224
WILLIAM MACNAE
Ceriops tagal, Lumnitxera racemosa and Xonneratia alba among mangrove
trees ; Barringtonia racemosa, Heritiera littoralis, Entada phaseoloides,
Derris trifoliata, Hibiscus tiliaceus and Thespesia populnea among
mangrove associates ; Cardisoma carnifex, Helice leachii, Scylla serrata,
Sesarnm meinerti, 8. smithii, Uca lactea and U . vocans among crabs;
Terebralia palustris among molluscs, and Periophthalmus kalolo among
fish.
One doubts if each of these species is uniform throughout such a
wide distribution. I have already mentioned the physiological variation
within Avicennia marina, and the structural and physiological variation
within Uca lactea. Structural variations also occur in U . vocans but
these affect the major chela alone and each or all of the three recognized
claw forms may be found within the population of any one area.
Chhapgar (1957) dividcd Indo-west-Pacific crabs into ten geographical groups which differ slightly from the above.
The spread of the mangrove species must have been by means of
sea-borne seeds or seedlings. How far can these be carried and still
remain viable? Guppy (1906) and Muir (1937) report that seedlings of
Rhizophora mucronata may still develop after 87 days afloat. Fruits and
seeds of Xylocarpus, Entada and other sea-shore plants can germinate
after floating for several days. Muir (1937) studied, over many years,
the seeds cast up on shores near Riversdale (almost midway between
Port Elizabeth and Cape Town on the south coast of Africa) and records
that two of the commonest seeds on the drift line were Entada phaseoloides and Intsia bijuga, both inhabitants of the landward fringe of
mangroves. These were still viable-they never germinated on the
Riversdale beaches, although they could be germinated indoors. The
nearest localities where Entada grows is the Bay of Lourengo Marques
while Intsia is not found south or west of Madagascar. Seeds or fruits
of Xylocarpus, Heritiera and Barringtonia racemosa were also cast up
but rarely in a viable condition. Muir recorded that in his searches for
seeds in drift he rarely found seeds or fruits of Sonneratia, Avicennia or
ofrhizophoraceous mangroves far from the estuaries in which the parent
trees were living. This observation seems odd and suggests that he may
have searched outside the season of dispersal.
The seedlings of the rhizophoraceous mangroves float horizontally
on sea water, inclined in estuarine water and vertically in nearly fresh
or fresh water. They have quite commonly been seen floating on the
waters of the Bay of Lourengo Marques, Mombasa harbour and strait,
and several other bays and estuaries visited by the author. Some of
these would be transported to, and stranded on, unsuitable shores but
some would reach suitable ground.
WILLIAM MACNAE
Ceriops tagal, Lumnitxera racemosa and Xonneratia alba among mangrove
trees ; Barringtonia racemosa, Heritiera littoralis, Entada phaseoloides,
Derris trifoliata, Hibiscus tiliaceus and Thespesia populnea among
mangrove associates ; Cardisoma carnifex, Helice leachii, Scylla serrata,
Sesarnm meinerti, 8. smithii, Uca lactea and U . vocans among crabs;
Terebralia palustris among molluscs, and Periophthalmus kalolo among
fish.
One doubts if each of these species is uniform throughout such a
wide distribution. I have already mentioned the physiological variation
within Avicennia marina, and the structural and physiological variation
within Uca lactea. Structural variations also occur in U . vocans but
these affect the major chela alone and each or all of the three recognized
claw forms may be found within the population of any one area.
Chhapgar (1957) dividcd Indo-west-Pacific crabs into ten geographical groups which differ slightly from the above.
The spread of the mangrove species must have been by means of
sea-borne seeds or seedlings. How far can these be carried and still
remain viable? Guppy (1906) and Muir (1937) report that seedlings of
Rhizophora mucronata may still develop after 87 days afloat. Fruits and
seeds of Xylocarpus, Entada and other sea-shore plants can germinate
after floating for several days. Muir (1937) studied, over many years,
the seeds cast up on shores near Riversdale (almost midway between
Port Elizabeth and Cape Town on the south coast of Africa) and records
that two of the commonest seeds on the drift line were Entada phaseoloides and Intsia bijuga, both inhabitants of the landward fringe of
mangroves. These were still viable-they never germinated on the
Riversdale beaches, although they could be germinated indoors. The
nearest localities where Entada grows is the Bay of Lourengo Marques
while Intsia is not found south or west of Madagascar. Seeds or fruits
of Xylocarpus, Heritiera and Barringtonia racemosa were also cast up
but rarely in a viable condition. Muir recorded that in his searches for
seeds in drift he rarely found seeds or fruits of Sonneratia, Avicennia or
ofrhizophoraceous mangroves far from the estuaries in which the parent
trees were living. This observation seems odd and suggests that he may
have searched outside the season of dispersal.
The seedlings of the rhizophoraceous mangroves float horizontally
on sea water, inclined in estuarine water and vertically in nearly fresh
or fresh water. They have quite commonly been seen floating on the
waters of the Bay of Lourengo Marques, Mombasa harbour and strait,
and several other bays and estuaries visited by the author. Some of
these would be transported to, and stranded on, unsuitable shores but
some would reach suitable ground.