Peter P. Fong

The zebra mussel (Dreissena polymorpha), a new pest in North America: reproductive mechanisms as possible targets of control strategies

Summary The zebra mussel (Dreissena polymorpha) has spread rapidly in temperate fresh waters of North America since its introduction into the Great Lakes in 1985 or 1986. It attaches to hard substrates, forming layers, occluding water intakes, encrusting and killing native mussels, filtering algae in competition with other planktivores, and possibly interfering with fish spawning. It reproduces prolifically, suggesting that an approach to its control may be by controlling its reproduction. Previous literature suggests that spawning in bivalves is regulated by both environmental and internal chemical cues. A suggested sequence is that phytoplankton chemicals initially trigger spawning; chemicals associated with gametes provide a species-specific pheromonal positive feedback for spawning; and the response to environmental chemicals is mediated internally by serotonin (5-HT). The role of 5-HT in zebra mussels is under investigation. Both males and females can be induced to spawn by either injection or extern...

The Effect of Salinity and Temperature on Spawning and Fertilization in the Zebra Mussel Dreissena polymorpha (Pallas) from North America

Zebra mussels have dispersed from their original site of introduction in the Great Lakes into the Mississippi River, Hudson River, and other watersheds in which they will encroach upon brackish water estuaries. To investigate their likelihood of reproductive success in such estuaries, we investigated the conditions of temperature, salinity, and acclimation under which spawning and fertilization could occur. Reproductive function of mussels that were acclimated to salinities up to 7.0 parts per thousand (ppt) at 12°, 20°, and 27°C for 1 to 21 days was tested. Reproductive function of non-acclimated mussels that had been maintained in fresh-water aquaria was also tested in various salinities. Spawning was induced by serotonin, previously demonstrated to elicit spawning of fertile gametes in fresh water. Successful fertilization was indicated by oocyte cleavage after adding sperm. Nonacclimated mussels spawned in salinities of 1.75 and 3.5 ppt at 12°, 20°, and 27°C, but not at 7.0 ppt. Fertilization using gametes from non-acclimated mussels occurred only in fresh water and at 1.75 ppt. Acclimation for as little as 2 days enhanced spawning. Fertilization rate in a salinity of 3.5 ppt improved within 4 days of acclimation and continued at a high level for as long as 21 days of acclimation. Although animals acclimated for 4 days in 3.5 ppt spawned readily when tested in salinities as high as 7.0 ppt, almost no fertilization occurred in 7.0 ppt. The reduction in fertilization at increasing salinities may be due in part to reduced sperm motility. Unfertilized oocytes remain intact for hours in fresh water; however, in salinities as low as 0.7 ppt, unfertilized oocytes tended to rupture within 2 hours. These data show that although sudden increases in salinity produce an immediate decrease in the reproductive capacity of zebra mussels, acclimation to brackish water can occur, and zebra mussels may be able to reproduce in brackish water below 7.0 ppt.

Serotonergic Ligands Induce Spawning but not Oocyte Maturation in the Bivalve Mactra chinensis from Central Japan

We examined the spawning sensitivity to serotonin and serotonergic ligands in the Japanese bivalve Mactra chinensis. Spawning was induced by both injected and externally applied scrotonin (5-hydroxytryptamine. 5-HT). The vertebrate 5-HT2 receptor agonist alpha-methyl 5-HT and the selective 5HT1A agonist 8-OH-DPAT were also effective at inducing spawning. However TFMPP (m-trifluoromethylphenylpiperazine, a verterbrate 5-HT1 receptor agonist) and 1-methyl-chlorophenyl biguanide (a vertebrate 5-HT3 agonist) were not effective spawning inducers. The 5-HT-induced spawning was blocked by mianserin (a vertebrate 5-HT2 antagonist). The rank order of potency of the agonists was: 5-HT > alpha-methyl 5-HT > 8-OH-DPAT > TFMPP > 1-methyl-chlorophenyl biguanide; these data support a growing body of literature invoking a mixed 5-HT1/5-HT2 pharmacological profile for serotonin receptors mediating reproductive processes in bivalves. However, neither 5-HT nor 8-OH-DPAT induced germinal vesicle breakdown (GVBD) in Mactra oocytes. Sperm induced GVBD in a high percentage of oocytes. This is the first report of a bivalve in which spawning, but not GVBD, can be induced by 5-HT. This result might be expected because Mactra spawns germinal vesicle oocytes that normally undergo GVBD upon fertilization, but is in contrast to the case of the closely related Spisula spp. in which serotonin induces both processes. The ability of 5-HT to induce spawning but not GVBD makes Mactra chinensis a model organism for studying spawning and meiotic mechanisms in bivalves.

Serotonergic mechanisms mediating spawning and oocyte maturation in the zebra mussel, Dreissena polymorpha

Summary The zebra mussel, Dreissena polymorpha, is a freshwater biofouling bivalve unintentionally introduced in the 1980s into North America from Europe. Oocyte maturation (germinal vesicle breakdown, GVBD) and spawning of the zebra mussel can be triggered with serotonin (5-hydroxytryptamine, 5-HT). In pharmacological experiments to characterize the receptor mediating spawning, the serotonin receptor agonists 8-OH-DPAT, TFMPP, and 1-(1-naphthyl)piperazine were effective at stimulating spawning; whereas, 2-methylserotonin and alpha-methylserotonin had no effect. In experiments with antagonists of serotonin receptors ketanserin and propranolol had no effect; mianserin, NAN-190, and cyproheptadine had partial inhibitory effects; and methiothepin was a very effective antagonist. Metergoline had mixed agonist/antagonist properties. Ergotamine was the most effective activator of spawning in females. Compared to serotonergic receptors in other organisms, the receptors that activate spawning in zebra mussels res...