|Reproduction Preferences Additional Information||
Gametogenesis and spawning:
- Eggs and early larvae develop within the female burrow, however post larvae are capable of active migration by crawling, swimming in the water column and passive transport by currents e.g. Günther (1992) suggested that post-larvae of Arenicola marina were transported distances in the range of 1 km.
- Wilde & Berghuis (1979b) reported 316,000 oocytes per female with an average wet weight of 4g.
Beukema & de Vlas, (1979) suggested a life span, in the Dutch Wadden Sea, of at least 5-6 years, and cite a life span of at least 6 years in aquaria. They also suggested an average annual mortality or 22%, an annual recruitment of 20% and reported that the abundance of the population had been stable for the previous 10 years. However, Newell (1948) reported 40% mortality of adults after spawning in Whitstable.
Adults reach sexual maturity by their second year (Newell, 1948; Wilde & Berghuis, 1979) but may mature by the end of their first year in favourable conditions depending on temperature, body size, and hence food availability (Wilde & Berghuis, 1979).
Factors influencing spawning:
- Germ cells released from gonads at meiotic prophase I.
- Spermatogenesis and oogenesis occur within the coelomic cavity. Sperm are released into the coelomic cavity in packets or sperm morulae. Release of gametes from the body cavity, and in the case of sperm by the prior breakdown of morulae, is under endocrine control by a 'maturation factor'. The 'maturation factor' is released by a neurosecretory organ, the prostomium (Bentley & Pacey, 1992; Pacey 2000). Sperm maturation factor stimulates breakdown of sperm morulae and spawning.
- Spawning takes place within the burrow.
- Spawning of gametes occurs due to rhythmic contractions of the body wall, and the gametes are released via the nephridia (Bentley & Pacey, 1992).
- Sperm motility is stimulated by the change in pH as the sperm are released into seawater (i.e. from pH 7.3 in the coelomic cavity to pH 8.2 in seawater).
- Spawned sperm are flushed out of the burrow by pumping activity of the male, whilst oocytes are retained in the horizontal shaft of the female's burrow.
- After spawning males fasted for 2 days while females fasted for 3-4 weeks, presumably to avoid ingesting eggs and larvae (Farke & Berghuis, 1979).
- Once spawned sperm remain motile for over 5 hours at 14 °C. (Pacey, 2000), form puddles on the sediment surface and are dispersed by the incoming tide. Eggs (oocytes) are retained in the females burrow (Bentley & Pacey, 1992).
- Sperm swim intermittently, perhaps in response to light, and Pacey (2000) suggested that this may be an adaptation to downward swimming towards the eggs.
Spermatogenesis, sperm maturation and oocyte maturation have been in studied in detail by Bentley & Pacey (1989), Bentley & Pacey (1992), Watson & Bentley (1995), and Watson & Bentley (1998). A comparative study of gametogenesis in Arenicola marina and Arenicola defodiens was carried out by Watson et al. (1998).
- Spawning usually occurs in late autumn or early winter but may occur in early spring (Pacey, 2000).
- Spawning is inhibited by temperatures above 13 or 15 °C (depending on study) (Bentley & Pacey, 1992).
- Synchronous spawning is associated with spring or neap tides suggesting a correlation with tidal or lunar cycles (Howie, 1959; Bentley & Pacey, 1992).
- Watson et al., (2000) examined Arenicola marina population on East Sands, St. Andrews and suggested that synchronous spawning was dependant on a number of environmental cues, i.e. once gametogenesis is complete (about late summer depending on population) a drop in sea temperature - of defined, but unknown magnitude - triggers endocrine stimulation of spawning. Synchronous spawning is then is triggered by spring tides, probably due to changes in hydrostatic pressure rather than lunar phase.
- Warm summer temperatures (ca May to July) may facilitate gametogenesis, due to increase metabolic rate and food availability, allowing the population to mature earlier and hence spawn earlier (Watson et al., 2000).
- Watson et al. (2000) suggested that the East Sands population spawned preferentially in clement weather (high pressure, low rainfall and wind speed), when sperm dilution (due to wave action) is minimal. Inclement weather coincident with spring tides resulted in the population wide spawning being aborted on the East Sands in 1996 (Watson et al., 2000).
- Individuals within a given locality may spawn synchronously, e.g. at East Sands, St. Andrews, over a period of 13 years observation spawning time varied by 5 weeks, but was synchronous over a period of 4-5 days (Watson et al., 2000).
- The exact timing of spawning varies between locations and some populations demonstrate protracted spawnings. For example, on sandy shores near St Andrews and Dublin spawning occurred between mid October to mid November, peaking in early November, whereas at Fairlie Sans, Millport spawning occurred between Apr and May and again in autumn (Howie, 1959; Bentley & Pacey, 1992). Dillon & Howie (1997) reported marked differences in timing of synchronous spawning or protracted spawnings in populations of Arenicola marina from the east coast of Ireland, even though separated by no more than 85 miles. The reported spawning periods of Arenicola marina were reviewed by Clay (1967; Table 1).