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Herbivory: algivores (seaweed-eaters)

hot buttons for algivory part of BCCR hot button for fishes part of algivores section in BCCR hot button for sea-urchin part of algivores in BCCR hot button for crustaceans/chitons/snails part of algivores in BCCR

This section of algivores deals with sea urchins; information on other seaweed-eating reef animals can be accessed via the icons.

There are only about half a dozen species of sea urchins on Caribbean coral reefs, but the comparatively large sizes and voracious appetites of 2 of these, the long-spined or black Diadema and the Barbados sea egg Tripneustes, belie their small taxonomic representation.

 
 

Herbivory: algivores (seaweed-eaters): sea urchins

 
 
seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of a Barbados sea egg Tripneustes ventricosus on a shipwreck

"This old shipwreck seems to be a popular spot for sea urchins. This particular species prefers seagrasses and it's not uncommon for them to be camouflaged in it...hard to imagine that a predator would be fooled, though." - St. Thomas 2007

NOTE Tripneustes ventricosus

 
 

photograph of sea urchin Echinometra lucunter in a creviceSea urchins have a complex apparatus known as an Aristotle's lantern that they use to feed on algae. The lantern is comprised of 20-or so calcaeous pieces, including 5 pointed teeth that can be extended out of the mouth. The biting action of the teeth is so photograph of mouth and teeth of sea urchin Echinometra lucuntereffective that nylon ropes can be bitten through and steel structures pitted. Each tooth is secreted in a soft sac at the top and worn away to a point at the biting end.


Sea urchins Echinometra
lucunter
in their preferred
crevice habitats 0.4X

 

Mouth of sea urchin Echinometra
lucunter
showing the 5 calcareous
teeth of the Aristotle's lantern 3X

 

photo composite showing position of the Aristotle's lantern in a sea urchinThe inner skeleton of a sea urchin is called a test. It is made up of many adjoining plates of crystalline calcium carbonate and is hollow like a squashed balloon. The feeding apparatus, Aristotle's lantern, is located in the lower central portion of the test, with the teeth facing outwards (see illustration). Each tooth is supported by a calcareous pyramid. The bite is made when the teeth are extended from the mouth. They flare open to enclose a piece of seaweed, then are pulled in and closed. On withdrawal, a chunk of alga is torn off and drawn into the mouth. The piece is then released and moved into the gut for digestion.

If you follow a line upwards from centre-most pointy
tooth, you can see the sac in which it is secreted.
The tooth is layered in such a way (like a beaver's
tooth) that it sharpens as it is worn. The supporting
pyramid is the large structure surrounding the tooth

 

photograph of aboral region of a sea urchin Echinometra lucunter showing location of anusInterestingly, the Aristotle's lantern changes in relative size depending upon the amount of seaweed food available in the environment. Where food is abundant, the lantern is relatively small; where it is scarce, the lantern increases in size. The maximum change in size in a black sea-urchin Diadema antillarum is about 50%, but a change of this magnitude is only obtained in experiments where sea urchins are kept for several months on varying amounts of food. The functional significance of the change is not clearly understood, but presumably a larger lantern relative to overall body size permits a relatively greater food intake. Thus, in conditions of low food availability, feeding is maximised. Levitan 1992 Ecology 73: 1597.

 


The gut in sea urchins terminates at the anus on the
upper or aboral surface. Shown here is Echinometra
lucunter
with some freshly released feces 0.8X

 


The unique relationship of lantern size to food availability has provided the basis for some interesting historical detective work. Specifically, a comparison of graph showing relationship of increasing population density in the Caribbean region with Aristotle's lantern size in sea urchins Diadema antillarumpresent-day lantern dimensions in the sea urchin Diadema antillarum with museum data for lanterns collected over a 100yr period photograph of sea urchin Diadema antillarumprovides a unique means to understand past interactions between human population density, associated fishing pressure, and algal abundance on 30 Caribbean reefs. Note in the graph that as human population-size increases, accompanied by greater fishing intensity, lantern size of Diadema increases. Levitan 1992 Ecology 73: 1597.

NOTE the effect of population density on lantern size is exaggerated here for visual effect. While lantern size in experiments can change up to 50%, increase in human population-density over the past 100yr is associated with only 6-9% increase in lantern size - a small but still statistically significant difference

 
 

cartoon drawing of Aristotle chatting with a SCUBA-diver about sea urchinsSo, what do we make of this observation? For a better idea of what is going on, let's turn to the man himself, Aristotle. His philosophy is empirical rather than metaphysical and is characterised by deductive logic ("just give me the facts, man"). With this in mind, how do you think Aristotle would explain the museum data on lantern dimensions in Diadema? Consider the possibilities below, then CLICK HERE for explanations.

He would conclude that more fishing by more people would reduce the number of sea urchin-eating fishes, leading to more sea urchins and thus to greater grazing pressure on the algae. With less algae the lanterns increase in size.

He would deduce that more humans would lead to increased nutrient enrichment in the form of agricultural run-off and sewage, and to more algae and more sea urchins.

He would conclude that a greater population of humans would consume more sea urchins.

He would conclude that more fishing by more people would reduce the number of herbivorous fishes, leading to more food for sea urchins and thus to more sea urchins, and ultimately to greater grazing pressure on the algae.

He would say the heck with it and simply swallow his hemlock poison.

 
 
seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of black sea urchins Diadema antillarum taken from a video

"I'm looking for a halo around these sea urchins. Halos are areas browsed free of algae extending out as far as the urchins would roam at night. Well... there might be a halo around this one. It looks like it has been eaten bare, doesn't it?." - Aruba 2004, St. Thomas 2007

NOTE Diadema antillarum

 
 

photograph of black sea urchin Diadema antillarum consuming a mound coral Montastrea sp.
It is not commonly known that sea urchins will eat coral.  However, black sea-urchins Diadema antillarum in Curacao and Bonaire commonly feed on living coral, most notably Acropora spp. (staghorn and elkhorn corals).  The urchins are active at night, during which time up to 8% of the population may be eating corals.  The immediate gustatory stimulus may be the taste of symbiotic algae within the coral, but an urchin’s digestive system is well adapted to process tissues of the corals themselves.  Sea urchins are one of the principal agents of bioerosion on reefs.
  Bak & van Eys 1975 Oecologia 20: 111.

 

 

Black sea urchin Diadema antillarum consuming
mound coral Montastrea sp. 0.5X

 
 


"Halos" are circular areas bare of algae or seagrasses caused by the grazing activities of Diadema sea-urchins and fishes that hide in the reef by day and come out to forage at night. Fidelity to a homesite crevice during photograph of sea urchin Diadema antillarum in an elkhorn coraldaytime for Diadema ranges from 30-80%. It is higher when photograph of several sea urchins Diadema antillarumpredators are abundant and lower when there are more sea urchins. Carpenter 1984 Mar Biol 82: 101.

 

Diadema antillarum
sheltering during
daytime in an
elkhorn coral 0.25X

 

 

 

Several black sea-urchins Diadema antillarum forage
at night in the halo region
around a patch reef 0.3X

 
 

photograph of sea urchins Diadema antillarum and Echinometra lucunterFollowing a Caribbean-wide die-off of Diadema sea urchins in 1983-84, overgrowth of algae was prevalent. In areas of Jamaica and elsewhere, studies show a return in densities of Diadema to levels approximating those prior to 1983-84. Accompanying the resurgance in sea-urchin numbers are decreases in amounts of fleshy algae, increases in coral recruitment, and generally a return to a coral- and algal turf-dominated reef ecosystem. Edmunds & Carpenter 2001 Proc Nat Acad Sci USA 98: 5067.

NOTE die-offs were noted from the Windward Islands in the east to the San Blas Islands of Panama in the west. On several reefs in the latter, for example, densities dropped from an estimated 14,000 individuals per hectare in June, 1982 to 0.5 per hectare in May, 1983. Average declines throughout the whole of the San Blas area was >94%. Lessios et al. 1984 Coral Reefs 3: 173.

And elsewhere in the Caribbean?

Florida Keys: in contrast to the situation in Jamaica described above, densities of D. antillarum in most of the Florida Keys never returned to pre-die-off levels. There was a second mass mortality in this area in 1991, but the general consensus is that the Florida Keys reefs have changed for the worse in recent decades, manifesting for black sea urchins in lower fertilisation success, poorer recruitment, and higher post-settlement mortality. Chiappone et al. 2002 Coral Reefs 21: 155.

St. Croix: 17yr after the 1983-84 die-off, D. antillarum on reefs in St. Croix have shown strong recovery, to levels more than 3X greater than pre-graph showing change in density of sea urchins Diadema antillarum in the San Blas Islands, Panama from 1980-2003die-off levels. The authors posit that a large and sudden influx of larvae in the late 1990s, possibly originating in Barbados, may have been the reason. Miller et al. 2003 Coral Reefs 22: 181.

Panama: 20yr following the mass mortality, populations of D. antillarum in the San Blas Islands remain at <6% of pre-mortality levels (see graph on Right). Of 2 possible explanations, either too few larval-producing adults upstream or continued presence of the pathogen, the author favours the former, mainly because existing individuals on San Blas reefs seem healthy. Lessios 2005 Coral Reefs 24: 125.

 


"Top down or bottom up?" Changes in seaweed abundance after 1983 in different regions of the Caribbean are generally attributed to a "top-down" effect caused by sea urchins. Thus, their demise in late 1983 was accompanied by an increase in % cover of seaweeds, while their return in the mid-1990s was accompanied by a decrease. The accompanying graphs show strong correlative evidence from the Discovery-Bay region of Jamaica in support of this idea or, at least, in the second part relating to their return. Note that macroalgal abundance decreases, presumably from sea-urch depredation, and that crustose and turf algal abundances increase, presumably taking advantage of the absence of the space-competing larger algae. Aronson & Precht 2000 Limnol Oceanogr 45: 251.graphs showing relationships of sea-urchin density with algal abundance in Jamaica during 1993-99

NOTE this a handy method to estimate the abundance of plant or animal species that grow attached to the sea bottom. There is no reference to vertical growth, only to the proportion of available substratum occupied, or "covered". An area where corals, for example, grow on half of the available area would be rated as having "50% cover"

The Left graph shows the relatively swift recovery
in stocks of sea urchins after 1996. The Right
graph shows a corresponding massive decrease
in abundance of macroalgae, and an increase in
abundance of low-growing crustose and turf algae

 

 

graph showing abundances of herbivorous fishes in Jamaica during 1993-99But is this the only explanation for the data? One other possibility, that there may have been an increase in number of seaweed-eating fishes in the late 1990s to account for the decreased % cover of seaweeds, is dismissed on the strength of evidence that neither of 2 dominant groups of algivorous fishes, parrotfishes and surgeonfishes, increased in abundance during that time in Jamaica (see graph). Aronson 1988 Bull Mar Sci 43: 93.

Similarly, an alternative "bottom-up" explanation, that nutrients required for growth of seaweeds may have increased throughout the late 1980s through increase in agricultural runoff waters or sewage pollution, is dismissed for lack of evidence.

 
  photograph of recovering population of black sea urchins Diadema antillarum in the yucatan area of MexicoRecovery of Diadema antillarum sea-urchin populations from their collapse in the 1980s has been slow but steady throughout the Caribbean region, and is favoured in shallow, wave-sheltered, rocky habitats. One such area is in Mahahual Bay, Yucatan, Mexico where boulder corals Montastraea annularis provide suitable habitat for a burgeoning population of black urchins. Algal cover is understandably low. Another correlative, one that caught the attention of the authors, is the low incidence of yellow-band and white-plague diseases in corals in the recovery area in comparison with nearby areas where Diadema is still absent. Jordan-Garza et al 2008 Coral Reefs 27: 295.

NOTE the authors think that this may owe to absence of disease-bearing algae in the Diadema recovery area, but such speculation may be unwarrented until other areas can be looked at
 
 


Herbivorous damselfishes inhabit the same general areas as sea urchins Diadema and, with their long protective spines would appear to be photograph of bicolor damselfish Stegastes partitusable to forage more-or-less where they like. So, the question arises as to whether the two groups of organisms compete for foraging space. Studies in the San Blas Islands of Panama show, in fact, that damselfishes and Diadema do potentially compete, but that there is less chance photograph of sea urchin Diadema sp. in a crevicefor this to happen in wave-surge areas because of the avoidance of such areas by the urchins. Perhaps their long spines get broken off in the water turbulence. Foster 1985 Diss Abstr Int Part B: Sci & Engineer 46: 155.


Bicolor damselfish Stegastes
partitus
in its algal garden 0.4X

 

Black sea-urchin Diadema sp 0.4X

 


Prior to its die-off from disease throughout the Caribbean in 1983-84 the black sea-urchin Diadema antillarum was an important component of reef communities. In that its activities had an inordinate impact on the distribution and abundance of other organisms, it is considered by some to be a keystone predator.

NOTE a species of high trophic status that exerts a disproportionately great influence on the species composition and diversity of a community. Its absence or removal will cause profound changes in the ecostystem to which it belongs. On a related matter, although the term "predator" is usually equated with carnivorous activities, because the ecological processes that regulate distribution and abundance of carnivores and herbivores are similar, it is not uncommon in the ecological literature to see the term applied to a herbivore

   
 

Just as in Jamaica, so the massive decline in numbers of Diadema during 1983-84 caused profound and interrelated effects in shallow reef communities around St. Croix. Carpenter 1985 Proc 5th Int Coral Reef Symp Vol 4: 53.

 
graph showing population crash of black Diadema sea urchins in 1983-84 in St. Croix graph showing change in algal biomass in St. Croix following the Diadema sea-urch crash of 1983-84 graph showing change in algal biomass in St. Croix following the Diadema sea-urch crash of 1983-84 graph showing increase in grazing intensity of fishes in St. Croix following the Diadema sea-urchin crash of 1983-84
The Diadema population "crashes" in late 1983-early 1984 Following the sea-urchin crash, algal biomass increases by over 400% Algal growth following the sea-urchin decline favours macroalgae at the expense of small filamentous and turf forms In response to increase in algal biomass following the crash, grazing intensity of algivorous fishes increases by 400%
  graph showing expected effect on diversity of herbivorous fishes after the Diadema sea-urchin crash of 1983-84The final graph in the above series is a result of an increase in abundance of herbivorous fishes in response to more plentiful food supply, but what would we predict be for the diversity of herbivorous fishes? The answer is that diversity would be expected to decrease as shown in the accompanying graph. This is because that as the overall number of herbivorous fishes increases, certain species would gain competitive dominance and eventually drive out the weaker species.
  In Jamaica, the pattern of response of algae to the demise of Diadema sea urchins is similar to that in St. Croix. Liddell & Ohlhorst 1986 J Exp Mar Biol Ecol 95: 271.
 
graph showing population crash of black Diadema sea urchins in 1983-84 in Jamaica graph showing effect of population crash of black Diadema sea urchins in 1983-84 in Jamaica on abundance of filamentous & turf algae graph showing effect of population crash of black Diadema sea urchins in 1983-84 in Jamaica on abundance of filamentous & turf alga
After the massive decline of Diadema in 1983 (density dropped from 7 per square meter to zero) Seaweed cover increases within a few weeks by over 100% At the same time, sponge & coralline-alga cover decreases, likely as a result of crowding by new growth of seaweeds
 
 

photograph of queen triggerfish Balistes vetulaLoss in 1983 of black sea-urchins Diadema antillarum, a favoured food of queen triggerfishes Balistes vetula in Belize, forces the triggerfishes to switch to a diet of crabs and chitons. Reinthal et al. 1984 Pubbl Staz Zool Napoli: Mar Ecol 5: 191.

 

 

 

Queen triggerfish Balistes vetula 0.33X

 

photogaph of toadfish Sanopus splendidus courtesy Anne Dupont, FloridaOther consumers of sea urchins, such as porcupinefishes, balloon-fishes, and grunts, must have been similarly affected by the sudden demise of Diadema, just as were 2 species of toadfishes in Panama. Robertson 1987 Copeia 3: 637. Photograph courtesy Anne Dupont, Florida.

 

 

 

Sanopus splendidus, a toadfish related to the 2 species
in Panama and also a consumer of sea urchins 1.4X

  graph showing effect of Diadema sea-urchin demise in 1983 on 2 species of toadfishes in Panama
In Panama, following the 1983-84 mortality of Diadema sea-urchins, population numbers of 2 toadfish species plummeted after remaining steady for several years. Both species almost exclusively preyed upon Diadema. Two and one-half years later, numbers of both species had recovered, in part by wholesale shift in diet to invertebrates by one toadfish species, and to fishes by the other. The delay in recovery of both species (see graph) suggests that the use of alternative food sources such as invertebrates and fishes takes a while to develop.
 
 
And now, for a summary: cartoon 1 of series with SCUBA-diver and philosopher Aristotle in conversation about keystone predators
cartoon 2 of series with SCUBA-diver and philosopher Aristotle in conversation about keystone predators cartoon 3 of series with SCUBA-diver and philosopher Aristotle in conversation about keystone predators
cartoon 4 of series with SCUBA-diver and philosopher Aristotle in conversation about keystone predators cartoon 5 of series with SCUBA-diver and philosopher Aristotle in conversation about keystone predators
cartoon 6 of series with SCUBA-diver and philosopher Aristotle in conversation about keystone predators cartoon 7 of series with SCUBA-diver and philosopher Aristotle in conversation about keystone predators
 
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