Effects of sand deposition on scleractinian and alcyonacean corals

:to.35:t
524
Table 3 Chlorophyll content
in four partially bleached alcyonacea.
Values in I1g chlorophyll
per 5 mm3 surfce tissue.
Lobophytum depressum was included
for comparative reasons.
Sarcophyton glaucum did not
bleach
Bleached area
Unbleached area
I-value
Significance
Lobophytum
depressum
-
3.13:t 1.03
L. venustum
1.41 :1:0.081
3.21 :1:0.63
-4.64
0.0001
Sinularia dura S. leptoclados
0.44:t0.22
1.06:t0.17
-5.36
0.001
Mucous sheets
Sasekumar 1979; van Katwijk et a1. 1993). In many reef
areas <strong>of</strong> the Indo-Pacific, a1cyonacea are commonly found
No mucous sheets comparable to those observed under nat- on the most inshore parts <strong>of</strong> the reefs (Tursch and Tursch
ural conditions were observed during the entire experi- 1982; Sheppard et a1. 1992), areas where one would anticment.
However, in all alcyonacea <strong>of</strong> the genera Lobophyipate sedimentation to be high. In South Africa as well,
tum and Sinularia <strong>sand</strong> particles stuck to the surface <strong>of</strong> the many alcyonacea are <strong>of</strong>ten found growing in <strong>sand</strong>y gullies
colonies on what appeared to be congealed mucus. In scle- with extremely high values <strong>of</strong> <strong>sand</strong> resuspension and reseractinia,
the presence <strong>of</strong> mucus was inferred by <strong>sand</strong> pardimentation.ticles stuck together and hanging from the edge <strong>of</strong> the col- However, while in the experiment alcyonacea were not
onies. This was most frequently observed in flat specimens. able to actively clear themselves from applied <strong>sand</strong>, in nature
they can be passively cleaned by water movement.
Therefore, they are more likely to be found in areas <strong>of</strong> high
water motion, e.g. exposed and elevated parts <strong>of</strong> reefs. In
Discussion
South African <strong>sand</strong>y gullies, they were mostly limited to
near vertical rock faces or areas <strong>of</strong> high water motion,
Even though the experiments were conducted in the labor- which allowed passive <strong>sand</strong> clearance. They generally faatory,
the results allow conclusions to be made about field voured elevated parts <strong>of</strong> the reefs which were exposed to
implications. Parallel observations in the field and local lower sedimentation levels as well as high water motion
community structure data (Riegl et al. 1995) supported la- (Riegl et al. 1995) which is likely to wash <strong>of</strong>f settled <strong>sand</strong>.
boratory findings.
In these reef areas, alcyonacea have the advantage <strong>of</strong> be-
The laboratory environment simulated extreme condiing able to survive physical disturbances caused by floattions,
such as high sedimentation and practically zero waing debris and catastrophic low tides better than scleractiter
motion in which the corals' attempts at <strong>sand</strong> clearing nia (Schuhmacher 1975; Benayahu and Loya 1977; Tursch
were completely unaided. In Natal, such conditions are and Tursch 1982; Dai 1991). These factors may lead to
likely to occur in winter months <strong>of</strong> May to July, where high, their dominance.
storm-created swells alternate with almost absolutely calm The alleged sedimentation tolerance <strong>of</strong> alcyonacea
seas and where no water movement is detectable on the (Schuhmacher 1975; van Katwijk et al. 1993) is mainly in-
reefs. Surge stirs up sediment which settles again in calm ferred by the position <strong>of</strong> the corals on the reefs, and ex-
seas and makes it necessary for corals to clean their surperimental pro<strong>of</strong> is inconclusive or lacking. There is, howfaces.
In other reef areas <strong>of</strong> the world, this situation would ever, the possibility that regional differences exist.
occur when strong sediment pulses are input by river dis- Certain aspects <strong>of</strong> colony morphology proved crucial
charge and not removed by currents and wave action for the coral's ability to cope with sedimentation. Spheric-
(Cortes and Risk 1985; Hodgson 1994). However, sediity <strong>of</strong> colonies is important in the early phases <strong>of</strong> <strong>sand</strong> shedment
grain sizes would be finer in such a situation (Cortes ding, as it obviously enhances run-<strong>of</strong>f and determines the
and Risk 1985).
ability <strong>of</strong> a colony to clear <strong>sand</strong> quickly.
The continuous <strong>sand</strong> application experiment repre- The morphology <strong>of</strong> alcyonacea ensured that at least
sented the worst possible case for the corals, an environ- some parts <strong>of</strong> the colony were not covered by sediment.
ment with high sedimentation but no water movement to The numerous radial ridges in Lobophytum venustum and
help clearing. The results allow us to judge the differential Sinularia dura, as well as the finger-like projections in
abilities <strong>of</strong> corals to cope with continuous <strong>sand</strong> cover over S. leptoclados, always kept a large portion <strong>of</strong> the corals'
a restricted period <strong>of</strong> time.
surface <strong>sand</strong>-free. The covered parts suffered necroses and
Both scleractinia and alcyonacea were well able to with- localized bleaching, especially under continuous sedimenstand
short-term episodic <strong>sand</strong> application, while contintation.uous <strong>sand</strong> application resulted in various stress responses In scleractinia, calyx size and meander length were not
or death <strong>of</strong> entire colonies. Overall, scleractinia coped bet- <strong>of</strong> obvious importance. Although an influence <strong>of</strong> calyx size
ter with continuous <strong>sand</strong> cover than alcyonacea. on the rate and efficiency <strong>of</strong> <strong>sand</strong> clearing is reported (Hub-
At first glance, this result appears strange when combard and Pocock 1972; Hubbard 1973; Stafford-Smith
pared to field situations and data from the literature. Some 1993), it is not clear why calyx size should have a strong
authors claim that alcyonacea are less susceptible to sedi- influence on <strong>sand</strong> clearance. The general behavioural rementation
than scleractinia (Schuhmacher 1975; Hong and sponse <strong>of</strong> most scleractinia to sedimentation was polyp in-
0.79
1.71
-1.7
0.13
1.0111