Distribution & gene flow
Research study 1

map showing distribution of cup corals Balanophyllia elegans along the Pacific west coastThe cup coral Balanophyllia elegans is distributed from Vancouver Island, British Columbia to Baja California, Mexico.  Interestingly, it lives intertidally/subtidally in the north, but only subtidally in the south (see map on Left). The point of submergence coincides with Point Conception, California, an area of temperature disjunction where mean annual sea temperatures rise from about 12oC to 17oC.  Is temperature a cause of the equatorial submergence?  This is tested by translocating adult cup corals from 19m depth to 3m depth in areas near La Jolla, California.  Some specimens are hung from surface buoys or attached to rocks in shallow water (3m depth), or to rocks in deeper water (35m), while others (CONTROL) are attached to concrete blocks in an area where cup corals are abundant (19m).  Each translocated set is monitored over the 1-yr duration of the experiment. 

diagram of arrays of cup corals used to study growth of cup corals Balanophyllia elegans at different locations Results indicate that not only are the corals able to grow in shallow water (albeit slowly: 1% increase in live mass in a year vs. 10% in controls, at 19m depth), but evidence of reproduction is seen in eggs and larvae being carried by females.  From these results the author suggests that neither early (planula/settled juvenile) nor later stages in Balanophyllia’s life history appear to be critical in excluding the species from shallow water.  However, the author also notes that temperature fluctuations measured subtidally during the course of the experiment will likely make the role of temperature as a factor in equatorial submergence of Balanophyllia difficult to interpret. Gerrodette 1979 Mar Ecol Progr Ser 1: 227.

NOTE  this is done by removing them with spatulas and fastening them to plastic plates with small sectioned-PVC collars (see diagram on Right)

NOTE  almost all of these shall0w-water inshore plates end up being washed away by waves or removed by curious sport divers

Research study 2

Limited dispersal as manifested by the crawling larvae of cup corals Balanophyllia elegans should result in genetic differences between populations proportional to geographic distances of separation.  Whether these differences would be evident on small geographic scale, such as 50km, is tested in a study using cup corals from 18 locations along the coast of Sonoma County, California.  Results show that these populations are significantly heterozygotic, subdivided both among localities separated by 1-50km and among patches separated by 4-8km.  This suggests that gene flow is restricted to immediately adjacent populations in accordance with a linear “stepping-stone model”.  Hellberg 1995 Mar Biol 123: 573; see also Hellberg 1994 Evolution 48: 1829.

NOTE  pattern of gene flow is inferred from frequencies of 8 polymorphic allozyme loci

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