ECOLOGICAL STUDIES AT LOUGH INE
255
south basins. P . vulgata does not occur above an average high water
and is virtually the only intertidal species; P . aspera is almost confined
to the sublittoral; the highest specimens recorded, in the South Basin,
would have been exposed to air only a t low water of neap tides (which
within the lough is lower than low water of spring tides). We have also
found P . aspera on sublittoral rocks surrounded by mud; unless they
can cross mud, they must have spent their whole lives below water.
Distribution in the Rapids is much as in the lough. P . vulgata extends
down to low water and very occasionally below it. P . aspera occurs on
sublittoral boulders on the Sill.
2. Variation in Relation to Level and Habitat
(a) Specijic characters. Both species of Patella are widely variable, and
we examined a number of characteristics to see whether there were any
consistent correlations with environment. Certain features, among which
are the shape of the radula teeth and the nature of the marginal tentacles, are species diagnostic, as described by Fischer-Piette (1935). To
these may be added the internal appearance of the shell which shows,
when not obscured by nacre, a grey or white head scar with orangeyellow radiating ribs in P . vulgata, and an orange head scar with white
or pale straw ribs in P . aspera. Nacre increases with size in both species,
more quickly in P . aspera, to hide the original colour. It would be
particularly interesting to know what are the relative advantages - if
any - of these specific characters. For instance, differences in the structure of the radula might be associated with differences in the food at
different levels on the shore.
(b) Radula length. Other features, while having a species-characteristic
component, also show varying degrees of environmental modification.
Most important of these is the radula length. As radula length depends
on the size of the limpet, the ratio radula length/shell length (termed
the ‘radula ratio’) is frequently used for comparative purposes (FischerPiette, 1955). This is not entirely satisfactory for a critical evaluation
of our samples because though the regressions of radula length on shell
length are approximately rectilinear they do not for either species a t
any level pass through zero. By analysis of covariance we have been
able to show that, for each species, radula length, after correction for
shell length, tends to decrease as level decreases, although the corrected
mean radula length for P . vulgata is substantially greater than that for
P . aspera a t all levels (see also Brian and Owen, 1952). The mean radula
ratio for P . vulgata ranged from 1.66 at its highest level (A) to 1.30 a t
its lowest level (G), and that for P . aspera from 1.17-at level E to 0.93
at level I ; individual values overlap for the two species, as already noted
by earlier workers.
I**
255
south basins. P . vulgata does not occur above an average high water
and is virtually the only intertidal species; P . aspera is almost confined
to the sublittoral; the highest specimens recorded, in the South Basin,
would have been exposed to air only a t low water of neap tides (which
within the lough is lower than low water of spring tides). We have also
found P . aspera on sublittoral rocks surrounded by mud; unless they
can cross mud, they must have spent their whole lives below water.
Distribution in the Rapids is much as in the lough. P . vulgata extends
down to low water and very occasionally below it. P . aspera occurs on
sublittoral boulders on the Sill.
2. Variation in Relation to Level and Habitat
(a) Specijic characters. Both species of Patella are widely variable, and
we examined a number of characteristics to see whether there were any
consistent correlations with environment. Certain features, among which
are the shape of the radula teeth and the nature of the marginal tentacles, are species diagnostic, as described by Fischer-Piette (1935). To
these may be added the internal appearance of the shell which shows,
when not obscured by nacre, a grey or white head scar with orangeyellow radiating ribs in P . vulgata, and an orange head scar with white
or pale straw ribs in P . aspera. Nacre increases with size in both species,
more quickly in P . aspera, to hide the original colour. It would be
particularly interesting to know what are the relative advantages - if
any - of these specific characters. For instance, differences in the structure of the radula might be associated with differences in the food at
different levels on the shore.
(b) Radula length. Other features, while having a species-characteristic
component, also show varying degrees of environmental modification.
Most important of these is the radula length. As radula length depends
on the size of the limpet, the ratio radula length/shell length (termed
the ‘radula ratio’) is frequently used for comparative purposes (FischerPiette, 1955). This is not entirely satisfactory for a critical evaluation
of our samples because though the regressions of radula length on shell
length are approximately rectilinear they do not for either species a t
any level pass through zero. By analysis of covariance we have been
able to show that, for each species, radula length, after correction for
shell length, tends to decrease as level decreases, although the corrected
mean radula length for P . vulgata is substantially greater than that for
P . aspera a t all levels (see also Brian and Owen, 1952). The mean radula
ratio for P . vulgata ranged from 1.66 at its highest level (A) to 1.30 a t
its lowest level (G), and that for P . aspera from 1.17-at level E to 0.93
at level I ; individual values overlap for the two species, as already noted
by earlier workers.
I**