Purely qualitative investigations of the plankton of an area cannot
provide sufficient i nformation for the full understanding of the seasonal
changes that oocur nor of the interrelation of the various planktonic
plants and animals. Quantitative data must be obtained before these can
be properly described. Except for the qualitative records by Scott (1906,
19(7) and Hiddell (1914) the plankton of waters off the north ooast of ales,
and. of the l-ienai straits in partioular, has tend d to be neglected. The
UlluSual hydrographic conditions of the enai .;)traits, which connect areas
whose fauna and flora show considerable differences (Crisp & Knight-Jones,
1953) , sUggested such studies would be rewarding.
In describing the various populations the policy in the present work
has been to give the details of the most numerous speoies of both plants
and animals individually and to summarize the others in various groups .
This simplifieD the description and discussion of the dynamics of the
population, which would be difficult to follo if all species were described
At an early stage in the investigations it became obvious that the
plankton of these coastal waters differed considerably from that of the
Irish es in general , which has been described by Willi8I:lson (1 956) . In
particular tho copepods Oi thonina .!!!!l!. (Giesbrecht) and terniIm acutifrons
, (Dana) were prominent species in the Menai Straits.
Euterpina acutifrons was found to have several points of interest.
Firstly, dioorphiam occurs in the malo; this has received considerable
attention and has been found to be of so significonce in the bi'oloey of
the speoies . Secondly, Euterpina is pelagic, while moat other harpacticoid
copepoda are benthio or littoral . Investigations of its a.evelop ant ( hich
was inadequately described by Tesch (1915)) .in culture allowed a consideration
of the adaptive modifications of Euterpina to a pelagic existence, of its
rate of grouth and development, and of the developmental aspects of
dimorphism in the male. The latter has been a controversial subject in
previous stuaies of copepod development (~ewell , 1912, 1929, 1940; Gurney,
1929; Coker, 1934) . Thirdly, ";uterpina has a world-wide distribution, mainly
c<""rc ... ~ " in the warmer seas. Consideration of this led to investigations of some aspects of the relationship between the environment and th devslopm nt of the species and also to a comparison of ita breeding season in the wat rs
round Anglesey with those reported in other latitudes .
The present work also includes studies of the larval d velopment of
Qithonina.nena another warm water form. Despite its being a very common
plankton animal, the larval. development of this species has tended to be
In the work as now presented the studies of these speoies are given
a considerable proportion of the total space, since the data provided on
their development , growth and breeding illuminate the general study of the
plankton which, in turn. forms a background to the detailed studies.
The form of preoentation of this thesis ms;y perhaps call for Bome
explanation. The secti0D:B into which it is divided have been prepared in
the form of separate papers intended for publication. This meane that
figures and tables are numbered oonsecutively within each section and not
continuously throughout the thesie, and aleo that a separate bibliography
will be found at the end of each section" It is hoped that this method of
presentation will cause no inconvenience to the reader.