Variable sperm size and motility activation in the pipefish, Syngnathus abaster; adaptations to paternal care or environmental plasticity?

Abstract

Like seahorses, some of the closely-related pipefish species (Family Syngnathidae) incubate their eggs within a male brood pouch. This has contributed to considerable confusion about sperm transfer mechanisms to the eggs; some authors have reported that ejaculates are released directly into water before they reach the eggs, while others have suggested that eggs are fertilised using spermatozoa deposited directly into the brood pouch via an internal sperm duct. Here we present anatomical evidence from the freshwater pipefish, Syngnathus abaster, showing not only that direct sperm deposition into the pouch is impossible, but that spermatozoa must somehow travel a significant distance (>4 mm) outside the body of the male, to reach and fertilise eggs in the pouch. We have also used several putative sperm-activating solutions to identify the type of environment most conducive to sperm activation. Spermatozoa released from the testis were active for a brief period (<5 min) in water or 150 mm saline, but showed prolonged (>25 min) motility in ovarian fluid. This suggests that spermatozoa are released into a mixture of ovarian fluid and eggs while the male and female are in close contact. Our data also suggest that the fertilisation mechanism is highly efficient (sperm : egg ratio <200 : 1) even though this pipefish species produces dimorphic spermatozoa (with long and short flagellae). The shorter (<40 microm) morphotypes were not capable of motility activation, and are therefore probably incapable of fertilisation. If so, the sperm : egg ratio reported here would represent an overestimate.

Intra‐ and intermale variability of mature sperm traits analysed in two brackish water populations of the pipefish Syngnathus abaster (Syngnathidae)

“…These values, which correspond to a good representation of the data without a real danger of bad interpretation, allow as to assert that (i) the two populations seem to be indistinguishable on the base on their spermatozoa (stress value = 0.16) and (ii) there is no polymorphism, being sperm not distinguishable into discrete classes either within a single male (stress value = 0.13) or within each populations (stress values = 0.14 and 0.2 for Sardinian and Venice populations, respectively). The latter datum does not seem to support the presence of polymorphic sperm, classified in three different groups, reported in the freshwater population of the same species S. abaster (Dzyuba et al 2008). First, the two studies differ in the methods performed to obtain measurable sperm.…”

“…First, the two studies differ in the methods performed to obtain measurable sperm. We used fixed spermatozoa, whereas Dzyuba et al (2008) used living cells. In addition, we have measured mature (apparently fully formed) and intact sperm (sperm with all their three portions clearly visible, i.e.…”

“…In addition, we have measured mature (apparently fully formed) and intact sperm (sperm with all their three portions clearly visible, i.e. nucleus, midpiece and flagellum) only, whereas Dzyuba et al (2008) measured all sperm when it was possible to obtain clear images. Because these methodological differences, spermatozoa with very short flagellum were discarded in our study, whereas Dzyuba et al (2008) included them in their data set, despite the analysis on motility of these sperm had a negative outcome.…”

“…However, Dzyuba et al (2008) reject this hypotheses for the shorter spermatozoa of Syngnathus abaster. These authors asserted that '.…”

“…The acquasperm are characterized by a spherical head and are typical of external fertilizing fish (Jamieson 1991). Moreover, three different morphotypes, differing in flagellum length, head length and head wide, have been reported in a freshwater population of Syngnathus abaster (Dzyuba et al 2008).…”