Spanioplon armaturum is of interest in a Northern Ireland context as it was originally described from Strangford Lough, which is therefore the type locality. It was described by Bowerbank in 1866 on the basis of specimens sent by Dickie. This sponge was a characteristic and moderately long-lived component of horse mussel beds in Strangford Lough until 20 years ago but appears to have declined rapidly in recent years. It grows into a cushion covering variegated scallop (Mimachlamys varia) shells in undisturbed horse mussel beds and can be considered to be an indicator of healthy status of these beds. Survey work concentrating on the sponge biodiversity of Rathlin Island in 2005 found several specimens of this species.

In brief

• In Northern Ireland this species is most abundant in horse mussel beds in Strangford Lough which have decreased significantly in the last 30 years.
• Lives in the sea at depths of 10-40 m and more with populations most common in sheltered sea loughs.
• Individuals probably live for several years.
• Scarce in Northern Ireland but most sponges are poorly known in Britain and Ireland.
• Threatened primarily by bottom fishing with trawls or dredges which disrupt the biogenic reef habitat.
• Reserve populations on the open coast could potentially recolonise the Strangford Lough habitats if conditions improve.

Species description: This sponge forms cushions or sheets on hard substrata. It is grey in colour with a semi-transparent surface and large exhalent canals leading to osculae where the feeding currents emerge. When growing on Chlamys varia these osculae are usually few in number and grow adjacent to the opening of the scallop shell, possibly thereby benefiting by a symbiotic relationship where the scallop and sponge support each other's feeding currents. Internally there is a silica spicule skeleton consisting of tornotes, styles and microspined straight acanthostyle spicules. The latter are very distinctive and can be used to positively identify this species. Encrusting specimens were redescribed by Burton under the name Hymedesmia stephensi but examination of Burton's material in the Natural History Museum, London confirms the synonymy.

Life cycle: Probably with planktonic larvae which could disperse long distances and enable quick recovery of populations.

Similar species: When in its encrusting phase, this species looks similar to several species of the genus Hymedesmia and can then only be identified by examination of a tissue sample for the characteristic spicules.

How to see this species: This species can be found attached to and enveloping living Mimachlamys varia in the horse mussel beds in Strangford Lough, where it was formerly a common member of this community.

Current status: Declining and scarce. This is a species primarily confined to horse mussel beds in Northern Ireland where it used to be present in densities of several specimens per 10m². Living Mimachlamys varia are now much rarer in these beds so there are fewer suitable hosts for the sponge. Investigations in 2003 confirmed that the horse mussel beds have declined considerably in the past 30 years and this species in particular seems to have become rare, it was not seen during the 2003 SLECI fieldwork.

Why is this species a priority in Northern Ireland?

• Criterion 2: Decline based on available data and expert judgement.

Threats:

• In rapid decline possibly due to physical impact of bottom fishing gear in its habitat. The target for the fishery is the queen scallop, Aequipecten opercularis, which lives in more open muddy patches between the horse mussel clumps, but requires stable biogenic reefs for successful recruitment. Both queen scallop and variegated scallop numbers are now very low compared with the situation 30 years ago and sponges have been particularly affected by the changes.
• Sponges may be very vulnerable to increased siltation which clogs their feeding mechanisms and may in turn protect the bivalves fron starfish predation, so healthy sponge populations may be an important indicator of health of these communities.
• It is unclear whether direct impact from trawling has disrupted the mussel beds and encouraged predatory starfish or whether smothering by stirred up bottom sediments is more to blame for their decline.

Action: Records of new sites for this species, with details of habitat and population are always valuable. Observations with a photograph can be recorded using iNaturalist, find the MBDP-NI iNaturalist project here.

Alternatively, records can be sent to CEDaR, National Museums Northern Ireland, 153 Bangor Road, Cultra, Holywood, County Down, BT18 0EU. Tel 028 9039 5257, e-mail cedar.records@nationalmuseumsni.org, or via the online portal.

Objectives:

• Establish appropriate management on historic sites.
• Monitor numbers present in horse mussel beds in Strangford Lough.
• Investigate recruitment and predation of this species.

Conservation:

• Strangford Lough is a Marine Protected Area with multiple designations.
• There is a Habitat Action Plan for Horse Mussel Beds.
• Rathlin Island is a Marine Protected Area with multiple designations.
• The Maidens are a Marine Protected Area with multiple designations.

Distribution Map from NBN: Spanioplon armaturum at National Biodiversity Network mapping facility, data for UK.

iNaturalist: Spanioplon armaturum at iNaturalist World Species Observations database.

WoRMS: Spanioplon armaturum at World Register of Marine Species. Accepted name: Spanioplon armaturum (Bowerbank, 1866). AphiaID: 133701.

Classification: Biota; Animalia; Porifera; Demospongiae; Heteroscleromorpha; Poecilosclerida; Hymedesmiidae; Spanioplon

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Picton, B.E., Keatley, L. & Morrow, C.C. (2026). Spanioplon armaturum. (Bowerbank, 1866). [In] Priority species, Marine Biodiversity Portal NI. https://www2.habitas.org.uk/marbiop-ni/priorityaccounts.php?item=C7530. Accessed on 2026-09-13