THE DEVELOPMENT OF ELECTRA CRUSTULENTA (PALLAS) 259
Membranipora seuratii (Canu), a species regarded by Borg
(1931) as a Mediterranean variety of E. crustulenta, was found
growing with true E. crustulenta in the Creek. Evidence has
been gained supporting the hypothesis of Hastings (MS) that the
breeding season of M. seurati differs from that of E, crustulenta.
Development of E. crustulenta (Pallas). The possibility
that the larva of Electra crustulenta might be the Cyphonautes,
C. barroisi Lohmann, has been suggested by Marcus (1940: 336),
Thorson (1946: 159), and Borg (1947: 365). C. barroisi was first
described (Lohmann, 1911: 38) from the North Sea; and Thorson
and Marcus gave its distribution in the Baltic Sea, showing that
it was the only planktonic Polyzoan larva extending as far east
as the Gulfs of Finland and Bothnia, where salinity is lowest. As
this distribution matched that of Electra crustulenta (see Borg,
1931: 18), the strong supposition that its larva was C. barroisi
lacked proof only in the observation of its metamorphosis.
Howes (1939: 383) recorded the occurrence of E. crustulenta
on Ruppia maritima in New England Creek, Essex, between the
River Roach and the sea, and found a Cyphonautes larva (un-
identified) in large numbers in the plankton in March. E.
crustulenta and C. barroisi were collected from the Creek on 15th
and 19th March 1957 respectively by Mr. G. Duncan Waugh,
and sent to Dr. D. Atkins, who presented specimens to the British
Museum (Natural History).
Thorson (1946: 157) notes that the numbers of all Cypho-
nautes larvae (irrespective of species) found in the Baltic "showed
a distinct minimum in February-March" (see text fig. 194), which
is very similar to the figures obtained for the North Sea and
illustrated by Rees (1954: 64, text fig. 12). C. barroisi was,
however, recorded (Thorson, 1946: 159) throughout the year
"with a maximum in winter"; and it is the only species found in
New England Creek in March. Further north, in 60° N. lat., on
the Baltic coast of Sweden, Borg (1947: 363) found evidence of
a breeding period in E. crustulenta from April to June, but he did
not obtain larvae. I have examined some of Borg's material and
confirmed that it is true E. crustulenta.
It thus appeared that an attempt to demonstrate the identity
of E. crustulenta and C. barroisi could very suitably be made by
collecting larvae from the Creek in March and endeavouring to
rear them through their metamorphosis at the Fisheries
Laboratory, Burnham-on-Crouch. Accordingly a week was spent
there in March 1959 and 3 days in March 1960. Collections were
made from the Creek, both of adult colonies growing on Ruppia
and of Cyphonautes from tow-nettings. Laboratory study of this
material has proved conclusively that C. barroisi is the larva of
E. crustulenta.
Howes described the water of New England Creek as "not
strictly brackish" as it has no direct connection with either the
sea or with the River Roach, being dammed at both ends. He
gave a salinity range during March 1934 and 1935 of 21.5-25.1‰,