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Why are there more species of coral and fish in tropical Indo-Pacific area than the Atlantic?

26 Mar

Historical reasons:

During the last ice age, sea levels and temperatures dropped, and previously underwater ledges were exposed to erosion, both by the wind and water currents.  This typically creates features that, once the Ice age ends and sea levels rise, are ideal settlement regions for warm water corals.  These corals become reefs that attract a large number of different marine life, creating simple but unique trophic interactions that ultimately result in speciation via geographic and ecological isolation. The longer it took for the Atlantic to warm, along with the smaller area and the shallower depths, led to the Indo-Pacific gaining a leg up on coral speciation and ultimately led to a greater biodiversity in that region. Even today, it remains warmer than the Atlantic, perhaps continuing to add to the speciation

Geographic location

The interconnection of the Malaysian, Indonesian, and Philippine archipelagos on the continental shelf provides a mechanism for the isolation and reconnection of Indo-Pacific populations as sea levels change.  Populations in Indo-Pacific oceanic archipelagos also undergo isolation and reconnection as current patterns change with changes in sea level.  In addition to increasing the rate of speciation, isolation can decrease the rate of extinction by protecting populations from disease and predation.  Therefore, the bigger size of the tropical Indo-Pacific, relative to the Atlantic, gives it an advantage for achieving high species richness. Geographic isolation prevents gene flow, so populations develop genetic differences.  This independent evolution leads to speciation.

Besides size, the Pacific Ocean is older than the Atlantic. The Pacific has many more islands than the Atlantic does – and these islands provide opportunities for isolation and speciation. Also, they provide for the development of more coral reefs in the shallow waters surrounding the islands.

Temperature and salinity are also huge factors in the diversity of corals and fish because with a higher salt content in the Atlantic, fish and coral have a tougher time adapting to such waters.

The East Indies Triangle appears to act as a center for the concentration of the majority of tropical marine families. This small triangle extends from the Philippines to Malay Peninsula to New Guinea.  The location of the East Indies triangle acts as a place of dynamic evolutionary radiation as well as a long-term net source of species diversity. This “center of origin” greatly contributes to the overall species diversity of the Pacific Ocean and makes the Indo-Pacific Ocean incredibly more diverse than the Atlantic Ocean.

The main reason that area within this triangle is more successful than other marine areas is because during the time period of the Ice Age, it remained a relatively decent place for marine organisms to live.  Native species were able to survive and continue to evolve, but more importantly, species from other areas migrated to this region. This could be because they had left their homes to find better areas to live in or just because they were brought by the currents and then were able to better survive in the optimal conditions that existed in this triangle.


The different water density causes water to rise and fall, creating the thermohaline system (or Conveyor belt).  There are several different types of upwelling; however, all types drive water circulation and allow nutrients and CO2 to reach different regions of the sea.  Upwelling allows oxygen to reach the deep abyssal plain, allowing organisms on the deep sea to survive.  When water evaporates in the cool air ice can form, which further increases the salinity of the water.  The surface water is then replaced by deep water that is rich in nutrients and CO2.  This causes high primary production and creates more phytoplankton, which creates an environment for fish to thrive.


ALL IMAGES FROM GOOGLE Invertebrate Zoology

Gladfelter, William B., Ogden, John C., Gladfelter, Elizabeth H. Similarity and Diversity Among Coral Reef Fish Communities: A Comparison between Tropical Western Atlantic (Virgin Islands) and Tropical Central Pacific (Marshall Islands) Patch Reefs. Ecology, Vol. 61, No. 5 (Oct., 1980), pp. 1156-1168
Birkeland C. 1990. Caribbean and Pacific coastal marine system: similarities and differences. Nature & Resouce 26: 9.
Briggs, John C. “Coincident Biogeographic Patterns: Indo-West Pacific Ocean” Evolution, Vol. 53 No. 2 April 1999.
Barber, P. H. and Bellwood, D.R. (2005). “Biodiversity Hotspots: evolutionary origins of biodiversity in wrasses in the Indo-Pacific and new world tropics. MPE 35: 235-253
World atlas of coral reefs (Mark Spalding, Corinna Ravilious, Edmund Peter Green, 2001)