There are four valid varieties of Pennatula phosphorea at the time of this assessment: P. phosphorea var. antarctica Kükenthal, 1910, P. phosphorea var. californica Kükenthal, 1913, P. phosphorea var. candida Marshall & Fowler, 1888, and P. phosphorea var. longispinosa Moroff, 1902. Recent molecular work indicates the Atlantic and Pacific P. phosphorea might represent different species (García-Cárdenas et al. 2019). Therefore, for the purposes of this assessment only the Atlantic populations are considered.

Pennatula phosphorea is assessed as Near Threatened. This sea pen species has a wide distribution in the Northeast Atlantic and is found in relatively shallow-water environments (15–155 m), which are vulnerable to damage by trawling and other activities. The species is impacted by bottom contact fishing and salmon aquaculture, which likely decrease fitness and population density. The decline is projected to continue. It is currently only possible to estimate the population size and extent of this species via underwater video, and technology with sufficient quality to identify species has only been available in the last 10–15 years. Therefore, it is not possible to quantify directly observed previous population decline, but it is suspected (A2(c)). The extent of damage is also inferred from trawling bycatch, which presents a proxy index for the extent to which specimens are removed from the population (A2(b)). In regions that are not protected from bottom trawling the decline is projected to continue. The generation length for this species is estimated at approximately 19 years based on the closely related species P. aculeata. A regional assessment for the Mediterranean listed this species as Vulnerable, based on suspected population decrease of 40% in the last 20 years and continuing decline, and there is evidence of similar decline in the North Sea over a time scale representing three generations for this species (1960s–2020s). These regions are inferred to reflect a global pattern of decline, and the inferred global decline is suspected to be at least 25%. Since the population is mainly found in areas not protected by deep water trawling bans, the global population may reflect similar ongoing rates of decline that will continue into the future.

Pennatula phosphorea, the species and its varieties, have been reported from European seas, Mediterranean, and both sides of the Atlantic but rarely in the West Atlantic. Specimens of this species have been reported from the Pacific, Indian Ocean, and Antarctica (Williams 1995), but confusion with other species makes the true geographic range uncertain. This assessment is therefore limited to the species found in the North Atlantic.

Information on population dynamics for P. phosphorea is not available. Densities can be variable depending on the location and even within a region (Greathead et al. 2007). For example, density reached 7.85 individuals per m² in the Central Fladen NCMPA (Northern North Sea, based on video data, Murray et al. 2016), but colonies were rare in trawl surveys conducted in Norway (Buhl-Mortensen et al. 2023).

Pennatula phosphorea is found in areas of bottom-contact fishing activity; however, the population density of this species is negatively correlated with fishing impact (Murray et al. 2016, Downie et al. 2021). The natural undisturbed population density of this species may be very high.

Pennatula phosphorea lives on soft bottom habitats, between 20–700 m, on sandy or muddy bottoms (Grasshoff 1989, Vafidis et al. 1994). Most studies on this species have been conducted in Scotland, where salinity and gravel content were shown to have an influence on its distribution (Greathead et al. 2015). The species was found more likely to occur in areas with low primary productivity in the UK continental shelf and Celtic Seas (Downie et al. 2021).

Pennatula phosphorea grows up to 40 cm in length with approximately half of it protruding above the sediment. As other sea pens, this species can be used as habitat by other species (De Clippele et al. 2015). Growth rates and longevities have not been studied for this species. However, the congeneric species Pennatula aculeata and other cold-water sea pens can have longevities of over 20 years (Murillo et al. 2018; Neves et al. 2015, 2018). Based on examination of growth rings in the congener P. aculeata (Greely 2022), this species likely reaches maturity at 7 or 8 years. An assumed longevity of approximately 30 years indicates an estimated generation length of 19 years.

Female P. phosphorea colonies can maintain a pool of oocytes throughout the year, which are broadcast-spawned during the summer (July and/or August) (Edwards and Moore 2008). Oogenesis and spawning may be related to seasonal cues and annual spawning might be a brief and synchronous event. Mean polyp fecundity increases with colony size, and presence of large oocytes (500 μm) indicate the production of lecithotrophic larvae. These reproduction patterns might not be representative of colonies found at different locations and depths.

Pennatula phosphorea produces blue/green bioluminescence when stimulated, and it can also contract when disturbed (Nicol 1958). Colonies are capable of withdrawing into the sediment (Mackie 1998). Extracts from this species have been found to show narcotic and anorectic properties, acting as feeding deterrents for the octocoral's main predator, the Dover sole Solea solea (Mackie 1998). The bacteria Alphaproteobacteria were observed for the communities associated with both tissues and mucus of P. phosphorea (Porporato et al. 2013).

Given that Pennatula phosphorea is a sedentary species that lives on soft bottom areas that can be exposed to bottom-touching fishing gear, fishing is one of the threats currently recognised for the species (Murray et al. 2016, Downie et al. 2021, Buhl-Mortensen et al. 2023). Bottom trawling impacts soft-bottom communities including sea pens by decreasing diversity and shifting their trophic structure in fishing areas (Vergnon and Blanchard 2006, OSPAR 2022). Long line fishing also destroys Pennatula phosphorea (Mytilineou et al. 2014).

Pennatula phosphorea has been considered potentially less vulnerable to trawling than other sea pen species, due to its capacity of withdrawing into the sediment (Downie et al. 2021). In the Bay of Biscay and Irish Sea area, populations of P. phosphorea overlap with fishing grounds of the lobster Nephrops norvegicus (Greathead et al. 2005, Le Joncour et al. 2023). A regional Red List assessment estimated that at least 50% of potential habitat had been affected by trawling in the last 40 years, with fishing activities still ongoing (Cerrano et al. 2015). Trawling is directly connected to reduction in sea pen abundance in the North Sea (Harrald et al. 2018). There is increasing recognition that trawling is detrimental to sea pens, but conservation efforts vary across the range of this species. In Scotland, P. phosphorea also overlaps with salmon aquaculture farms (Wilding 2011, Wilding et al. 2012). Other sea pen and coral species show declining growth rates and damage in proximity to salmon cages (Kutti et al. 2015). As for other corals, climate change has the potential to change the range of suitable habitats for the species (Morato et al. 2020), and ocean acidification might affect their calcium carbonate skeletons.

There is no known use or trade for this species.

Sea pens are indicators of Vulnerable Marine Ecosystems (VME). If P. phosphorea has longevities comparable to those of its congeneric species P. aculeata (i.e., over 20 years in age, Murillo et al. 2018), then recovery from damage could take decades. No conservation sites have been identified specifically to protect the species; however, conservation areas to protect sea pens exist. Sea pens have been reported in several Special Areas of Conservation and they are protected under the ‘Mud in Deep Water Habitat Action Plan’ of the UK Biodiversity Action Plan (UKBAP) (Greathead et al. 2007).

Fields of sea pens shallower than 200 m depth are included in the OSPAR threatened and/or declining habitat "Sea pens and burrowing megafauna". However, monitoring in this way lumps multiple vulnerable habitats together and cannot discern declines specifically in P. phosphorea. This species cannot be distinguished from Pennatula aculeata on video data and thus its full distribution and extent are likely not appreciated. Further research into distribution and occurrence (or not) of P. phosphorea in protected areas is recommended. Research into age and other life history parameters would allow a better understanding of the susceptibility and resilience to threats of this species. Research into the impact of ongoing trawling, particularly where the species provides habitat, is required.