Reproduction
 
photograph of solitary tunicate Ascidia paratropa

drawing of solitary tunicate showing features of reproductive systemsTunicates are hermaphroditic, with ovary and testis usually maturing at the same time.  However, although eggs and sperm may be released together from the atrial siphon, self-fertilisation is not common.  Breeding season is usually in summer but can be all year long in some species. 

 

 

 

Solitary tunicate Ascidia paratropa with the opening
to the branchial siphon just visible at the top 0.7X

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  Spawning
  Reproduction involves spawning dealt with here, while BROODING, LARVAL LIFE, SETTLEMENT & METAMORPHOSIS, and EARLY JUVENILE are considered in other sections.
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Research study 1
 

Experiments at Corona del Mar, California on Ciona (see photograph of C. intestinalis below) show that egg and sperm release occurs in early morning in response to change from darkness to light.  Cross-fertilised eggs in 100 experiments result in 100% normal cleavage, while self-fertilised eggs in 5 experiments mostly fail to cleave (mean = 28% cleavage).  In addition, many instances of abnormal development are associated with self-fertilisation. The author provides the results of many different kinds of treatments, for example, whether eggs are washed or not, whether they are old or young, whether cleaned glassware is used, and whether A eggs are crossed with b sperm or B eggs with a sperm, and so on, but the results are many and variable and interpreting them is confusing.  Perhaps they may be useful to someone with interests in embryological development of these hermaphrodites.  Morgan 1941 Biol Bull 80 (3): 338.

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Research study 2
  The colonial ascidian Botryllus schlosseri is an introduced species from the east coast of North America.  Studies in Massachusetts using allozyme markers show that sperm concentrations decline rapidly within 50cm of a source colony.  This, combined with limited larval dispersion, generally within 1m of the parent colony, suggests the potential for genetic diversification and inbreeding in a population.  In fact, detailed study using allelic and genotypic frequency-analysis of 3 polymorphic allozyme loci indicates a bit of both, namely, an absence of genetic diversification, but the presence of inbreeding.  The author discusses the genetic and population implications of these results.  Grosberg 1987 Evolution 41: 372; see Grosberg 1991 Evolution 45: 130 for references to further research on B. schlosseri.
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Research study 3
 

photograph of tunicate Molgula pacificadrawing of life cycle of tunicate Molgula pacificaMolgula pacifica is unique among tunicates on this coast because it has no tadpole larval stage.  It is hermaphroditic and apparently can fertillise itself, although whether this happens regularly is unclear.  After the eggs are released from the parent they rupture and release a sticky secretion that attaches them to the sea bottom on contact.  Metamorphosis takes place within the egg membrane and hatching is to a tail-less juvenile that attaches to the substratum and grows to an adult. Because the egg remains in a habitat favourable to survival, such a strategy increases the chance of successful settlement, but chance for long-distance dispersal to new habitats is minimal.  Young et al. 1988 Biol Bull 174: 39; see also Bates & Mallett 1991 Can J Zool 69: 618; diagram from Bates 1994 Am Zool 34: 333.

NOTE most other solitary tunicates hatch to a tadpole larva, then metamorphose later, a developmental schedule different from that found in Molgula

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Research study 4
 

photograph of solitary tunicate Ciona intestinalisThe solitary tunicate Ciona intestinalis is hermaphroditic but does not normally self-fertilise. Eggs are released into the seawater where they are fertilised by sperm from another individual. The eggs are negatively buoyant and sticky; sometimes they are released in mucous strings that may adhere to the bottom or get stuck up on the adult.  Tadpole larvae hatch in 1-2d (at 13°C), are non-feeding, and may spend up to 6d swimming about. However, if a larva originates from an egg string, it may forego swimming and settle within moments of hatching.  Although preference for settling near to adults by free-swimming larvae cannot be ruled out, the localisation caused by sticky eggs may explain the aggregated nature of some populations of CionaSvane & Havenhand 1993 Mar Ecol 14: 53. Photograph courtesy Angus Jackson and the Marine Biol Assoc UK, Plymouth, England.

Ciona intestinalis with simulated egg
string emerging from the atrial siphon 1X

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Research study 5
 

comparison of gonadal indices over a 1yr period in 3 species of west-coast tunicates on regular and out-of-phase light cyclesThe effect of light cycles on gametogenesis and spawning in 3 species1 of Puget-Sound ascidians is investigated by a researcher at Shannon Point Marine Center, Washington.  For each species the author collects adult individuals from the field, allocates them haphazardly to 4 screened, artificially lighted aqaria and rears them for 11-13mo periods (depending upon species) beginning in October.  Two of the aquarium tanks are exposed to ambient daily light cycles and ambient daylengths through the test period; the other two, to daily light cycles set 12h out of phase and daylengths set 6mo out of phase with those in the other 2 tanks.  Each month gonads are sampled2 for determination of gonad indices3 and other features.  Results for the 3 species are shown on the Left.  Under a normal light cycle, all species followed patterns reported for field populations, Boltenia reproducing more-or-less continuously, and Styela and Chelysoma spawning primarily in spring.  Alteration of the light cycle has little consistent effect on gametogenesis in Boltenia and Styela, but strongly modifies that in Chelysoma (see graphs).  Overall, the results suggest that other factors, such as temperature, are likely contributing more to gametogenetic control than light.  The author suggests that experiments over longer term and with inclusion of different factors may be required to resolve the issue.  Bingham 1997 Invert Biol 116 (1): 61.

NOTE1  these are Boltenia villosa, Styela gibbsii, and Chelyosoma productum

NOTE2  for reasons presumably of shortage of research specimens, monthly sample sizes are notably small: no more than 1-2 for each species (actual details of sample sizes are contradictory in different parts of the report).  The reader should interpret the results with this in mind

NOTE3  calculated as % of total body mass excluding tunics consisting of gonads.  Oocyte volumes are also determined but not presented here in this summary

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