Percnon spp. crabs are active omnivores with a strong appetite for
both microalgae and macroalgae.
Most herbivorous snails, like Astraea sp. sold in the marine
aquarium trade have small, fine radula suitable only against microalgae
(so, it's not surprising that they leave macro's alone, save to help
clean epiphytic microalgae off of them).
Other herbivorous molluscs, particularly sea-hares (even the smaller
ones found in the Philippines), have more robust radula that allow them
to break into tougher macroalgae, and drain algal fluid in a vampire-like
fashion.
Regarding thallic disintegration in Chaetomorpha:
Several factors may be implicated, beginning with inappropriate pH,
salinity and photoperiod. Chaetomorpha benefits from a dark period,
especially when most other parameters are less than desirable.
Chaetomorpha can briefly endure severe swings in temp/chemistry
and salinity (as both tide and seasonal rains can indce), but prolonged
exposure to inhospitable conditions (like in a badly parametered reeftank)
is something else entirely.
When conditions are bad, many alga resort to fragmentation as a survival
means: any fragments swept away by current would have a chance of
finding a more hospitable locale to colonize. When conditions are
EXTREMELY bad, sexual or asexual bailout can result --all viable algal
(nuclear) material is released in hope of seeding a better locale, leaving
behind an empty husk.
Most Chaetomorpha anchor readily to hard substrate in good
conditions, as in the wild. Refusal to anchor can be taken as a sign that
tank conditions are less than ideal for the alga (though admittedly, if he
alga is being (abused) as a nutrient scrubber, the alga's natural comfort
isn't exactly the top priority.
hth.
horge
