106
THE ESSEX NATURALIST.
Mollusca : Cardium edule, var. lamarkii, Mya arcuana,
Littorina saxatilis tenebrosa, Hydrobia ven-
trosa, Phytia myosotis myosotis, Embletonia
pallida.
Polyzoa : Membranipora crustulenta var. fossaria.
Vertebrata: Anguilla vulgaris, Gasterosteus aculeatus,
Gobius niger, Gobius minutus, Platichthys
flesus, Zoarces viviparus.
The zooplankton shows the same type of annual fluctuation
as in sea-water but the productivity is very much less.
The fauna list shows that the majority of inhabitants are
marine forms and almost certainly forms which are descendants
of ancestors trapped when damming was completed. It is of
interest to examine what factors determine their presence and
continued existence in the creek and to compare their present
environment with sea-water from which it may be assumed the y
originated.
The historic factor is important, in that, although similar
environments may occur in other places, the fauna of New
England Creek will clearly depend upon the local fauna at the
time of its formation as an artificial lagoon. Not only will the
species present be local species, but the relative numbers of
individuals of different species trapped may quite well deter-
mine the future nature of the fauna. A single species im-
prisoned in large numbers may increase rapidly numerically to
the detriment and even final elimination of other scarcer species.
Change in osmotic pressure has a selective action on marine
organisms, some, e.g., Palaemonetes varians, Platichthys flesus,
being able to live in very dilute sea-water, while the majority are
unable to withstand even small dilutions; further, while adults
of a species may be euryhaline, the young and the ovigerous
females may not; the selective action will therefore apply first
to adults, but in time there will be a further elimination of forms
which cannot breed. The salinity of the creek varied from
21—28‰ compared with sea-water of 35‰, with a corresponding
reduction in osmotic pressure. Resistance to reduced osmotic
pressure is accompanied by increased oxygen requirements, but
there was no shortage of oxygen.