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Diversity
At A Glance (pdf)
This
column aims to introduce interesting species of Hong Kong flora and fauna
that might be encountered during fieldwork. Distinctive physical characteristics
and some interesting ecological facts are included for each example.
Macrobrachium
hainanense
by Sukhmani
Kaur Mantel
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Fig. 1. A large
Macrobrachium hainanense. |
Macrobrachium
hainanense, known locally as the Hainan Swamp Shrimp (®ü«nªh½¼)
is a nocturnal shrimp belonging to the family Palaemonidae that you might
encounter in forested upland streams in Hong Kong. These shrimps can be
distinguished from other shrimps in these streams by their large chelae,
due to which they get their name (Macro = large; brachium
= arm). Palaemonid shrimps are present in tropical Asian and Neotropical
streams, and species of Macrobrachium are found throughout the
tropics, primarily in freshwaters. In Tai Po Kau Forest Stream (TPKFS
in Tai Po Kau Nature Reserve) and Tai Shing Stream (TSS in Shing Mun Country
Park), the shrimps are generalist predators that feed primarily on slow-moving
or sedentary benthic macroinvertebrates. Tagging of shrimps, using individual
numbered markers, followed by release in Tai Po Kau Forest Stream pools,
was conducted to study their growth rate. Smaller shrimps grew faster
than larger ones and growth rate was higher during the summer period.
At the age of two years both sexes began breeding and females produced
20-75 eggs per brood between April and August. Males lived longer (up
to four years) and grew larger (up to 79 mm total length) than females,
which lived for only three years. Since these shrimps are relatively large
and predatory in nature, it might be expected that they are important
in structuring stream communities (Fig. 1). Experiments were, therefore,
conducted in pools of the two streams (TPKFS and TSS) that compared communities
of benthic invertebrates in pools with shrimps to pools without Macrobrachium
shrimps. Shrimp removal did not result in any significant change in the
community, probably since these pools also contain benthic predatory fishes
that might have cropped the excess prey made available by removal of the
shrimps. Interestingly, however, this shrimp feeds on large quantities
of Brotia hainanensis, a snail that is abundant in the streams,
with the gut of one shrimp containing 74 small snails! It is therefore
believed that the shrimp is probably responsible for the high mortality
of small-sized Brotia.
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