THE BIOLOGY OF WOOD-BORING TEREDINID MOLLUSCS
429
limiting factor in distribution is apparently temperature. Therefore,
species characteristic of higher latitudes are restricted in range, incapable of spreading into sub-tropical or tropical areas as may be seen
in the case of Bankia setacea of eastern and northern Pacific, B. gouldi
of northern and western Atlantic and B. martensi of southern South
America and South Africa. Similarly the tropical species B. carinata,
B. campanellata and B. bipennata, though established all round the
world are restricted by the temperature factor and have not spread
beyond the subtropical zone.
While discussing the dispersal of marine species with long-term
larviparous young such as the members of the genus Lyrodus and
Teredo furcifera, T . clappi, T . somersi and T . bartschi, Turner (1966)
enumerates the reasons for their world-wide distribution : (1) in common
with other shipworms the adults can be transported great distances in
ships or floating wood; (2) the young are protected within the parents
during the early critical stages of development ; (3) the larvae are not
spawned unless optimal conditions for their survival exist ; (4) being
further developed, the young are less sensitive when extruded; ( 5 ) the
larvae are ready to settle shortly after they are extruded and so are not
carried away from the floating log or ship from which they emerged ;
and (6) most wooden ships and pieces of driftwood are covered with a
good growth of hydroids, bryozoans, algae and other organisms which
form a protective " forest cover within which the larvae can swim
until time of settlement ".
The wide distribution of Teredo navalis, which is a short-term
larviparous species, is because it is both eurythermal and euryhaline
while the restricted distribution of the larviparous species T . poculifer
is due to its brackish-water habitat. Protection to the larvae is afforded
by retaining them until the late veliger stage within the mantle cavity
and through an ephemeral free-swimming stage which lasts only for
about 24 h.
Experimental studies on the dispersal of shipworms have been few.
By far the most common ways have been by driftwood or through the
agency of the wooden hulls of ships, etc. Some believe that Teredo was
brought to Holland, England, and other European countries by wooden
vessels from the tropics, possibly from India (Brown, 1816). However,
fossil records do not support this view (Deshayes, 1860; Jeffreys, 1861).
There is no doubt that extended and intensive intercourse between
nations in the long years of maritime activity contributed to the spread
of this menace in widely separated places. Historical records indicate
that marine borer attack had increased in Pacific coast harbours with
the increase in shipping, the chief agents in transport being the hulls
429
limiting factor in distribution is apparently temperature. Therefore,
species characteristic of higher latitudes are restricted in range, incapable of spreading into sub-tropical or tropical areas as may be seen
in the case of Bankia setacea of eastern and northern Pacific, B. gouldi
of northern and western Atlantic and B. martensi of southern South
America and South Africa. Similarly the tropical species B. carinata,
B. campanellata and B. bipennata, though established all round the
world are restricted by the temperature factor and have not spread
beyond the subtropical zone.
While discussing the dispersal of marine species with long-term
larviparous young such as the members of the genus Lyrodus and
Teredo furcifera, T . clappi, T . somersi and T . bartschi, Turner (1966)
enumerates the reasons for their world-wide distribution : (1) in common
with other shipworms the adults can be transported great distances in
ships or floating wood; (2) the young are protected within the parents
during the early critical stages of development ; (3) the larvae are not
spawned unless optimal conditions for their survival exist ; (4) being
further developed, the young are less sensitive when extruded; ( 5 ) the
larvae are ready to settle shortly after they are extruded and so are not
carried away from the floating log or ship from which they emerged ;
and (6) most wooden ships and pieces of driftwood are covered with a
good growth of hydroids, bryozoans, algae and other organisms which
form a protective " forest cover within which the larvae can swim
until time of settlement ".
The wide distribution of Teredo navalis, which is a short-term
larviparous species, is because it is both eurythermal and euryhaline
while the restricted distribution of the larviparous species T . poculifer
is due to its brackish-water habitat. Protection to the larvae is afforded
by retaining them until the late veliger stage within the mantle cavity
and through an ephemeral free-swimming stage which lasts only for
about 24 h.
Experimental studies on the dispersal of shipworms have been few.
By far the most common ways have been by driftwood or through the
agency of the wooden hulls of ships, etc. Some believe that Teredo was
brought to Holland, England, and other European countries by wooden
vessels from the tropics, possibly from India (Brown, 1816). However,
fossil records do not support this view (Deshayes, 1860; Jeffreys, 1861).
There is no doubt that extended and intensive intercourse between
nations in the long years of maritime activity contributed to the spread
of this menace in widely separated places. Historical records indicate
that marine borer attack had increased in Pacific coast harbours with
the increase in shipping, the chief agents in transport being the hulls