Vicky Wilkins, Vicky Knight and Alan Gray
Nicola Weber and Sam Weber
Facilitators / Compilers/s:
As its name suggests, Garypus titanius, the Giant Pseudoscorpion, is the largest known species in this order reaching lengths of 12 – 15 mm, compared to 3 – 8 mm in most other species. and is endemic to Ascension Island. G. titanius is known only from the seabird nesting station of Boatswain Bird Island (BBI), a 5 hectare islet 300 m off the eastern coast of Ascension Island. It was first collected by Eric Duffey in 1958 and has been regularly recorded since, including most recently in a survey in 2014 plus an ad hoc collection in 2016. A brief survey in 2014 suggested it is more common on the rocky slopes below the summit plateau than on the flat, guano-covered plateau itself. Current threats to G. titanius need more research but invasive species have already reached the island and Periplaneta americana has colonised BBI, this species will have significantly increased the predator load and directly predate G. titanius and will compete with it for invertebrate prey. Garypus titanius is assessed as Critically Endangered since it has a very restricted geographic range with extent of occurrence (EOO) and area of occupancy (AOO) of 4 km2 . Present at only one location where the invasive threat is present. There is also an inferred continuing decline in habitat quality relating to invasive invertebrates, impacting the EOO and AOO.
Garypus titanius is endemic to the island of Ascension in the South Atlantic Ocean, where it is restricted to the Boatswain Bird Island, a 5-hectare islet 300 m off the eastern coast of Ascension Island. Observations conducted in October 2014 confirmed the continued presence of G. titanius on the summit plateau, as well as on the more accessible western slopes of Boatswain Bird Island. This species was easily located by searching under loose rocks and boulders (AIBAP 2015). However, no baseline abundance data have been collected from which to infer population trends. It has been speculated that G. titanius once occupied the seabird nesting colonies of the main island of Ascension but was exterminated there by introduced predators such as mice and the centipede Scolopendra morsitans (Ashmole and Ashmole 1997, 2000). Its current distribution may therefore be a refuge, limited to the last fragment of suitable habitat not reached by these predators. A second possibility is that the decline in seabird nesting on the main island due to predation by feral cats during the 19th and 20th centuries contributed to the disappearance of G. titanius from these areas. Now that feral cats have been eradicated and large seabird colonies have re-established on the mainland it may therefore be worth exploring whether G. titanius can be induced to expand its range through an introduction programme.
No population data is available
Garypus titanius is only found amongst the dense seabird nesting colonies of Boatswain Bird Island (BBI), a steep, rocky, guano covered islet lying 300 m off the coast of Ascension Island. Unlike pseudoscorpions elsewhere, which tend to be tiny, cryptic species, G. titanius hunts for its invertebrate prey in the open on the surface of the guano. It is thought to be mainly active at night, preferring to shelter from extremes of temperature in crevices and under rocks during the day (AIBAP 2015). In 1995 Garypus titanius was observed searching for prey on the surface of the guano after dark on the summit plateau, (Ashmole and Ashmole 1997). The diet of G. titanius has not been studied in detail but is likely to include many of the insect and arachnid ectoparasites and scavengers that dominate the invertebrate community of BBI’s seabird colonies (AIBAP 2015). Duffey observed G. titanius as the most abundant of the endemic pseudoscorpions on BBI and feeding on carrion beetles such as Dermestes marmoratus and generalist feeders such as Alphitobius laevigatus (Duffey 1964). Duffey reported that captive individuals would consume houseflies (Musca sp.), suggesting G. titanius is capable of tackling quite large prey (Duffy 1964). The evolution of large size and rare foraging behaviour in this species may have arisen because there was a limited faunal diversity on Ascension Island prior to human colonisation in 1501 (AIBAP 2015, Ashmole and Ashmole 1997). This could have enabled the original pseudoscorpion colonisers to occupy a different ecological niche to relatives elsewhere that tend to occupy niches where competition and predation is more intense. However, it is also possible that a large predatory pseudoscorpion arrived on Ascension from elsewhere but subsequently became extinct throughout its former range. Species of Garypus are typically found under rocks on the seashore, so it seems likely that the ancestors of G. titanius reached Ascension by rafting on driftwood (Ashmole and Ashmole 2000). The reproductive ecology of G. titanius has never been described, but much can be inferred from studies of related species. In other Garypid pseudoscorpions, mating occurs without physical contact. Males deposit a package of sperm called a spermatophore onto the substrate which is somehow located by females and drawn into a genital opening called a gonopore. Eggs are fertilised and begin developing internally and are then transferred to an external brood sac where they mature until protonymphs emerge. These remain with the female for a short period and eventually disperse for a solitary existence. Nymphs pass through three moult cycles before reaching maturity, after which they do not moult any further. Many species spin protective, silken cocoons in which to moult or develop their brood sac (AIBAP 2015).
G. titanius originally existed in an environment with low faunal diversity and very limited competition and predation. However, this situation is changing as a result of species introduced by people. At least one species of ant and several large species of cockroach colonised Boatswain Bird Island including Periplaneta americana (S. Stroud pers comms) sometime between the surveys of Duffey (1958) and Ashmole and Ashmole (2000) and are now highly abundant. Even modest additions to the relatively simplistic invertebrate fauna of Boatswain Bird Island will fundamentally alter community structure and impact the Pseudoscorpions present particularly a top predator such as G. titanius. These invasive invertebrate species can directly predate on the species as well as compete for invertebrate prey. Moreover, Ashmole and Ashmole (2000) speculated that predation by mice and larger, introduced invertebrates such as centipedes, e.g. Scolopendra morsitans, may have been responsible for exterminating G. titanius altogether on the main Island of Ascension.
In-place research and monitoring
Action Recovery Plan : Yes
In-place land/water protection
Conservation sites identified : Yes, over entire range
Percentage of population protected by PAs : 91-100
Occurs in at least one protected area : Yes
Invasive species control or prevention : Yes