[Coral-List] substrate parasites

[Charles Birkeland]

The very thin 1 mm coral-killing sponge *Terpios hoshinota* (with
photosymbiont cyanobacteria) grows up to 4 times faster on living coral
than on reef rock. However, if the reef rock or coral skeleton is
air-blasted clean of microbes or living tissue, the sponge grows faster on
the sanitized rock (Plucer-Rosario 1987 Coral Reefs 5:197-200). *Terpios*
seems to tend to be found where coral is abundant and doing well. For
example, an area 1000 m in length of otherwise healthy coral reef was
mostly covered by *Terpios*, but *Terpios* was not common in northwest Guam
where a crown-of-thorns outbreak removed most of the corals (Bryan 1973
Micronesica 2: 237-242). Living coral seems to be a more preferred
substrate for *Terpios* than bare CaCO3, possibly because of microbes on
apparently bare space.


Likewise, didemnid tunicates with carpet-morphology and photosymbionts
(Prochloron) are often found on living corals, despite plenty of bare reef
space around. They also seem to grow larger on living coral (see Fig. 1 in
1981 Bull. Mar. Sci. 3:171 or see color photos temporarily on my Facebook).
Why competing with coral for space when there is plenty of free space? 1 –
just to be mean? 2 – the challenge of competition is a thrill? 3 – Living
coral is an easy substrate for carpet-morph didemnids. Tunicates are the
only animals that produce cellulose and corals surely cannot digest the
cellulose in ascidian tunics. Note *Trididemnum solid*um overgrowing the
stinging fire coral *Millepora* and the *Mycetophyllia* near the top of
Judy Lang’s competitive hierarchy. I wonder if the thrill of competition is
more spicy with *Millepora* and *Mycetophyllia*, but rather bland with
*Acropora
*and* Porites.*


In ecology, one organism overgrowing another is competition for space. But
natural history is concerned whether the interaction is negative or
positive for the organisms involved. Two negatives is competition. But in
the interactions above, the coral is affected negatively while the *Terpios*
and the carpet-morph didemnids are positively affected by the gift of
preferable substrate from their host. A positive/negative relation is
predation or parasitism. *Terpios* and carpet-morph didemnids might be
considered “substrate parasites” rather than competitors. That is a stuffy
term. “Space parasites” would be preferable, but I fear that if searching
on Google, we would be referred to science-fiction movies.