In the present research several aspects of the reproductive biology of the marine hermaphroditic bryozoan, Celleporell hyalin (Linnaeus, 1767) were investigated. First (see Chapter 2), aliquots, of different ages from a stock of allosperm suspension were used to fertilize a series of virgin ramets, so characterizing the decay in fertility of released sperm and any effects of sperm ageing on subsequent embryogenesis and larval metamorphosis. The effect of temperature on the above variables was also investigated. The fertile half-life of C. hvalina sperm was about 1-2 h, although significant decay in fertility occurred within a few minutes after release. Sperm ageing showed no deleterious effects on embryogenesis, larval viability, or metamorphosis. No clear effects of temperature on sperm ageing and fertilization success were found. Allosperm. storage was studied in colonies of C. hyalina. (L. ) under several experimental conditions (see Chapter 3). Recipient virgin colonies were exposed to sexually compatible allosperm suspension and the appearance of the last newly ovulated oocytes in the coelorn. was used to assess duration of sperm storage. The same experiments examined continuation of brooding cycle and brooding success throughout the period of allosperm storage. Similar obserVations were conducted on wild colonies of C. hyalina taken from the field and kept in reproductive isolation in the laboratory. Production of progeny in females zooids budded beyond the original colonial growing edge was taken as evidence of sperm movement. The results of the present study show that recipient colonies continue producing coelomic oocytes up to 5 weeks after exposure to allosperm suspension. Moreover, the progeny were produced not only by female zooids present at the moment of allosperm dosage but also by female zooids, budded later, beyond the limit of the original growing edge. Since oocytes were not present in control colonies exposed to selfsperm, the results of the present study indicate that recipient colonies store sexually compatible allosperm and transport them within the colony in order to produce viable progeny. The effect of water flow on both sperm release and fertilization success in colonies of C. hyalin (L. ) was studied (see Chapter 4). Maximum numbers of released sperm were found at low and zero water velocities. Moreover, protruded male lophophores were observed only under those conditions. Fertilization success was studied in virgin colonies of C. hyalina (L. ) exposed to compatible allosperm suspensions under different feeding activity and water flow conditions. Fertilization success was higher in colonies with more active feeding autozooids than in those with fewer feeding autozooids. High water flow conditions induced reduction in the proportion of protruded lophophores, and reduced the frequency of ovicells bearing progeny. Moreover, in all the experiments offspring were concentrated in areas of the colony bearing active feeding autozooids. The results of this study suggest that sperm release take place under similar conditions that enhance cross fertilization, with a possible role of feeding activity in bringing sperm to the proximity of receiver colonies. Sperm competition and female choice was investigated in virgin colonies of C. hyalin (L. ) exposed to sexually compatible allosperm cocktails (see Chapter 5). A microsatellite-based. genotyping system was used to determine paternity. Progeny were mainly the product of outcrossing. In a few cases, a small proportion of progeny was attributed to selffertilization. These results suggest that outcrossing is the main reproductive strategy in this species and that neither selective female choice nor sperm competition occur in C. hyalin . Cryptic incompatibility allowing a flexible mating strategy to produce out-crossed progeny Z) in the presence of allospenn and selfing when they were absent was not found. The effects of mating sequence and temporal interval between matings (2 or 48 h) on sperm precedence in double-mated individuals were studied (see Chapter 6). Paternity was determined by using a microsatellite-based genotyping system. Settled colonies produced after short mating showed evidence of sperm mixing and first-male precedence. However, last colonies produced after both short and long mating intervals showed evidence of first-male precedence. When analyses were conducted using all the sampled progeny, low incidence of paternity by the second sperm donor (P2) and absence of self fertilization were found. No effect of mating order on success of the second donor was found. Prevalence of outcrossing was also found. These results suggest that first-male precedence in C. hyalin may promote outcrossing under sperm limitation conditions, by acceptance of the first compatible allosperm to become available. The effects of exposure to sperm suspensions and stressors on sexual allocation in colonies were studied (see Chapter 7). In the first experiment the effect of a waterborne factor on receptor colonies was studied. Adult colonies were exposed to compatible allosperm suspensions that had been filtered through 0.45 Vrn pores potentially able to remove cellular sized particles. As a control, receptor colonies were exposed to non-filtered allosperin suspensions. Appearance and growth of oocytes occurred only in the coelorn. of the control colonies. The active factor is not a dissolved molecule, but a particle that can be removed from water by filtration through 0.45 [im pores. This result plus the absence of developing oocytes in the receptor colonies exposed to similar concentration of selfsperm, suggest the operation of self/honself recognition and an important and active role of allosperm in initiating colonial reproductive investment in C. hyalina. Prevention of colonial growth and others stressors were associated with production of basal male zooids. In other experiments, exposure to sperm suspensions of different degrees of genetic relatedness showed a virtual absence of production of progeny in those colonies exposed to closely related sperm (i. e. self and halfsibs). Finally, in experiments with sexually immature colonies, the onset of sexual reproduction was triggered by exposure to allosperm, resulting in the production of female zooids even before the appearance of male zooids. Contacts between colonies of different genetic relatedness were studied under laboratory conditions (see Chapter 8). Moreover, observations were made on colonies growing on their natural substrata. Five types of responses were observed, from total fusion to overgrowth. Maximum degree of fusion, or morphological fusion, was manifested as morphologic interconnection between the ad oining colonies (i. e. production of coalescent zooids). Fusion occurred in all contacts between colonies of the same genotype, between parental colonies and their progeny and, between full and half sib colonies. In most cases the production of coalescent zooids was found. Absence of fusion occurred in all contacts between unrelated colonies and between some of the half sibs. Observations on wild colonies growing in contact with each other failed to reveal any incidences of coalescence. Non-aggressive overgrowth was confined to dead areas of one colony overgrown by zooids of the other healthy colony, independent of the genetic relatedness of the pairs. Differences in the fusibility between isocontact and allocontact suggest that colony specificity exists in C. hyalin , as has been found in other sessile colonial marine organisms.