The benthic ecology and community structure in Lyttelton Harbour ...

65.
situation must extent over the entire area of the
community. These ru1es are based on the defini ons
proposed by Petersen (1924) and later modified by
Sparck (1937). Hemiplax and Virgularia fulfil these
. conditions adequately in this case and both are very
easy to recognise, there being no 'other species of the
same genera in the harbour.
Characterizing species of the second order
may be defined as those species t~at are found only in
some parts of the community where they again must be
found in at least 50 percent of all samples and make up
at least 5 percent of the biomasso Taking up this role
are ~~ plebe jus and Micrelenchus huttoni.. These
two gasteropods, one a carnivorous whelk and the other
a herbivore, ve distributions that are somewhat
governed by the availability of fond but nevertheless
are widespread over the community area. They correspond
to the concept of nmosaics" proposed by Davis (1925).
The third part to this compound definition is the
third order characterizing species that must be found in
large quantities in up to 70 percent of the samples.
It must also be included within the framework of the
present community.. However it is not restricted to the
region under investigation but also participates in
the species complement of the surrounding regions.
Into this category Pontophilu5 australis falls. This
very mobile shrimp may be caught in very large numbers
in the muddy regions but also sporadically outside these
areas. While Pontophilus must be included in the
community as presented, it cannot be used in defining
the region as a typical mud bottomed association.
The fourth class of inhabitant is termed
associated animals, usually transitory, or those making