The black spiny Caribbean urchin Diadema antillarum is a formitable looking creature. It is basically a pin cushion with black hypodermic needles for spines. It seems reasonable to conclude that its spines are an adaptation to deter predators, and moreover, that they would be fairly effective. In fact, many Caribbean reef scientists assume few predators can eat Diadema. For example, Harbone et al (2009) recently stated;
“Urchins are particularly susceptible to unregulated ‘plagues’ because only a few specialist predators can overcome their defensive spines“
But surprising as it might seem, a wide range of fishes and invertebrates consume Diadema and could control it’s behavior and population densities. (I love these natural history surprises that defy logic and human biases.)
Predators of Diadema include: snapper, jacks, porcupinefishes, trunkfishes, grunts including black margate, porgies, triggerfishes, pufferfish, large wrasses, parrotfish, octopuses, lobsters, large gastropods and even small crabs (which eat juvenile Diadema).
The classic paper on predators of Diadema on Caribbean reefs is Randall et al. (1964). This paper, published before I was born, is a masterpiece of natural history and an invaluable documentation of the ecology of Diadema before it was wiped out by a disease in the early 1980s. Randall et al. reported;
Predators of D. antillarum include 15 fishes of the families Balistidae, Carangidae, Diodontidae, Labridae, ostraciidae, Sparidae, and Tetraodontidae, two gastropod of the genus Cassis, and the spiny lobster (Panulirus argus).
Some interesting excerpts from Randall et al:
Two larger wrasses, the Spanish hogfish (Bodianus rufus) and the puddingwife (Halichoeres radiatus), appear to feed directly, on Diadema without depending on the efforts of another predator. The senior author watched a large B. rufus eating Diadema at St. Croix and noted that it ate spines which it could have discarded. It took a piece of test into its mouth from which several spines projected, one of which was very long. The spines were drawn in gradually, apparently by the action of the pharyngeal teeth, and completely consumed.
Two large pomadasyid fishes, the black margate (Anisotremus surinamensis) and the Spanish grunt (Haemulon macrostomum), feed heavily on Diadema as adults. The lips and mouths of these fishes nearly always show purple dots indicating the sites of entry of Diadema spines, and the bones around their mouths are stained purple, probably because of the continuous tatooing action of the spines.
The authors have observed Diadema antillarum preyed upon by the two helmet shells Cassis madagascariensis and Cassis tuberosa in the Virgin Islands (Schroeder, 1962). When these gastropod encounter an urchin on which they wish to feed, they elevate the foot anteriorly, creep forward, and fall upon the prey, pinning it beneath. Within about 10 minutes the proboscis rasps a hole in the test about 6 to 10 mm. in diameter for feeding. The helmets may remain on top of the urchins for an hour or more. At times Diadema was found completely crushed beneath them. Surprisingly, the spines rarely penetrate the foot of these large gastropod.
D. antillarum have a suite of known consumers. Common predators include diverse finfishes: triggerfishes (balistids), jacks (carangids), wrasses (labrids), pufferfishes (tetradontids and diodonids) and grunts (haemulids), among which the queen triggerfish (Balistes vetula) has been identified as the most important fish. Invertebrate predators include spiny lobsters, king helmet snails and fighting conch. Small carnivores, such as small crabs and fireworms may prey upon newly settled juvenile D. antillarum. The significance of these micropredatorson D. antillarum population dynamics has yet to be explored. – from the Diadema Workshop Report 2004
There was an email thread about Diadema predators that went around among a group of 20 or so reef ecologists during the holidays. The highlights are below. I found the discussion fascinating. It really emphasized the importance of unpublished yet key natural history information in understanding reef dynamics and management. It also reminded me how knowledgeable and experienced these senior scientists are. I guess you do get something out of doing many thousands of dives over 3-4 decades! (other than hearing loss and a crooked spine)
Martin Moe: On predation, I’m sure that predation has a great effect on reducing the number of juvenile Diadema that settle and survive on various reef areas. I think, however, that the substrate upon which the late larvae settle has an even greater effect on the numbers of settling larvae that survive to become small, stable, feeding juveniles that actually have a chance to avoid predation and become reproductive adults. Apparently differential predation on Diadema due to fishing effort had little observable effect on Diadema populations in the recent past when fishing effort was spotty and Diadema populations were high throughout the tropical Western Atlantic.
I base this speculation on the results of my May 6, 09 larvae rearing run. Settlement and metamorphosis from day 40 through day 55 produced many thousands of early juveniles that settled out on many different types of substrates including sand, bare rock, algae covered rock, shells, algae strands and plastic. Of the thousands of early juveniles only about 100 survived past the 6 to 10 day early juvenile phase when internal organs and feeding apparatus had developed. By far, the substrate that produced the best survival was acrylic strips with coralline alga and hard plated green algae. I assume that diatom and bacterial growths were also present. Filamentous algae and sediment coated surfaces did not appear to favor survival. I am sure that some survival occurred on other substrates but I am not sure which of these other substrates were effective. Once the feeding juveniles were established, they moved to many other substrates and there was no mortality in these juveniles that I was aware of. I was very surprised at the almost total lack of survival of early juveniles on natural substrates that I assumed would be excellent substrates for early survival and growth.
Les Kaufman: The more intact hard coral-dominated reefs in the Indo-Pacific help to place things in perspective. On these reefs, herbivorous fishes are larger and more abundant than in the Caribbean (today), by many fold and up to at least one order of magnitude- a big difference. Under these circumstances, urchin predators are also large and abundant, and urchins- indeed all motile macroinvertebrates- are very hard to find. They are still there, but their movements are severely curtailed by the array of large, powerful invertivores moving about, especially by day. In this milieu, fishes are the primary herbivores, and here, a marine reserve will not have the effect we are worried about for Diadema in the Caribbean.
So, the negative rebound from a marine reserve (through a predator-Diadema cascade) is a transient. If fish populations were farther along in their recovery, fish herbivory would cover for the decrement caused by predation on urchins. Or alternatively, the Caribbean may actually have always been on a different trajectory. Do you think?
Jamie Bechtel: Yes – It would appear that micropredation may have played a key role in preventing recovery of the diadema population. My dissertation, which is now old and dusty; basically, diadema was found to be influenced by the entire echinoid complex – and it was only found when there were other echinoids something like 96% of the time. It was pretty astounding. The theory being that diadema larvae – juveniles had to land on bare substrate cleared by other urchins or be eaten by crabs. Implications of course for the role of bare substrate in phase transition.
Bill Precht: I always find these little tidbits that everybody adds quite enjoyable as they really fill in the picture.
The commonly held belief is that are very few predators of Diadema. This, even though Randall showed that there were at least 15 reef fishes that consumed adults not to mention ALL the micro-predators you all mention.
John Valentine: To add some observations to all of this: Based on our work in the keys, and hours of video tapes of predation on small urchins, we found that most of our views are overly simple. In the lower keys it is small wrasses (when urchins are small), hogfish and saucereye porgies. In the northern keys their was an attack sequence that began with small wrasses who picked at the prey (without much success) followed by attacks by either hogfishes or, oddly enough, redtail parrots.
Redtails attacked urchins in virtually every location we placed urchins, fore and back reef and at horseshoe. I would add the urchins were echinometra as we found no small diadema. there were larger ones around but mostly at Little Grecian and once in a while at White Banks. And in Hawkschannel, it was Cassis feeding on Lytechinus. The urchins seemed to be aware of their presence and crawled to the top of the cages we had in place at the time.
Rich Aronson: Specialist is probably not the right word anyway. All the fishes that eat Diadema, including queen triggerfish, are invertivores that include urchins in their diets. Queen triggers eat Diadema preferentially when and where they are abundant but switch to other skeletonized prey when Diadema are scarce or absent.
Boom-bust cycles appear to be a general feature of echinoderm ecology. II think Tom Ebert remarked on that somewhere, years ago.
Les Kauman: John (V) redial (chrysopterum) or redband (aureofrenatum)? The latter has a high penchant for carnivory, be interesting to know if that is true of chrysopterum too and under what circumstances.
John Valentine: It was redband. We have struggled to find about much in the literature beyond its grazing, and grouping as a herbivore. Any suggested readings would be appreciated.
Brian Keller: In DB olden days, on several occasions I witnessed parrotfish chomping Diadema spines down to nubbins (sorry Les – no idea what species!). I also observed broken “nubbin Diadema” tests, but did not witness the perpetrator.
Les Kaufman: Actually redbands are voracious carnivores, were always among the first to show at a deliberate urchin “kill” in the old days, and Rich Aronson and I share a favorite terminally rejected manuscript on this topic.
Les Kaufman: For what it’s worth, octopus have an astonishing ability to handle fully spined Diadema. I’ve got footage someplace of an octopus (perhaps a briarium) draping a Diadema, its oral web gracefully (and one would think painfully) tented by the spines. I don’t remember that instance leading to predation, but draping is often or maybe even usually (Rich?) an action pattern related to foraging.
Rich Aronson: Right you are Les: octopuses, especially O. briareus, hunt by extending their webbing over prey. They also pounce on rocks and coral heads, enveloping them with their webbing, and then insert their arms into crevices to hunt on spec. Roger Hanlon and others have looked into this behavior pattern.
Despite our recent obsession with regional and global forcing, it sure is nice to chat about natural history once in a while.
Additional observations and comments are welcome!
Harborne A, Renaud P, Tyler E, Mumby P (2009) Reduced density of the herbivorous urchin Diadema antillarum inside a Caribbean marine reserve linked to increased predation pressure by fishes. Coral Reefs 28:783-791
Randall JE, RE Schroeder and WA Starck II (1964) Notes on the biology of the echinoid Diadema antillarum. Carib. J. Sci. 4: 421-433