On
25 October 1995, 190 University of Hong Kong Ecology students collected
benthic invertebrates from Shap Sze Heung stream as part of their 1st year
ecology field course. Not a single snail was collected. This is puzzling
because most of the other Hong Kong streams have numerous snails clinging
to their rocky substrates. Even streams with large predatory crab populations
have numerous thick-shelled pulmonate snails such as Sinotaia quadrata (Viviparidae) and Melanoides
tuberculata (Thiaridae) coexisting with the crabs (Dudgeon, 1992).
The stream was revisited on 31 March 1996, and most of its length from the
Sai Sha Road to the headwaters on Ma On Shan was traversed in a concerted
search for aquatic gastropods. None were found. There are no records of
Mollusca reported from this stream.
The
Shap Sze Heung stream arises about 400m above sea level on the slopes of
Ma On Shan. The stream is about 3.4km long, and its drainage area is composed
predominantly of volcanic rocks and acid soils with a pH of around 5.5.
Shade trees reduce light penetration to the stream and consequently periphyton
production is low.
It
was postulated that a lack of calcium in combination with crab predation
was the reason for the complete absence of gastropods in this unpolluted
stream.Water and soil samples removed from the stream were tested (Tables
1 & 2). The total dissolved calcium concentration in Shap Sze Heung
stream was only 0.654 ppm (as opposed to 2.33 ppm in Pokfulam Reservoir
stream where snails are common members of the benthic community).
Table
1: Pokfulam Reservoir stream (4 Apr '96)
No crabs observed but freshwater snails common
| pH |
7.5 |
| Conductivity |
120-276 microS/cm |
| Turbidity |
1-2
NTU (Nephelometer turbidity units) |
| Dissolved
oxygen |
7.69-9.82
mg/L |
| Temperature |
19°C |
| Salinity |
0 ppt |
| Calcium
conc. |
2.33 ppt (water) 0.878 mg/g (sed.) |
Table
2: Shap Sze Heung stream (31 Mar '96)
No
snails observed but freshwater crabs common
| pH |
<6.64 |
| Conductivity |
42-50 micros/cm |
| Turbidity |
0 NTU |
| Dissolved
oxygen |
7.5-9.89
mg/L |
| Temperature |
18°C |
| Salinity |
0 ppt |
| Calcium
conc. |
0.654 ppt (water) 0.292 mg/g (sed.) |
All
shelled molluscs maintain a continual net uptake of calcium from their environment.
The calcium is sequestered as CaCO3 which is the major mineral
component of the shell. In freshwater, calcium is the major dissolved cation.
It ranges in concentration from 1-3 mg/L in streams with little carbonate
rock in their watershed to 400 mg/L in streams flowing over or through carbonate-rich
deposits (Russell-Hunter et al., 1967). According to McMahon
(1983), approximately 95% of all freshwater gastropods are restricted to
waters with calcium concentrations greater than 1-3 mg/L.
Instead
of snails, the dominant herbivore in the Shap Sze Heung stream was the freshwater
crab, Cryptopotamon anaculothon Aquarium studies with this crab and
the common pulmonate snail, Biomphalaria straminea, indicated that the
crab was capable of consuming this thin-shelled snail without difficulty.
Ng
(1991) stated that little is known about the food habits of freshwater crabs
but the available information suggests that they are predominantly omnivorous
scavengers. In aquaria they prefer animal over plant matter.
Most
aquatic snails in Hong Kong are spiral-shelled species such as Sinotaia quadrata,
the thiarids Melanoides tuberculata, Thiara scabra, and Brotia
hainanensis,
and the physids Radix plicatulus and Physelia acuta. Flat-sided
planorbid snails such as Biomphalaria straminea and Hippeutis cantonensis
are thin-shelled pulmonates depending on atmospheric rather than dissolved
oxygen, whereas the viviparids and thiarids are thick-shelled prosobranchs.
It is notable that pulmonate snails, which are vulnerable to crab predation,
are scarce in habitats where freshwater crabs are found, although prosobranchs
may abound in such waters. Pulmonates, by contrast, build up dense populations
in streams where crabs are not present (Dudgeon and Corlett, 1994). Some
streams may lack crabs because they are polluted by agricultural wastes
(which provide abundant food for snails) - the low dissolved oxygen levels
in such streams are a problem for the crabs but not for the air-breathing
pulmonates, which survive quite well under such conditions (Dudgeon, 1992).
In running waters in Hong Kong where crabs are common, Sinotaia quadrata and Melanoides tuberculata
are the only abundant snails (Dudgeon, 1992). Although the evidence
is circumstantial, selection of thin-shelled prey by crabs probably limits
pulmonate abundance in flowing waters where crabs and snails co-occur (Dudgeon
and Corlett, 1994).
In
conclusion, a simple field trip with 190 enthusiastic 1st year Ecology students
was the stimulus for this article. It is likely that the combination of
a dense population of freshwater crabs and low level of dissolved calcium
has resulted in a Hong Kong stream where no snails are able to survive.
Thick-shelled snails which normally co-occur with the crabs require high
concentrations of calcium to produce their shells, while thin-shelled snails
which can survive at low calcium concentrations are impacted by crab predation.
Acknowledgements
I am grateful to Dr. Peter K.L. Ng for identifying the
Shap Sze Heung stream crabs, and the 1st year Ecology class of the University
of Hong Kong for stimulating this study.
MIKE DICKMAN
References
Dudgeon,
D. (1992). Patterns and processes in stream ecology: A synoptic review of
Hong Kong running waters. Die Binnengewaser 29:1-146, Stuttgart.
Dudgeon,
D.and Corlett, R,(1994). Hills and Streams. An Ecology of Hong Kong. Hong
Kong University Press,234pp.
McMahon,
R.L. (1983). Physiological ecology of freshwater pulmonates. W.D. Russell
(ed.) The
Mollusca Vol. 6 pp. 359-430. Academic Press, Orlando, Florida.
Ng, P.K.L. (1991). The Freshwater Crabs of Peninsular Singapore. Shinglee Publishers
Ltd, Singapore, 156pp.
Russell-Hunter,
W., Apley, M.L., Burky, A.J. and Meadows, R.T. (1967). Interpopulation variations
in calcium metabolism in the stream limpet, Ferrissia rivularis (Say).
Science 155:338-340.
