Reproduction & development

diagram of generalised life cycle of a jellyfishJellyfishes have separate sexes.  Gonads develop in the lining of the digestive cavity.  Fertilisation may take place in the open water, or sperm may swim into the gastrovascular or digestive cavity of a female and fertilise the eggs in situ.  In pelagic species a planula larva swims and crawls for a time, then develops into a polyp known as a scyphistoma.  The scyphistoma begins to bud off small medusae or ephyrae, in a process known as strobilation, and these ephyrae develop into juvenile jellyfishes. photograph of jellyfishes Aurelia aurita

This pattern differs for sessile jellyfishes, mainly in the absence of strobilation. The accompanying diagram summarises the major phases in the life cycle of the west-coast species Aurelia labiata.  Drawings modified from Lucas 2001 Hydrobiologia 451: 229 Gröndahl 1988 Mar Ecol Progr Ser 45: 87.

NOTE  sometimes referred to as a scyphopolyp

Jellyfish Aurelia labiata showing gonads
in the gastrovascular pouches 0.6X

  The accounts below deal with sessile and free-swimming jellyfishes separately.
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  Sessile jellyfishes
Research study 1

photograph of a sessile jellyfish Manania gwillamiSessile jellyfishes such as Manania have separate sexes.  Eggs are produced in springtime from cells that line the digestive cavity.  Sperm swim into the digestive cavity, fertilise the eggs, and about a day after fertilisation, planula larvae develop. The larvae are about 0.1mm in length and consist of 16 large cells covered with a layer of smaller cells.  Unlike planulae of other cnidarians that swim about for a day or two, the planulae of sessile jellyfishes creep along the sea bottom in a sort of accordian fashion until they find a suitable spot to settle.  As each accordian-like movement takes about 2min to complete, final settlement and initiation of metamorphosis usually occurs close to the adult.  Metamorphosis in Manania begins with a rearrangement of the large cells mentioned above to surround a central digestive cavity, but what happens later is not known.  Otto 1978 p. 13 In, Settlement and metamorphosis of marine invertebrate larvae (Chia & Rice, eds.) Elsevier NY.

NOTE  the large cells are endodermal and will form the gut; the small cells are ectodermal and will form the outer epithelium

Sessile jellyfish Manania gwilliami 2X

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Free-swimming jellyfishes: Aurelia spp.

This section on free-swimming jellyfishes: Aurelia spp. is complemented by a separate section dealing with FREE-SWIMMING JELLYFISHES: other species.

Research study 1

longitudinal X-sectional view of a strobilating scyphistoma of a jellyfish Aurelia auritaAn early study done on Aurelia in Northumberland, England provides a detailed account of the process of strobilation. Prior to separation of the ephyra from the “parent” polyp, it is attached by only a fine strand of connecting tissue.  Final separation may be by contraction of tissues at the site of joining, by physical action of currents, or perhaps a combination of the two. Note in the drawing the contiguous form of the gastrovascular cavity, which extends from the polyp into all the budding ephyrae.  Not all ephyrae released become juvenile jellyfishes; some may drop to the sea bottom and develop into new polyps, which eventually bud off their own ephyrae.  Before strobilating and when conditions for growth are good, the polyps bud off new polyps and, since a polyp may live for several years, densities in areas favourable for growth may be quite high.  After the last ephyra is released in the spring season, the polyp may die or over-winter to commence growing and budding in the following spring-time. Note that polyps may be either male or female, and all ephyrae produced from a single polyp are genetically identical and, hence, of the same sex.  Percival 1923 Q J Microsc Sci photograph of scyphistomae of a jellyfish67: 85.

NOTE  the species is not mentioned by the author, but is presumably A. aurita. The life-cycle features described here can be applied to the endemic west-coast species A. labiata

NOTE  also known as transverse fission

Scyphistomae of a jellyfish. Densities may be locally
great owing to the propensity of the polyps to bud
new polyp when conditions are good for growth 4X

Research study 2

drawing of a scyphistoma of a jellyfish Aurelia aurita showng production of stolons and new budsdrawing of scyphistoma of jellyfish Aurelia aurita showing bud and stolon formationThere are actually 3 modes of reproduction in scyphozoans: 1) sexual by means of gametes produced by free-swimming adult medusae, 2) asexual budding and pedal-stolon production in scyphistomae that develop from fertilised eggs, and 3) asexual transverse fission in scyphistomae (strobilation) that give rise to a series of juvenile medusae (ephyrae).  The second mode, or stolon production, is described for Aurelia aurita scyphistomae collected at Pacific Grove, California.

Buds and stolons develop from the wall of the lower body/stalk (see drawing on Left). Stolons themselves later grow buds, which may crawl away from the parent until the stolon connection is stretched thin and breaks (see drawing upper Right).  Stolons are also used in locomotion of the polyp, extending outwards from the polyp base with the remainder of the polyp following.

drawing showing types of polyps produced from regenerating segments of a scyphistomaScyphistomae have good powers of regeneration and new polyps will form from fragments, as long as they are not too small and have both body layers (epidermis and gastrodermis) present.  The schematic on the lower Right shows regeneration from transverse segments and a gradation of polyp-forming potential from top to bottom of the original polyp. Note the stolon-forming tendency in segments c-e and the base-forming tendency in segment g. As shown in the photo in Research Study 1 above, budding has the potential to greatly increase the number of scyphistomae in a given area.  Gilchrist 1937 Biol Bull 72: 99.

NOTE  all budded polyps and stolons are genetically identical to the parent polyp

NOTE  although cited here as A. aurita, this species is likely to have been A. labiata

Research study 3

Scyphistomae of Aurelia aurita collected from the Gulf of Mexico can be maintained in the laboratory on a diet of live brine shrimp for at least 3yr.  Depending upon season, mature medusae are produced in 4mo.  The author divides the life cycle into 9 somewhat arbitrary stages, 4 of which are shown below: Stage 1 ephyra, newly released from scyphistoma; Stage 3 ephyra, 13mm bell diameter; Stage 5 juvenile, 20mm; Stage 9 adult, 40mm, reached in as little as 40d depending upon culture conditions:  Spangenberg 1965 J Exp Zool 159: 303.

photograph of a newly released ephyra of jellyfish Aurelia aurita courtesy Spangenberg 1965 J Exp Zool 159: 303 photograph of late ephyra stage of jellyfish Aurelia aurita courtesy Spangenberg 1965 J Exp Zool 159: 303 photograph of a juvenile medusa of jellyfish Aurelia aurita courtesy Spangenberg 1965 J Exp Zool 159: 303 photograph of a young adult of jellyfish Aurelia aurita courtesy Spangenberg 1965 J Exp Zool 159: 303
Research study 4

graph showing population density effects on budding of polyps in jellyfish Aurelia auritaIn favourable conditions, jellyfish scyphistomae bud off junior versions of themselves and densities of polyps may become quite high.  A study on scyphistomae of Aurelia aurita in Virginia shows that not only do conditions of low temperature and starvation decrease the extent of budding, but so does increased density. Note in the graph that populations with high initial densities have lower growth rates than populations with lower initial densities.  For example, a culture that starts off at 5 polyps multiplies to about 35 polyps after 21d, or a 7-fold increase. In comparison, a culture with 100 polyps at the start multiplies only about 2-fold in the same time. The author suggests that a water-soluble substance may be released that inhibits growth of the polyps.  Inter-polyp competition for food does not seem to play a role.  Coyne 1973 Chesapeake Sci 14: 55.

NOTE  at 24oC, fed on brine shrimp Artemia salina every 3d at a standardised “per-polyp” ration

Research study 5

Laboratory observation of settling Aurelia aurita planulae indicate that more than 90% settle on the undersides of objects.  Thus, the resulting scyphistomae hang with their oral surfaces downwards as commonly seen in the field.  There is no significant preference of the planulae for rough or smooth surfaces, but there is a trend for selection of grooved over smooth surfaces.
Brewer 1978 Estuaries 1: 120.

NOTE  textural differences in substrata are created by sanding or cutting grooves into 2x2cm plastic coverslips

Research study 6

Research on the life cycle of moon jellies Aurelia labiata in Roscoe Bay, British Columbia reveals that eggs are held in special brood sacs and hatch to planulae in autumn (Oct-Nov).  The planulae are released in March and ephyrae appear in June.  The medusae mainly stay in the Bay throughout their 1-2yr lifespan, although some are washed out through tidal flushing over a shallow gravel sill at the Bay’s mouth.  Albert 2005 J Mar Biol Ass UK 85: 575.

NOTE  these are not described by the author. Apparently, though, in A. labiata they are located on the manubrium, while in A. aurita they are on the oral arms

NOTE  late summer temperatures are about 18oC and salinities about 23ppt

Research study 7

map of Roscoe Bay, British ColumbiaThe author of Research Study 6 above later writes that despite being entrained in a twice-daily tidal exchange involving about one-quarter of the total water volume in Roscoe Bay, Aurelia labiata appear to reside there year-round and to maintain stable breeding aggregations.  Apparently, on being moved out of the bay on ebb tides, the medusae sink from the ebbing turbulence into still or counter-current water below.  The counter current is created by the hydrodynamics of the ebb flow of water over a shallow gravel sill at the mouth of the bay (see map).  Later, when the tide begins to flood into the bay, the medusae rise up, join the flood stream, which is less turbulent than the ebbing one, and are carried into the bay.  Another related observation is that medusae are rarely to be stranded on beaches in the bay during ebbing tides.  The reason for this appears to be that when they touch the bottom in shallow waters they immediately rise to the surface, and there they are entrained in the ebbing stream that is at the surface.  In this way they are carried out of the intertidal zone.  The study is apparently the first to show that scyphomedusae are able to utilise vertical migration to remain in a location subject to tidal currents.  The author remarks that the strategy of vertical migration tends to enhance survival by keeping the population in a favourable location, and keeps males and females in close proximity.  Albert 2007 J Sea Res 57: 281; Albert 2009 J Sea Res 61: 140.

NOTE  the author is able to monitor numbers and depths of the jellyfish by use of a floating viewing box.  Some vertical tows of a  conical drift net are also done

NOTE  is it possible that A. labiata in Roscoe Bay are using vertical migration within the bay to avoid being washed out with the tides?  There are many reports of jellies in other parts of the world being able to hold their positions in inner parts of estuaries by migrating to the moving surface waters during flood tides and to the still waters near the bottom during ebb tides.  However,  later observations by the author show that vertical positions of the medusae do not differ significantly during flood and ebb tides.  Albert 2010 J Sea Res 64: 422.

Research study 8

photograph of scyphistoma of a jellyfishhistogram showing effects of temperature and salinity on bud and ephyra production in scyphostmae of jellyfishes Aurelia labiataJellyfish blooms that are becoming increasingly common throughout the world may proximally be regulated by number and asexual-budding activity of benthic scyphistoma stages.  Yet, little is known about environmental effects on this activity, a deficiency that could be important in this time of global warming.  Laboratory experiments on polyps of Aurelia labiata at Shannon Point Marine Center, Washington address this.  The researcher counts ephyrae released from polyps under 9 conditions of temperature and salinity (in the dark), and under 9 conditions of photoperiod and light intensity at ambient temperature and salinity.  Polyps under all combinations survive well 83-100%).  Results show that ephyrae production is highly responsive to temperature conditions, for example, from 0 per polyp at 7oC to 42 per polyp at 15oC over a 20wk period (see histogram) but, overall, it is amount of light (longest photoperiod and highest light intensity) that has greatest effect on strobilation. In the field, light increases seasonally much faster than seawater temperature, suggesting to the author that light may be the more important environmental cue for seasonal initiation of strobilation.  Purcell 2007 Mar Ecol Progr Ser 348: 183.

NOTE  as a reminder, a polyp known as a scyphistoma results from settlement of the planula larva.  The polyp feeds and grows for several years, and seasonally buds off juvenile jellyfish in a process known as strobilation

Research study 9

graph showing strobilation intensity of moon jellies Aurelia labiata in Cornet Bay Marina, Washington over 3 years of studyIn a follow-up study, researchers at Shannon Point Marine Center, Washington undertake a comprehensive field study of effects of temperature, salinity, precipitation, and light intensity on strobilation rates in Aurelia labiata at Cornet Bay Marina, Washington.  The study is done on 15 populations of scyphistomae growing on the undersides of covered floating docks of the marina, that average 58% cover (9 polyps . cm-2) over the 3yr duration of study.  Although the population varies little photographs of strobilating and non-strobilating polyps of moon jellyfish Aurelia labiata, courtesy Purcell et al. 2009 Mar Ecol Progr Ser 375: 139in density, the times and amounts of strobilation and magnitude of all physical factors vary significantly among years.  Highest frequency of strobilation (63%) occurs in Year 2 (see graph) and correlates significantly and positively with pre-strobilation water temperatures and light levels, and negatively with low salinity.  Of these, water temperature is most important.  Unlike what is known from other studies on Aurelia spp., polyp density is not a significant factor in determining strobilation frequency. As these same factors of temperature and light are correlated with enhanced plankton growth that represents food for both polyps and ephyrae, the authors consider them as likely synchronizing cues.  The considerable value of the study is its use of data collected in situ. Purcell et al. 2009 Mar Ecol Progr Ser 375: 139.

NOTE  in laboratory experiments a reduction in salinity from 27 to 20ppt retards the seasonal onset of strobilation by 20d and reduces the number of strobilating polyps by 60%


Non-strobilating scyphistoma polyps (top)
and strobilating ones (below) 1X

Research study 10

histogram showing settlement preferences of larvae of jellyfishes Aurelila labiata for various dock-building construction materialsAnother study by researchers at Shannon Point Marine Center, Washington focuses on settlement preferences of Aurelia labiata planulae for 6 different dock-building materials.  The study is really outside the purview of the ODYSSEY but, as the results may be useful for marinas and portside managers concerned with jellyfish blooms, they will be presented here briefly.  Medusae are spawned in the laboratory and settlement of planulae onto replicates of the 6 materials recorded after a 1mo period.  Results, shown in the accompanying histogram, may be of interest to port and dock authorities interested in ways of minimising general biofouling and possibly jellyfish blooms.  Hoover & Purcell 2009 Hydrobiologia 616: 259.

NOTE  most floating docks in Washington State are apparently constructed from combinations of 6 primary materials:  Polystyrene++ matprp (blue), Polystyrene foam (white beads), Hard plastic (enclosing flotation material), Shrink-wrap sheeting (enclosing flotation material), Wood (pressure treated with copper and zinc arsenate), and Rubber (ordinary tire rubber)

NOTE  a 2mo field study produces similar results to the laboratory one

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