Is seasonal breeding in Retusa obtusa (Montagu) (Gastropoda: Opisthobranchia) merely the consequence of seasonal breeding in its prey, the mudsnail Hydrobia ulvae (Pennant)?
Berry AJ., Radhakrishnan KV., Coward K.
Retusa aobtusa (Montagu) lives little > 1 yr with most young appearing in Feb-May, followed by death of adults in May-Jun. In the Forth estuary, the main cohort of young feed on foraminifera (≈ 200 μm) until Jul-Aug when they switch to newly settled young Hydrobia ulvae (starting at ≈ 300 μm). Most breeding occurs in Jan-Mar. By May, early stages of spermatogenesis occurred in gonads of R. obtusa as small as 1.1 mm. Fully developed spermatozoa were found in gonads of most snails > 2.0 mm, and these were stored in seminal receptacles in snails > 2.5 mm. By Sep, even snails of 1.9-2 mm had mature spermatozoa in gonads and seminal receptacles, long before any substantial breeding occurred. At no time of year did snails < 2.6 mm have oocytes > 85 μm diameter. Fully grown oocytes (170-330 μm diameter) first occurred in gonads of snails > 2.7 mm in Aug, again long before any substantial breeding. From Dec, gonads of snails > 2.7 mm become progressively dominated by such full-grown oocytes rather than by spermatogenic tissue. In the laboratory, R. obtusa of 3.0-5.2 mm laid very few eggs between late Jul and early Dec, reaching a mean ≈ 0.3 eggs·d -1 only in mid-Nov - early Dec 1990. Subsequently snails of similar sizes released markedly more eggs: ≈ 1·d -1 during mid-Dec 1990 - mid-Jan 1991 and, briefly, 4-5.5·d -1 in Feb and Mar. Larger R. obtusa died off in May. Manipulation of laboratory water temperature, daylength and tidal regime over periods of up to 42 d failed to show significant effects on spawning rates except that some unnatural regimes (e.g., 15°C) adversely affected survival and spawning. Although R. obtusa of 3-4.5 mm bore fully developed oocytes and spermatozoa in autumn, lowered temperature and shortened daylength did not bring forward the faster spawning characteristic of Dec-Mar. It is possible that breeding activity is not directly timed by familiar environmental signals but instead simply follows a requisite period of growth and accumulation of large oocytes in an annual cycle imposed by the provision of young Hydrobia as prey to those predators at the right size to swallow them when they first appear in Jul-Aug. © 1992.