Tropical Tidal Flat Benthos Compared Between Australia an Central America
289
nemertean preying on fiddler crabs is an indication for further predatory
effects in tropical sediments. Furthermore, macrobenthic predators and
deposit feeders can affect bivalve recruitment. Vargas (1988) found higher
recruitment rates within the exclusion cages and Dittmann (1996) found
higher recruitment of bivalves in shrimp exclusions, similar to the competitive exclusion reported by Peterson (1977). Predatory snails could also
account for the low bivalve numbers in many tropical tidal flats (Lee 1978;
own observations).
Competition may have significant effects on community structure in tidal
flats, although this process is generally less important in soft sediments
compared to rocky shores (Reise 1985; Peterson 1991). In tropical tidal flats,
competition has not yet been investigated, but observations in northeast
Australia showed intraspecific density regulation in terebellid polychaetes
and echiurids. There also seem to be adult-juvenile interactions in M. longicarpus, where juveniles gathered in separate groups from adult soldier crabs,
and in T. australiensis, where juveniles were found in different areas of the
tidal flat. Tamaki and Ingole (1993), however, found intraspecific facilitation
between juvenile and adult Callianassa japonica in tidal flats in Japan. Interspecific competition could explain the absence of tube-worms in colonies of
fiddler crabs or the inhibition of feeding activities of T. australiensis when
soldier crabs were active on the sediment surface. All these cases require
further investigations.
13.5 Conclusions
This comparison of tropical tidal flat benthos between Australia and Central
America has revealed a high degree of faunal similarity despite the different
biogeographic areas. Species numbers were high, although not as high as
expected for tropical sites. Furthermore, comparing several tidal flats in each
area showed a high degree of variation in species diversity. Few species were
abundant and abundances were generally low and very variable with higher
abundances recorded in tidal flats from the Pacific coast of Central America
than Australian sites. In all tidal flats compared here, polychaetes, crustaceans
and molluscs accounted for most of the species. The rank order of abundance
revealed site-specific species compositions and dominance structures of
macrobenthic organisms. Large-scale spatial distributions were related to
environmental gradients in Australian tidal flats and indications exist for a
similar zonation of life forms in a tidal flat in Costa Rica. Deposit feeders
dominated the benthic fauna numerically, while suspension feeders were rare.
Related species often had similar ecological roles and only few cases were
identified where an ecological counterpart did not exist in both Central
289
nemertean preying on fiddler crabs is an indication for further predatory
effects in tropical sediments. Furthermore, macrobenthic predators and
deposit feeders can affect bivalve recruitment. Vargas (1988) found higher
recruitment rates within the exclusion cages and Dittmann (1996) found
higher recruitment of bivalves in shrimp exclusions, similar to the competitive exclusion reported by Peterson (1977). Predatory snails could also
account for the low bivalve numbers in many tropical tidal flats (Lee 1978;
own observations).
Competition may have significant effects on community structure in tidal
flats, although this process is generally less important in soft sediments
compared to rocky shores (Reise 1985; Peterson 1991). In tropical tidal flats,
competition has not yet been investigated, but observations in northeast
Australia showed intraspecific density regulation in terebellid polychaetes
and echiurids. There also seem to be adult-juvenile interactions in M. longicarpus, where juveniles gathered in separate groups from adult soldier crabs,
and in T. australiensis, where juveniles were found in different areas of the
tidal flat. Tamaki and Ingole (1993), however, found intraspecific facilitation
between juvenile and adult Callianassa japonica in tidal flats in Japan. Interspecific competition could explain the absence of tube-worms in colonies of
fiddler crabs or the inhibition of feeding activities of T. australiensis when
soldier crabs were active on the sediment surface. All these cases require
further investigations.
13.5 Conclusions
This comparison of tropical tidal flat benthos between Australia and Central
America has revealed a high degree of faunal similarity despite the different
biogeographic areas. Species numbers were high, although not as high as
expected for tropical sites. Furthermore, comparing several tidal flats in each
area showed a high degree of variation in species diversity. Few species were
abundant and abundances were generally low and very variable with higher
abundances recorded in tidal flats from the Pacific coast of Central America
than Australian sites. In all tidal flats compared here, polychaetes, crustaceans
and molluscs accounted for most of the species. The rank order of abundance
revealed site-specific species compositions and dominance structures of
macrobenthic organisms. Large-scale spatial distributions were related to
environmental gradients in Australian tidal flats and indications exist for a
similar zonation of life forms in a tidal flat in Costa Rica. Deposit feeders
dominated the benthic fauna numerically, while suspension feeders were rare.
Related species often had similar ecological roles and only few cases were
identified where an ecological counterpart did not exist in both Central