The site of sperm storage in C. hyalina was investigated using thymidine-methyl-3H. Failure of sperm donor colonies to take up the radiotracer and incorporate it into spermatozoa appears to have prevented the identification of the site of storage of exogenous sperm in this species. Potential ways of overcoming this problem in future studies are discussed.
Considerable variation in the ability of colonies to self-fertilise was observed
among twenty one geographically isolated populations of C. hyalina. Results
ranged from a complete lack of self-fertilisation activity in isolated colonies from
most populations to comparable levels of embryo production by isolated and
grouped colonies from the Woods Hole population. This study also included
colonies of two other Celleporella species, C. angusta and C. carolinensis.
Isolated colonies of C. carolinensis were found to suppress the development of
female zooids, and therefore to be incapable of self-fertilisation. All isolated
colonies of C. angusta, however, were found to produce embryos and release
abundant viable larvae in numbers comparable to those released by grouped
colonies.
Mating trials to assess the level of reproductive compatibility between
geographically isolated populations revealed the presence of at least seven good
biological species, and perhaps several incipient species within C. hyalina sensu
lato. All mating trials between populations occupying separate phylogenetic
clades (based on mitochondrial DNA sequencing) were unsuccessful. Mating
trials between populations occupying the same phylogenetic clade gave results
ranging from complete reproductive compatibility to complete reproductive
incompatibility.
Morphological analysis appeared to reveal the presence of at least four
morphologically distinct populations within C. hyalina sensu lato, and revealed a
number of potentially useful taxonomic characters. However, the presence of
cryptic species within C. hyalina sensu lato could not be ruled out. The problems
associated with the failure to recognise cryptic species are discussed. These
studies suggest that the use of a concordant suite of unrelated morphological,
reproductive and molecular genetic characters may be necessary for the
discrimination of species in bryozoan groups.