St. John's Jellyfish (Calvadosia cruxmelitensis)
Key Details
| Taxonomic Groups: | Invertebrate > coelenterate (=cnidarian) > Cnidarian |
| Red List Status: | (Not Relevant) [(not listed)(nr)] |
| D5 Status: | |
| Section 41 Status: | (not listed) |
| Taxa Included Synonym: | Lucernariopsis cruxmelitensis |
| UKSI Recommended Name: | Calvadosia cruxmelitensis |
| UKSI Recommended Authority: | (Corbin, 1978) |
| UKSI Recommended Qualifier: | (none specified) |
| Red List Citation: | (not listed) |
| Notes on taxonomy/listing: | (none) |
Criteria
| Question 1: | Does species need conservation or recovery in England? |
| Response: | Yes |
| Justification: | First described in 1978, historical population trends are unknown but there has been a decrease in recordings with an estimated reduction of population size of 90% from 1970s to 2005 by experts. Recent population trend is unknown. |
| Question 2: | Does recovery/ conservation depend on species-specific actions? |
| Response: | Yes |
| Justification: | H. servus is almost entirely confined to southern and eastern England with just a single historic record from Wales. The data do not suggest any indication of a decline and it is now known from 15 hectads since 1980 (10 hectads in the 2016 status review) The species has a predominantly coastal distribution and the only recent inland records are from the East Anglian Breckland. H. servus is strongly associated with early-successional habitats on dry sandy soils. At a landscape scale, it is a species that would benefit from untargeted habitat management to increase early successional habitat mosaics. At the current time it is not a priority for recovery actions. |
| Question 3: | At a landscape scale, would the species benefit from untargeted habitat management to increase habitat mosaics, structural diversity, or particular successional stages? |
| Response: | No |
| Justification: | Not relevant to marine species |
Species Assessment
| Current step on the Species Recovery Curve (SRC): | 3. National Monitoring Plan agreed and implemented |
| Recovery potential/expectation: | Unknown |
| National Monitoring Resource: | Opportunistic - insufficient |
| Species Comments: | There is very little evidence for pressure impacts and little on the autoecology, population dynamics and trends. This species occurs on vegetation in the lower intertidal and subtidal. In contrast to most species of Stauromedusae it is rarely attached to seagrass (Zostera spp). but, is often found on the algae Chondrus crispus, Mastocarpus stellatus, Ulva spp and Corallina officianalis. |
Key Actions
Key Action 1
Proposed Action: Develop an effective monitoring protocol and census and monitor populations at key sites.
Action targets: 3. National Monitoring Plan agreed and implemented
Action type: Targeted monitoring
Duration: >10 years
Scale of Implementation: ≤ 50 sites
High priority sites: Recorded from the south-west of England, from Swanage to north Devon, and the Isles of Scilly, where it has declined, occasional records elsewhere.
Comments: Monitoring should be focussed on known existing populations, particularly within MPAs, as a part of site management. Also targeted surveys at known former sites. Surveys within protected areas could form part of 6-year monitoring and could monitor for additional stalked jellyfish and other key species. Citizen scientists could potentially be used as the species occurs in shallow waters, although it is the smallest of the family of stalked jellyfish (1.2cm diameter, up to 0.8cm in height) and is distinctive.
Key Action 2
Proposed Action: Develop a research proposal to understand ecology and drivers of change.
Action targets: 4. Autecology and pressures understood
Action type: Scientific research
Duration: 3-5 years
Scale of Implementation: Not applicable
High priority sites: Informed by monitoring, disparate study sites and lab research required.
Comments: Further research is required to understand ecology and factors that influence their appearance and disappearance. There is very limited knowledge of the biology and ecology of this species (particularly reproductive strategy and minimum viable population level), its full distribution, and what is most effecting its survival/ apparent decline. Recovery may be successful via re-introduction of breeding populations but understanding of reproduction and development of captive breeding is first necessary. Understanding of conditions for successful breeding and rearing are of wider interest/significance.
Key Action 3
Proposed Action: Promotion of the group, encouraging increased awareness, recording and monitoring, and studying patterns of behaviour and factors that influence their appearance and disappearance.
Action targets: 5. Remedial action identified
Action type: Education/awareness raising
Duration: 3-5 years
Scale of Implementation: National
High priority sites: Sites and stakeholders identified from first two actions.
Comments: General awareness raising may increase reporting, monitoring and action to reduce any identified pressure. Action can apply to all stalked jellyfish.
Acknowledgment:
Data used on this website are adapted from Threatened species recovery actions 2025 baseline (JP065): Technical report and spreadsheet user guide (Natural England, 2025). Available here.