Cup corals seem to space themselves out during settlement, thus minimising effects of intraspecific competition.  But what about interspecific competition with other anthozoans?  This is examined in a study in southern California between several corals, including the orange cup-coral Balanophyllia elegans, the brown cup-coral Paracyathus stearnsii, another coral Astrangia lajollaensis, and a corallimorpharian Corynactis californica.  In areas of the Hopkins Marine Life Refuge in Monterey County, California these 4species occupy >50% cover on hard substrata at a depth of 5-15m. 

Laboratory and field experiments at Hopkins Marine Station, Calfiornia demonstrate that C. californica is competitively dominant over B. elegans. Corynactis influences the distribution and abundance of the subordinate species by reducing its fecundity and larval survival, and by altering its recruitment patterns. The graph on lower Left shows that physical contact with C. californica significantly reduces reproductive output of B. elegans. In field experiments, removal of polyps of C. californica for 2-yr periods produces a slight increase in density of B. elegans juveniles and adults. 

When Balanophyllia is paired with each of the other 3 species over 6-mo periods in the laboratory, it is damaged by all of them (see graphs on Right). Thus, in this assemblage of Californian sessile and sedentary cnidarians, Balanophyllia elegans is competitively recessive. Chadwick 1991 Mar Ecol Progr Ser 70: 39.

Ever wonder why cup corals Balanophyllia elegans are generally found on clean, rocky areas devoid of algal growth?  Researchers from University of California, Los Angeles and UC Santa Barbara investigate interactions of cup corals with seaweeds and sea urchins at Anacapa Island, California and discover several competitive interactions.  First, abundances of Balanophyllia and percentage cover of algae are inversely correlated (see graph). Not only are the coral polyps overgrown by kelp holdfasts, filamentous algae, and encrusting coralline algae, but the situation is exacerbated by continual brushing of the polyps by larger seaweed species that causes the polyps to retract (see photographs).  Chronic retraction allows adventitious algal spores to settle and coralline algae to overgrow the polyp bases.  The greater the water motion, the greater the adverse effects.  In areas of high sea-urchin densities and active grazing, however, the effects of algae are ameliorated or removed, and the cup corals flourish.  Coyer et al. 1993 J Exp Mar Biol Ecol 167: 21.

NOTE  4 sea-urchin species occur on the Island, with red urchins Stonglylocentrotus franciscanus being the most abundant.  Balanophyllia elegans is the most abundant of 3 scleractinian corals

NOTE  to determine these interactions, the investigators use both natural and artificial algae in manipulative experiments, some involving translocations and settlement plates

Cup corals Balanophyllia elegans overgrown by
coralline (top) and filamentous (bottom) algae

In other areas, such as San Juan Islands, Washington, cup corals Balanophyllia elegans are competitively dominant over an aggressive overgrowth competitor, the colonial tunicate Trididemnum opacum.  On subtidal rock walls in this region B. elegans may grow in densities of over 30 individuals ^. 0.25m^-2, and 25% of these may be involved in competitive interactions with T. opacum. Researchers employ manipulative field experiments and pairwise interactions in the laboratory to examine the possible roles of size refuge, aggressive behaviour, allelochemicals, and aggregation in preventing overgrowth.  Results of field experiments in which cup corals are capped with epoxy resin to kill them but leave their skeletons intact, show that about 68% of the killed corals are overgrown versus 0% of control corals, indicating that height alone is no barrier to the tunicates.  Other laboratory experiments show that aggressive tentacular behaviour and nematocyst stinging are the operative deterrents to overgrowth of the cup coral by the tunicate. Only tunicates in tentacular contact with cup corals evidence signs of tissue damage - not ones separated by mesh barriers - suggesting that defensive chemicals are not involved.  Bruno & Witman 1996 J Exp Mar Biol Ecol 207: 229.

NOTE  chemicals produced by one species to defend against another species

NOTE  this is done to ensure that height of the corals is not a factor preventing their overgrowth