The Marine Environment of the Pitcairn Islands - Pew Environment ...

Table 7. Specimens of Crustacea collected from
Henderson by R.A. Irving and J. Jamieson in 1991;
these still have to be identified to species.
Taxon Probable no. of species
Majinae 1
Xanthidae 9
Grapsidae 4
Portunidae 2
Ocypodidae 1
Scyllaridae 1
Palinuridae 1
Anomura (hermit crabs) 4
Thalassinidae (lobsterlike)
1
Natantia (shrimp-like) 1
Alphaeidae 1
Stomatopoda 1
Hapalocarcinidae
(gall crab)
1
Cirrepedia: Thoracica 2
Cirrepedia:
1
Acrothoracica
Total probable
no. of species: 31
CRUSTACEANS
Collections of crustaceans were made from Henderson
by R.A. Irving and J. Jamieson in 1991. However, many of
these specimens have still to be identified to species (they
currently reside at the Natural History Museum, London).
Table 7 sets out the probable number of species within this
collection, all of which were collected from Henderson.
Other crustaceans which have been identified from
Henderson are included in Appendix 5. The marine
Ostracoda have been examined by Whatley & Roberts
(1995) who identified 33 species from Henderson (seven
samples), Pitcairn (one sample) and Oeno (one sample),
though more recent work by Whatley et al. (2004) has
increased the number of species from these three islands
to 47. Whatley & Roberts (1995) report that 25 of these
species are likely to be endemic.
MOLLUSCA
A total of 326 taxa of marine molluscs have been recorded
from Henderson Island (out of the 502 taxa recorded for
the Pitcairn Islands as a whole). Preece (1995a), as part
of the Sir Peter Scott Commemorative Expedition to the
Pitcairn Islands in 1991/92, made considerable additions
to the earlier collections of Harald Rehder (in Rehder &
Randall, 1975; Rehder & Wilson, 1975; Rehder in Fosberg,
Sachet & Stoddart, 1983) and Gustav Paulay (Paulay,
1989).
Plate 31 (top) & 32 (above). Two species of the hermit
crab family Paguridae which spend most of their time on
land at Henderson but still need to shed their eggs in the
sea. (top) The strawberry hermit crab Coenobita perlatus
(Photo: J. Jamieson); and (above) the coconut crab Birgus
latro (Photo: M. Brooke).
Preece (1995a) added considerably to the known
molluscan fauna by searching for and collecting many
small gastropod species in particular. The figure of 502
is by no means comprehensive. Indeed, Preece (1995a)
estimates the shallow-water marine molluscan fauna of
the Pitcairn Islands together to be composed of about 700
species. A list of all molluscs recorded from Henderson
(and the other islands in the group) is included in
Appendix 5.
The gastropod Engina rosacea was first described from
Henderson by Smith (1913) and has yet to be reported at
any other locations. A species of octopus ‘Octopus’ oliveri,
thought to have been a new species when found at
Henderson in 1991, has also been recorded from Pitcairn
(Plate 22). However, it is still not clear as to whether it
belongs to the genus Octopus (Norman & Hochberg,
2005) – hence the inverted commas.
STARFISH, SEA URCHINS AND
SEA CUCUMBERS (Echinodermata)
A total of 32 species of echinoderms (sea urchins [12
spp], starfish/sea stars [4 spp], brittlestars [6 spp] and sea
cucumbers [10 spp]) are listed from Henderson by Paulay
(1989). These species, together with their IUCN Red
List status (all of which are ‘Not Evaluated’), are set out
in Appendix 5. Noteworthy species are commented on
below.
Plate 33. The nudibranch Dendrodoris tuberculosa, off
North-West Beach, Henderson. This is one of the largest
species of nudibranch in the world and this particular
specimen (at ~25 cm long) was almost twice the size
of the previous largest recorded individual (Edmunds &
Preece, 1996). (Photo: R.A. Irving)
Plate 34. The purple slate pencil urchin (Pitkern: fetuei)
Heterocentrotus trigonarius in among various corals.
North end of East Beach, Henderson. (Photo: M. Brooke)
Plate 35. The red slate pencil urchin Heterocentrotus
mammillatus. Intertidal pool, Down Isaacs, Pitcairn.
(Photo: R.A. Irving)
24 The Marine Environment of the Pitcairn Islands: Henderson
The Marine Environment of the Pitcairn Islands: Henderson
Slate pencil urchins Heterocentrotus trigonarius
and H. mammillatus
The distribution of the purple slate pencil urchin
H. trigonarius (Plate 34) is widespread throughout
Polynesia where it is found in the surf zone on reef fronts
and on outer reef flats (Paulay, 1989). At Henderson,
Oeno and Ducie populations are unusually dense (up to
one to two individuals per square metre in places) and
consist of very large individuals which have a different
colour pattern (uniformly dark blue violet) and shape to
those in central Polynesia. They occur in waters up to 3 m
deep in the surf zone and on the fore-reef. Paulay (1989)
suggests that their abundance within these Islands is at
least in part the result of lack of human predation, as the
species is frequently harvested by Polynesians elsewhere.
Interestingly, H. trigonarius is not found at Pitcairn.
The red slate pencil urchin H. mammillatus (Plate 35) has
a much more limited distribution than H. trigonarius,
extending from Ducie westwards to Raroia in the
Tuamotus (Paulay, 1989). Typically, it lives in crevices on
fore-reef from 10 to 20+ m depth at Pitcairn, Henderson
and Ducie, though on occasion it may be found in
shallower depths.
The washed-up spines of both of these urchins on the
strandline of Henderson’s beaches have recently started to
be collected by Pitcairners and made into jewellery for sale
locally and for export.
Long-spined sea urchin Diadema savignyi
Sea urchins are often nocturnal (they may be preyed upon
by triggerfish and pufferfish in particular, despite their
protective spines) and usually hide away during daylight
hours, tightly wedged under rocks or in crevices. However,
in exposed situations, such as at Henderson, where there
are insufficient hiding places, they cluster together to form
large aggregations. These aggregations may consist of
as many as 2,000 individual D. savignyi, spine to spine,
covering areas up to 100 m 2 in depths of about 8–30 m
(Plate 36). D. savignyi is also found at Ducie and Oeno.
Crown-of-thorns starfish Acanthaster planci
In general, asteroids (starfish) and ophiuroids (brittlestars)
are rare at Henderson. Prior to 1991, the only record of
the crown-of-thorns starfish Acanthaster planci within the
Pitcairn Islands was from the lagoon at Ducie in 1970/71,
where Rehder and Randall (1975) reported encountering
just one individual. In December 1991, however, Irving
(1995) found it at Henderson and counted 150 individuals
in an area of approximately 100 m 2 within the depth
range of 32–38 m off the North-West Beach (Plate 37).
Acanthaster planci is known to be a voracious predator
of corals and its presence at Henderson was a cause for
concern. Large numbers of the starfish are known to
suddenly appear on a reef, gorge themselves on live coral
polyps – which then leaves the coral dead – and then
disappear as mysteriously and rapidly as they arrived. Mass
coral mortality events, such as was reported by Rehder
& Randall (1975) at Ducie in 1971, may well have been
caused by outbreaks of A. planci in the past.
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