RWD response

robin bruckner arbruckner at
Wed Sep 17 16:43:15 EDT 1997

To: T. Goreau, J. Cervino and other coral list readers
From:  Andrew and Robin Bruckner, University of Puerto Rico

Response to comments from T. Goreau  et al. posted on the Coral List:

We regret that our remarks have been interpreted as criticism rather 
than as an avenue for the advancement of the general coral reef 
community, which is why we chose to post this information on the Coral 
List.  We were simply responding to items appearing on the list, in the 
newspaper and in Science.  We were unaware of any manuscript in review 
on RWD (and any data it may contain), and believe we have commented 
strictly upon our own observations. There are some points Goreau et al. 
addressed that deserves further comment.

Goreau et al. have not acknowledged that we are speaking specifically 
about Sparisoma viride; they are correct in their generalization that 
parrotfish graze predominantly on algal turf.  They are incorrect when 
they state that parrotfish rarely if ever bite Colpophyllia natans 
tissue.  Hours of observation, photographs and video footage will 
confirm that the occurrence of S. viride biting Colpophyllia natans is 
NOT rare, although it may be an infrequent event, and possibly cyclic in 
its emergence.  Sparisoma viride DO attack coral tissue, repeatedly, at 
the interface of tissue and denuded skeleton.  Our data will 
substantiate that these fish are capable of destroying entire colonies 
rapidly in a manner resembling RWD, and that often the damage is a 
result of two or more fish.  Is it a coincidence that this new disease 
and stoplight parrotfish share a characteristic denuding of coral tissue 
and degradation of skeletal material, and have manifested on the reefs 
at the same time?  

Having six years of continuous field experience specifically studying 
coral disease in the Caribbean, we are very familiar with the signs of 
YBD, now RWD, and other diseases, as well as damage caused by various 
corallivores, and we certainly know the difference between coral tissue 
and algal turf.  We have been monitoring colonies of Montastraea spp. 
with YBD on an offshore island since 1995.   Signs of coral destruction 
Goreau  et al. attribute to RWD have manifested only infrequently in 
Puerto Rico.

We are currently following several colonies of Colpophyllia natans 
exhibiting signs of rapid tissue and skeletal degradation from an area 
we survey routinely - this is the first incidence of intense 
predator-mediated damage we have observed on this reef in the four years 
we have been studying this site.  One of these colonies has received 
over 150 bites ON LIVE TISSUE in four hours of observation on four 
consecutive days; damage was inflicted by three terminal phase stoplight 
parrotfish.  On a second colony, two original lesions existed; one 
lesion increased in diameter by two centimeters from one day to the 
next.  We placed utility mesh over the top half of the colony, covering 
the two lesions; the lower half of the coral was exposed.  Four days 
later, two new sites of tissue and skeleton destruction had initiated on 
the same colony, outside of the caged area.  No further progression of 
tissue or skeletal destruction has been observed to the two lesions 
under the cage.  In contrast, one new lesion outside the caged area has 
continued to increase in diameter daily.  Video of these lesions confirm 
that one terminal phase and one initial phase S. viride are biting on 
live coral tissue repeatedly at the interface of the lesion, returning 
at 3-10 minute intervals.

This discrepancy generates some interesting questions that require 
further study:

1)  Are stoplight parrotfish grazing diseased tissue?
2)  Are these fish responsible for transmitting a disease?
3)  Why have so few people observed such extensive and obvious damage?  
Is this a new phenomenon?

It is possible that RWD/parrotfish lesions are not new.  Since many 
coral reefs are suffering from decline, and more and more interest is 
being taken in them, researchers might simply be noticing more damaged 
and dying coral.  S. viride damage is too obvious to be missed, if you 
catch the fish in the act.  Algal colonization is a rapid process and 
obscures the causes of decline, as Goreau et al. point out.  Without 
frequent observation, it would be impossible to tell whether this 
phenomenon existed in the past simply on the basis of old photographs. 
Goreau et al. refer to the same study team we cited that noticed  "white 
spots"  inflicted on Montastraea annularis by S. viride.  We are not 
suggesting that stoplight parrotfish continuously bite at live coral; 
instead, they infrequently take bites on live coral, grazing most often 
on algal turf .  If approximately 9% of their bites are taken from live 
coral, as observed by researchers from University of Groningen, 
considering that these fish spend a large part of the day feeding this 
would amount to a substantial amount of damage. 

Our observations from corals in Bonaire and Puerto Rico indicate that 
large terminal phase S. viride are extremely cautious, and easily 
disturbed.  Initially, we attempted to monitor colonies with signs of 
RWD from closer than 2 m; parrotfish approached the colony, but swam off 
without biting.  While it is crucial to examine a coral up close to 
identify signs of disease, it is also imperative to "stand back" and 
look at things from several perspectives.   

Goreau et al. have erroneously ascribed our YBD and RWD findings as 
characteristic of Bonaire; we would like to clarify that the low 
incidence of RWD mentioned was from Curaçao.  We recorded a higher 
proportion of RWD in Bonaire. Although we believe the two diseases are 
unrelated, we also have observed RWD and YBD on the same colony.  As 
Goreau et al. point out, there are many disease signs that might be 
confused with YBD, including white plague, bleaching, predator damage, 
and other factors which may possibly result in overestimates as well as 
underestimates.  We are aware of work in progress to assemble waterproof 
photographic cards with characteristic signs of specific diseases so 
more divers will be able to identify them.  After studying and 
photographing thousands of corals with disease, we feel rigorous 
training is necessary before anyone other than coral reef specialists 
can collect meaningful, accurate data on coral disease. 

Goreau et al. are correct in their analysis that after a coral dies and 
is colonized by algae, it is difficult to determine a coral's cause of 
death. From our experience, frequent and continual monitoring is 
necessary to follow the progression of disease; diseases are plastic, 
and their abundance and severity can change rapidly.  Without frequent 
surveys one cannot definitively attribute the mortality of a coral to a 
specific factor. 

We are excited to see the GCRA web site, and look forward to 
contributing photographs and video footage for public comparison with 
other sites in the Caribbean.  This type of rapid information sharing is 
desperately needed to eliminate confusion and ensure a rapid response to 
such newly emerging phenomenon, which are affecting coral reefs today.

Andrew and Robin Bruckner
Department of Marine Sciences
University of Puerto Rico
PO Box 908
Lajas, PR  00667

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