new holes. By day copepods sometimes gather in a dense swarm near the coral: a feast. An octopus may come hunting, by day and at dusk, camouflaged or crawling over the coral and feeling into holes with its arms: a snug hole is safer. At night long-spined sea urchins graze over the coral head.
With a mouse you never need the buttons: click your own fish to dash home, to hide or find a hole when you're homeless, or (as a male in your hole) to start or stop courting; during a fight, click anywhere to bite or brace. A small label by the pointer says what a click will do. Keys: Space to bite or brace, H to go home, M for this menu.
Questions, ideas, or something not working? Spinyhead is being tested: tell us on its odd.place discussion page, and like it if you enjoy it.