Beautiful intruders: the misplaced lionfish

Lionfish (Pterois volitans and P. miles) populations have drastically exploded in the western Atlantic and the Caribbean in the past decade, and not without attracting some attention. The trouble is that these gorgeous fish sporting an array of venomous spines are invasive species.  They naturally occur in the Indo-Pacific but have been introduced to Florida via aquarium releases and are now potentially causing significant changes to marine ecosystems, the inhabitats of which have not evolved with this fish.  They can now be found from Costa Rica and Venezuela up the US eastern seaboard to Rhode Island, a truly impressive extent considering the first individual was found offshore of Florida in 1985.  Recently, I was fortunate enough to dive in Roatan, Honduras on my honeymoon and lionfish were a relatively common sight, despite their efforts to hide among the barrel sponges on the benthos.  They could potentially spread well into the southern hemisphere, along the the coast of South America, based on the lethal minimum water temperature [pdf] for this fish (10 C).  Lionfish feed upon the larvae of reef fishes, undercutting the next generation of fishes.  They can spawn year-round and release buoyant egg masses that can float in the currents for weeks, ensuring a wide distribution.

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Seamount mythbusting

Seamounts—mountains beneath the waves—may not be conventionally thought of as large habitats on the global ecological stage, compared to say, forests or estuaries.  But there’s quite a bit of them to say the least, perhaps in the hundreds of thousands. The global seamount biome, or the total area represented by seamounts within the global ocean, has been shown to be larger than the whole of Australia.  Large seamounts have a global area of nearly 10 million square kilometers, more than wetlands, seagrass, or temperate grassland biomes. This is actually a conservative number:  if smaller seamounts (rising between 1000-1500 meters from the seafloor) are included, the global seamount area rises to 28.8 million square kilometers (Etnoyer et al.2010).

Unidentified cnidarian from Davidson Seamount. MBARI, NOAA-OER.

Fewer than 200 seamounts have been intensively sampled.  Even so, the ecological structure and evolution of seamount biological communities has been expounded upon and debated for more than a few years now.  For example, it is a possibility that seamounts, as submarine islands, are ecologically isolated to a degree and function within the island biogeography paradigm, having high levels of endemic species (species that are found no where else).  A new study in Marine Ecology (Rowden et al. 2010) has   compiled the evidence for and against the existing concepts in seamount ecology and serves as a healthy breath of context.

Unsupported:

Seamounts function as islands and have hydrological mechanisms, e.g. Taylor columns, that limit dispersal of larvae, and have high numbers of endemic species.  Although, it should be noted that cryptic speciation (micro-endemism) has been recently indicated on seamounts.

Plausible:

Seamounts as stepping-stones for the dispersal of species (seamounts provide relatively shallow substrate in the open ocean), host increased numbers of species and biomass, and are highly productive.

Seamounts are distinct from other deep-sea habitat at the same depth, have high species richness, are population sources for continental slope sinks, and can act as refugia from dire oceanic-basin scale events.

Supported:

Seamounts are vulnerable to fishing (remember the precipitous collapse of the orange roughy fishery?) and to bottom trawling.

With the scarcity of seamounts actually visited and sampled, keep in mind that Unsupported and Plausible concepts could very well be supported (or not) in future studies.  For one, seamount endemism is a hotly debated topic, and more research is surely ongoing.  This work is interesting not only for providing a review of the evidence, but by doing so, it highlights the need for additional research in the deep ocean and gently suggests that some operating ideas for seamount ecosystems may not be as cut in stone as previously thought, or at least have more variability from seamount to seamount.  This is a great example of science being an evolving body of work and methodology, not only a compilation of knowledge.  As a former professor of mine once told me, “what we know is not as important as know we know it.”

One idea that Rowden et al. 2010 list as Plausible is that seamounts could shelter biota from environmental changes.  The threat of ocean acidification—the alteration of seawater carbonate chemistry—not only impacts shallow corals, but is thought to put deep-sea corals at risk.  By using predicted values for environmental parameters, for example, aspects of seawater chemistry, and global habitat suitability models, Tittensor et al. (2010) show that seamount summits are consistently less impacted by ocean acidification than the surrounding benthos under various IPCC scenarios.  The researchers also point out that the largest areas of suitable habitat for deep-sea corals are around New Zealand and in the North Atlantic.  These areas are largely within various countries’ Exclusive Economic Zones, rather than the high seas, suggesting that habitat conservation measures taken by individual nations could have large effects for these potential oceanic refuges.

References:
Etnoyer PJ, Wood J, Shirley TC (2010). How large is the seamount biome? Oceanography, 23 (1), 206-209
Rowden, A., Dower, J., Schlacher, T., Consalvey, M., & Clark, M. (2010). Paradigms in seamount ecology: fact, fiction and future Marine Ecology, 31, 226-241 DOI: 10.1111/j.1439-0485.2010.00400.x
Tittensor, D., Baco, A., Hall-Spencer, J., Orr, J., & Rogers, A. (2010). Seamounts as refugia from ocean acidification for cold-water stony corals Marine Ecology, 31, 212-225 DOI: 10.1111/j.1439-0485.2010.00393.x

Plight of Staghorn Coral

Staghorn coral (Acropora cervicornis). Wikipedia Commons, photo taken by Alessandro Donà in Bonaire, 2007. Creative Commons Attribution ShareAlike 3.0 License

With any luck, we’ll be discussing a specific anthozoan (or maybe groups of anthozoans) each week.  Anthozoa is a class of marine organisms within the phylum Cnidaria, and consists of corals, anemones, and sea pens.  There are over 6000 anthozoans (that we know of—likely lots more, particularly in the hugely undersampled deep-sea), and most Cnidarian species in existence today are within this class.

Acropora cervicornis (staghorn coral) is a scleractinian coral species—meaning it is a reef-building coral that forms a calcium carbonate skeleton in the form of aragonite (for more on aragonite and ocean acidification, you can go here)—that occurs in pretty much all of the greater Caribbean.

A. cervicornis is among the faster growing corals in the Caribbean and provides reef framework that adds to habitat complexity on the larger coral reef ecosystem—helping to give habitat to all sorts of organisms found on reefs, including other invertebrates and reef fish.  But it’s probably not accurate to say that A. cervicornis is a major reef-builder throughout the Caribbean presently.  This species is listed by the International Union for Conservation of Nature as critically endangered.  Populations of this coral have declined over 80% in the past 30 years; the main culprit causing this enormous die-off is white-band disease (WBD).  While the cause is unknown, WBD causes tissue decay, eventually peeling away from the skeleton on afflicted colonies.  This exposed skeleton can be rapidly colonized by algae in short order, potentially causing other problems for the coral.  But not all A. cervicornis colonies are affected by WBD, with research indicating that 6% of  staghorn coral genotypes are resistant.  Larger mechanisms are undermining this and other corals worldwide as well:  altered ocean chemistry, increased thermal stresses via climate change and more intense El Nino/Southern Oscillation events, sedimentation, dominance of fleshy macroalgae due to grazer die-offs or overfishing…I could go on, but I think you get the picture.

A. cervicornis can reproduce asexually or sexually.  Asexual reproduction via fragmentation allows local propagation—this is one reason why hurricanes and other storms are really an integral facet of coral reef spatial ecology.  Storms and other mechanical stressors provide a means for branching corals to fragment and thus reach other local areas.  Despite this, reefs may have lower resilience due to anthropogenic stress and may not be able to recover from storms as quickly as they once were able.   Sexual reproduction through broadcasting larvae is needed for dispersal across any real distance and for the maintenance of genetic diversity.  A recent study published in the Public Library of Science (Hemond and Vollmer 2010) looked into the genetic connectivity of A. cervicornis in Florida and discovered a potential future genetic bottleneck.  The investigators, using mitochondrial DNA sequences, found that the A. cervicornis population in Florida was genetically diverse—the good news—but may be isolated from larval inputs from other populations in the Caribbean.   Acropora species within the Caribbean are known to have restricted gene flow and thus, reduced connectivity, among populations that are farther away than 500 km from each other.  However, these recent results indicate that the Florida Keys population appears to be isolated from even the Bahamas (<200 km), the Gulf Stream possibly acting as a dispersal barrier (remember, larval dispersal is dependent upon oceanographic conditions like currents).  In the here and now, these findings indicate the dependence of A. cervicornis in Florida on self-recruitment; conservation programs are called for in order to manage these populations as separate ‘unit’.  There is another, longer-term consequence of this lack of larval input.  Disease has reduced populations to a fraction of what they once were, and even with Florida’s relatively high diversity within this coral population, genetic drift may produce a future bottleneck, potentially putting the genetic diversity of this population at risk.

The following references and those cited therein were drawn upon for this post.  They would serve as nice starting points for more information.

Aronson, R., Bruckner, A., Moore, J., Precht, B. & E. Weil 2008. Acropora cervicornis. In: IUCN 2009. IUCN Red List of Threatened Species. Version 2009.2. <www.iucnredlist.org>. Downloaded on 10 February 2010.

Fautin, Daphne G. and Sandra L. Romano. 2000. Anthozoa. Sea Anemones, Corals, Sea Pens. Version 03 October 2000. http://tolweb.org/Anthozoa/17634/2000.10.03 in The Tree of Life Web Project, http://tolweb.org/

Hemond, E., & Vollmer, S. (2010). Genetic Diversity and Connectivity in the Threatened Staghorn Coral (Acropora cervicornis) in Florida PLoS ONE, 5 (1) DOI: 10.1371/journal.pone.0008652

Humann P, DeLoach N. Reef Coral Identification:  Florida, Caribbean, Bahamas. 2nd Ed. New World Publications. Jacksonville, FL. 2002.

Vollmer SV, Kline DI, 2008 Natural Disease Resistance in Threatened Staghorn Corals. PLoS ONE 3(11): e3718. doi:10.1371/journal.pone.0003718

Evolution while you wait

Well, only if you’re not the foot-tapping type.  Those crazy, evolutionary-thought provoking finches are at it again.  Peter and Rosemary Grant, evolutionary biologists at Princeton, authored a study recently published in the Proceedings of the National Academy of Sciences that provides a view into evolution that no one’s really ever had before.  The researchers used Darwin’s finches–thus named for their profound influence on Charles Darwin– as a model system and tracked the descendants of an immigrant finch for 28 years on the Galapagos Island of Daphne Major.

Wikipedia Commons, John Gould

The newcomer, a  large, male Geospiza fortis, had very likely come from the nearby island of Santa Cruz, as determined by genetics.  This particular bird, dubbed 5110 (maybe if they named him Fred, they would’ve gotten attached) , had a slightly different beak and sang a slightly different song than the local Geospiza fortis population. The Grants followed this individual’s offspring, which also sported beaks and songs that resembled their parents’ more than the other finches, for almost the next three decades.  In a later generation, a drought occurred on the island and all but one mating pair (a brother and a sister) perished.  In the generations since, their descendants only began to breed within their population, reproductively isolating themselves.

So why the isolation?  Why no breeding across populations?  Likely, this has to do with the music, according to the Grants.   Finches’ songs aren’t genetically passed down–no finch instinctively sings one song or the other.  They learn songs from their parents.  When 5110 originally came to town, he attempted to copy the local’s songs, but this wasn’t a perfect conversion.  If you’re an I Love Lucy fan, think of Ricky Ricardo slowly developing a New York accent.  He still smugly sings Baba Lu like no native New Yorker.  5110’s kids (and their kids and so on) developed their dad’s song style; and along with ecological factors related to their new beak morphology, this musical difference is perhaps what led to the reproductive isolation.

Isolation of a population via a barrier to interbreeding can lead to speciation, which is  essentially how new species come about–thus, this could be the first step in 5110’s lineage evolving into a new species.   Evolution in action!  Now if only we could figure out when a divergent population becomes a new species… Or what exactly a species should be defined as (the species problem)…

ResearchBlogging.org

Grant, P., & Grant, B. (2009). Inaugural Article: The secondary contact phase of allopatric speciation in Darwin’s finches Proceedings of the National Academy of Sciences, 106 (48), 20141-20148 DOI: 10.1073/pnas.0911761106

(You probably won’t be able to view the article unless you have a PNAS subscription.  But you can snag the abstract here.)