information on the biology of species and the ecology of habitats found around the coasts and seas of the British Isles

St John’s jellyfish (Calvadosia cruxmelitensis)

Distribution data supplied by the Ocean Biogeographic Information System (OBIS). To interrogate UK data visit the NBN Atlas.



This stalked jellyfish is the smallest member of its family. Calvadosia cruxmelitensis has a translucent, maroon, broad funnel-shaped bell that can reach 1.2 cm in diameter and 0.8 cm in height. The bell is divided by hollow septa. The reproductive gonads are thick and linear, and arranged inside the bell in halves which join at the base and extend in a linear fashion to the arms. The 8 arms are arranged in a circle and well-developed with up to 35 tentacles each. Each of the tentacles has a rounded head. Unique to the Calvadosia cruxmelitensis is the arrangement of the nematocyst organs, which appear as white spots in the shape of a 'maltese cross', from which the species name 'cruxmelitensis' is derived. A distinguishing feature of Calvadosia cruxmelitensis is its stalkless appearance due to the base of the bell involuting around the stalk that is 0.8 cm in height and attached to the substratum by a broad basal disc. Tentacle anchors are absent in Calvadosia cruxmelitensis compared to Haliclystus salpinx.

Recorded distribution in Britain and Ireland

Calvadosia cruxmelitensis is recorded from the south-west of England, from Swanage to north Devon, and the Isles of Scilly and the Atlantic coasts of Ireland, with occassional records from Pembrokeshire and the Llyn Pennisula in Wales, the Firth of Lorn in Scotland.

Global distribution

Calvadosia cruxmelitensis is only recorded from the British Isles.


Calvadosia cruxmelitensis inhabits moderately-exposed rocky shores in the low intertidal and shallow sublittoral zones. In contrast to most species of Stauromedusae it is rarely attached to Zostera spp. but is often found on the macroalgae Chondrus crispus and Mastocarpus stellatus.

Depth range

Lower eulittoral to shallow sublittoral

Identifying features

  • Stalk-less appearance.
  • 8 arms.
  • Up to 35 tentacles on each arm.
  • Translucent, maroon colour.
  • White spots on surface of bell form a distinctive maltese cross.
  • No tentactle anchors.

Additional information

The nematocyst capsules are large, vivid white against the maroon of the body, and arranged in the pattern of 'maltese cross' (Corbin, 1978). The species name 'cruxmelitensis' refers to this 'maltese cross' pattern (Corbin, 1978).

Information on the ecology of Calvadosia cruxmelitensis is limited. Information from similar species or other members of the group (Stauromedusae) was used to complete this review. Information relevant to sensitivity assessment was particularly lacking. Sensitivity assessments are based on proxies (e.g. the sensitivity of its preferred substratum) and expert judgement where possible. Miranda et al. (2010, 2016a&b) provide detailed studies of the morphology, histology and taxonomy of the Stauormedusae.

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Further information sources

Stauromedusae UK

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Biology review


Authority(Corbin, 1978)
Recent SynonymsLucernariopsis cruxmelitensis (Corbin, 1978)


Typical abundanceHigh density
Male size rangeup to 0.8 cm
Male size at maturityup to 0.8 cm
Female size rangeup to 0.8 cm
Female size at maturityup to 0.8 cm
Growth formRadial
Growth rateNo information
Body flexibilityHigh (greater than 45 degrees)
MobilityTemporary attachment
Characteristic feeding methodPredator
Diet/food sourceCarnivore
Typically feeds onCopepods, amphipods, ostracods, small juvenile decapods, polychaetes and gastropods
SociabilityNot relevant
Environmental positionEpifloral
Is the species harmful?No

Biology information

Population dynamics.
Stauromedusae are annuals that develop seasonally, reproduce, and die back until the following year, although they have a complex life cycle that may include resting or overwintering stages (see ‘life history’ below) (Berrill, 1962; Corbin, 1978,1979, Zagal, 2004a; Miranda et al., 2012). Although the number of studies is limited, Stauromedusae exhibit seasonal periods of high abundance in both the northern and southern hemispheres (Miranda et al., 2012, Table 2).  In Wembury, the UK, Corbin (1978, 1979) noted that Haliclystus auricula exhibited its highest abundance in mid-summer, Calvadosia (as Lucernariopsis) campanulata in autumn and Calvadosia (as Lucernariopsis) cruxmelitensis in winter, based on 23 years of observations. Between 1953 and 1974, Corbin (1979) also observed years with exceptionally high counts of Calvadosia campanulata in 1962 and 1974, of Calvadosia cruxmelitensis in 1968 (when ca 2000 individuals were found) and of Haliclystus auricula in 1972 and 1973. The annual mean numbers of indivudals was 37 in Haliclystus auricula, 39 in Calvadosia cruxmelitensis but 5 in Calvadosia campanulata (Corbin, 1979). In southern Chile, Zagal (2004a) noted a summer peak in abundance in Haliclytus auricula, with a maximum density of 1,405 individuals/m2, after which it disappeared in winter.  The stauromedusae adult phase is the only conspicuous phase of the life cycle. Although it is small and often camouflaged on its algal substratum, the other life stages (planulae and stauropolyps) are small and hard to observe (Corbin, 1979; Miranda et al., 2012). The environmental cues for the seasonal growth and exceptional years are unknown but their abundance coincides with the greatest algal cover and is probably correlated with optimal conditions for feeding and hence growth (Zagal, 2004a; Miranda et al., 2012).

Stauromedusae are passive predators that catch food using stinging nematocysts. In southern Chile, Zagal (2004b) reported that the prey of Haliclystus auricula consisted mainly of gammarid amphipods, chironomid fly larvae, ostracods (seed shrimp), juvenile decapods crustaceans and gastropods, and that the smaller medusae took the smallest prey. These prey are typical mobile grazers and scavengers in seaweed canopies. In South Georgia (subantarctic) Haliclystus antarcticus preyed mainly on calanoid copepods, amphipods and, in one case, an errant polychaete (Davenport, 1998). Davenport (1998) concluded that they took both benthic and planktonic prey. It is probable that Calvadosia spp. take similar prey, depending on size.

Davenport (1998) also noted that Haliclystus antarcticus in South Georgia (subantarctic) was preyed on by aeolid nudibranchs and the fish Notothenia rossii (the marbled rockcod).

Habitat preferences

Physiographic preferencesEnclosed coast / Embayment, Strait / sound
Biological zone preferencesLower eulittoral, Sublittoral fringe, Upper infralittoral
Substratum / habitat preferencesMacroalgae
Tidal strength preferencesWeak < 1 knot (<0.5 m/sec.)
Wave exposure preferencesModerately exposed, Sheltered
Salinity preferencesFull (30-40 psu)
Depth rangeLower eulittoral to shallow sublittoral
Other preferences

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Migration Pattern

Habitat Information

Haliclystus auricula, Calvadosia (as Lucernariopsis) cruxmelitensis and Calvadosia (as Lucernariopsis) campanulata grow on a variety of macroalgae on the lower half of the shore, in pools or 'moving with the waves' 'at the tides edge' (Corbin, 1979). Corbin (1979) noted that Calvadosia cruxmelitensis rarely settled on Zostera and that no specimen was seen attached to a solid substratum. It occurred most frequently on Chondrus and Gigartina, frequently on Ulva, Gracilaria, Furcellaria, Polyides, Corallina, Ceramium and other feathery macroalgae, infrequently on Codium, Cystoseira and Calliblepharis, but also on a wider range of other macroalgae. In 25 years of observation, only one specimen was noted on Zostera or Fucus and none on Halidrys, Laminaria, Chorda, or Himanthalia. He also noted that Calvadosia cruxmelitensis is perfectly camouflaged on Corallina, as the white tips of the pink branches match the stauromedusan's colouration; a camouflage enhanced by the presence of encrusting Lithothamnium (Corbin, 1978).

Life history

Adult characteristics

Reproductive typeAsexual, Gonochoristic (dioecious), Sexual
Reproductive frequency Annual episodic
Fecundity (number of eggs)No information
Generation time<1 year
Age at maturity<1 year
Life span<1 year

Larval characteristics

Larval/propagule typePlanula
Larval/juvenile development
Duration of larval stage2-10 days
Larval dispersal potential Very limited (<1 m)
Larval settlement periodWinter

Life history information

Stauromedusae have a potentially complex life cycle with sexual and asexual stages, although the larval and early stages have been observed in few genera (Haliclystus and Stylocoronella) (Miranda et al., 2010, 2012). In addition, Miranda et al. (2010) concluded that the hydrozoan Microhydrula limopsicola was a life-stage of Haliclystus antarcticus (based on morphology and molecular markers) and, therefore, suggested that the ‘microhydrula’ was part of the Stauromedusan life cycle.

  • Individual medusae are dioecious. Spawning in Haliclystus stejnegeri was induced by exposure to light after an 8 hour dark period, while spawning was more intense in Haliclystus salpinx rather than induced under the same light regime (Otto, 1978).
  • Eggs were in diameter 35 µm in Haliclystus stejnegeri and 40 µm in Haliclystus salpinx (Otto, 1978).
  • Once fertilized the embryos become extremely sticky, stick to the substratum, and develop into planulae within 24 hours at 12-15°C (Otto, 1978).
  • The non-ciliated benthic planulae settle within 1-3 days, on an available substratum, or already settled planulae, and many form aggregations of 1 to 8 planulae (Otto, 1978, 1979).  They develop nematocysts within a week. In Haliclystus octoradiatus planulae settle in aggregations of 3-20 larvae (Miranda et al., 2010).
  • Otto (1979) noted that in one culture, the planulae underwent cell division and developed gastric cavities after several weeks, but was unable to stimulate further development in culture.
  • Miranda et al. (2010, 2012) suggest that the planulae develop into the microhydrula stage, which further develops into the stauropolyp (the juvenile or intermediate stauromedusa).
  • The planula develops into the fully developed stauropolyp in ca 15 days in Haliclystus octoradiatus (Wietrzykowski, 1912; cited in Miranda et al., 2012).
  • The stauropolyp develops apically into the stauromedusa (Kikinger & von Salvini-Plawen, 1995; Miranda et al., 2010, 2012), in about 2 months in Stylocoronella (Kikinger & von Salvini-Plawen, 1995).

Asexual reproduction can occur at several stages. The adult stauromedusa can bud ‘frustules’ from the upper part of the animal, while frustules can also bud from special tentacles on the stauropolyp and from the ‘microhydula’ stage (Kikinger & von Salvini-Plawen, 1995; Miranda et al., 2010, 2012).  Kikinger & von Salvini-Plawen (1995) noted that ‘frustules’ divided and then encysted, and suggested that they were ‘resting stages’. Otto (1979) also noted that the lack of further development within her cultures suggested that the larvae had entered an overwintering stage, especially as larvae did not develop immediately after settlement in the field.  Miranda et al. (2012) suggested that the ‘microhydrula’ stage might represent the ‘resistant’ stage of Otto (1978). 

Miranda et al. (2012) noted that stauromedusae appear in specific seasons and then disappear. Adults are small (1-4 cm) and often camouflaged against the background of macroalgae and their abundance may be underestimated (Corbin, 1979; Miranda et al., 2012). However, the tiny planulae (ca 100 µm in length) and stauropolyps (0.3-0.8 mm in height) are more difficult to find and have only been documented for nine of the 51 known species of Stauromedusae (Miranda et al., 2010). It was also suggested that there might be a subtidal 'reservoir' population (Gwilliam, 1956, cited in Miranda et al., 2012). The ‘microhydrula’ stage of Haliclystus anatarcticus was collected at a depth of 31 m, and the stauropolyp of Haliclystus octoradiatus can detach itself from the substratum, even in calm water.  However, no other evidence was found for seasonal migration of planulae, stauropolyps and stauromedusae to or from deeper water (Miranda et al., 2012). Field observations suggest that Stauromedusae disappear for several months before the young stages appear, which suggests that encystment occurs in the field (Otto, 1979; Miranda et al., 2012). Also, Wietrzykowski (1912, cited in Miranda et al., 2012) noted young polyps in April and mature stauromedusae in July. Miranda et al. (2012) concluded that three months from December to March would be adequate for planulae to become mature stauromedusae.

In addition, although a single stauropolyp only develops into a single adult (sexual) stauromedusae, the ‘microhydula’ and ‘stauropolyp’ stages can create numerous asexual ‘frustules’. Frustules and planulae provide the potential to create a 'resevoir' of resistant or overwintering stages (Otto, 1978, 1979; Kikinger & von Salvini-Plawen, 1995; Miranda et al., 2010) towards the end of the season, which may then develop when favourable conditions return the following year resulting in the seasonal peak in abundance.  But frustules 'from asexual reproduction' may also rapidly develop many new polyps, contributing to the seasonal peaks in abundance, and in particular, exceptional ‘blooms’ in some years (Miranda et al., 2012).  Miranda et al. (2010) also noted that intense asexual reproduction was consistent with the low genetic diversity of the Haliclystus antarcticus populations they studied and, provide Stauromedusae with the potential to develop large populations in isolated areas.

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Biological pressures

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Additional information

Importance review


UK Biodiversity Action Plan Priority
Species of principal importance (England)
Features of Conservation Importance (England & Wales)



Importance information

This species was found in often high numbers (ca 2000 individuals in one shore search in 1968; Corbin, 1979) on shores in south-west England but is now rarely seen (Hiscock et al., 2011). Hiscock et al. (2011) suggested that the population had declined by 90% from the 1970s to 2005, although the reason for the decline was unknown.


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This review can be cited as:

Tyler-Walters, H. & Richards, S. 2017. Calvadosia cruxmelitensis St John’s jellyfish . In Tyler-Walters H. and Hiscock K. (eds) Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 18-06-2018]. Available from:

Last Updated: 22/02/2017