Pseudosquilla oculata (Brullé 1837)

Pseudosquilla oculata (Brullé, 1837)

Squilla oculata Brullé,1837: planche unique: fig. 3; 1839:18.

Pseudosquilla oculata.—Manning,1977:103,figs. 32,33,55.

MATERIAL.—Operation Origin: Site 0, English Bay, found under a rock,22 m: 1 female [53].

Other Collections: Jourdan (1976), Ascension Island, from dead Cymatium shell, 20 ft (6 m): 1 female [cl 10].—Jourdan (1976), English Bay, under rocks, 20 ft (6 m): 1 female [51].—Jourdan,English Bay,20–30 ft (6–9 m): 1 male [79], 1 female [96].—McDowell, English Bay, 30–50 ft (9–15 m): 1 female [56].—Olson (1970), McArthur Point, sandy bottom tide pool: 1 female [31].

SIZE.—Total lengths of male, 79 mm; of females, 31–96 mm.

HABITAT.—Littoral, in rocks around tide pool, and sublittoral to 22 meters; one specimen taken in Cymatium shell.

DISTRIBUTION.—Indo-West Pacific from Hawaii to the western Indian Ocean, and amphi-Atlantic, including Ascension and Saint Helena slands; littoral to about 60 meters.

ZOOGEOGRAPHICAL CONSIDERATIONS

With its geographical isolation (Figure 47), almost 1300 kilometers from the nearest land mass, St Helena, and more than 2200 kilometers from the nearest continental land mass, Brazil, Ascension had to have been colonized chiefly by pelagic larvae,the teleplanic larvae characterized by Scheltema (1968,1986). As Ascension is a volcanic island, with limited habitats (no grass flats or coral reefs), only some groups could colonize it Yet,no fewer than 74 species of decapod Crustacea have become established on the island since its emergence in the the Pleistocene. Not surprisingly, 59 of the 74 species of decapods found there (80%), occur elsewhere in the Atlantic. Slightly more species are found only in the western Atlantic than in the eastern Atlantic, but larvae from both sides of the Atlantic and the central Atlantic as well have contributed to the Ascension decapod fauna.

Briggs (1974:95) and Pawson (1978:5,6) have discussed the current patterns affecting Ascension, pointing out that it lies in the westward flowing South Equatorial Current, at the northern edge of the South Atlantic Gyre. It must occasionally come under the influence of the eastward flowing Equatorial Countercurrent or the Equatorial Undercurrent (Scheltema, 1971:287, 1977:81; Pawson, 1978:6), so that transport mechanisms exist to carry pelagic larvae to Ascension from both sides of the Atlantic. This is reflected in the faunal composition of the island. Ascension and St. Helena, and especially the latter, also are situated so that they come under the influence of the Benguela Current, flowing northeastward from southern Africa, providing a way for larvae of Indo–West Pacific species that have survived the journey around the Cape of Good Hope or larvae originating from southwestern Africa to reach these isolated oceanic islands. The unique absence of cyclonic storms in the South Atlantic (Darlington, 1957:18) eliminates that distributional factor from consideration. Both islands also lie in the path of the southeast trade winds, driving surface water from southwest Africa toward the equator (Olson, 1973, fig. 1; Briggs, 1974, fig. 4–1; Edwards and Lubbock, 1983, fig. 1). These factors may account for the number of Indo-West Pacific species found at both Ascension and St. Helena, including the occurrence in the Atlantic of several species otherwise known from the Indo–West Pacific: Alpheus crockeri and Percnon abbreviatwn at Ascension, Planes marinus, Petalomera wilsoni, Calappa bicornis, and Mursia cristimanus at St Helena, and Metalpheus paragracilis at both islands.

Scheltema (1971:306; 1977:87) provided estimates of the number of days required for trans–Atlantic drift across the South Atlantic, 60–154 days from the Gulf of Guinea to Brazil in the South Equatorial Current, 96 days from Brazil to the Gulf of Guinea, in the Equatorial Countercurrent. To reach Ascension from each area would take only about half the time, 30–77 days westward, 48 days eastward. Garth (1965:44) quoted C.O’ D. Iselin (in litt.) in saying that “it is safe to say that the velocity of ocean currents is about twice that shown on current charts,” so it is possible that much less time would be needed for larvae to reach Ascension from either side of the Atlantic.

Hines (1986:449) summarized the average duration in days of larval periods for several crab families, as follows: Grapsidae 39, Majidae 30, Portunidae 45, and Xanthidae 31. Thus the duration of larval life for at least some members of all of these families would allow them to remain in the plankton to colonize Ascension from either side of the Atlantic. Curiously, Hines also found (p. 450) that “among marine species mere was no significant relationship between the extent of the range and duration of the zoeal period or duration of the total larval period.” This is similar to the finding of Brothers and Thresher (1985), who concluded that in coral reef fishes breadth of distribution may not correlate with duration of pelagic developmental stages.

More specific data has been provided by several authors. Wilson and Gore (1980) reported that the minimum time needed for completion of larval development of Plagusia depressa was 60 days, and that this might account for its amphi–Atlantic distribution. Rice and Provenzano (1966) reported that Dromidia antillensis required 23–25 days to reach the megalopa stage, and that a megalopa took another 14.5 days to molt. They also noted that Gurney (1924:191) had described a dromiid larva from the central Atlantic [22°06′S, 39°40′W] that closely resembled their larvae of D. antillensis. Gurney (1938:296) identified a larva from 3°17′S, 29°57′W in the South Adantic as an Enoplometopus. Rice, Ingle, and Allen (1970) studied the larval development of the Mediterranean Dromia personata and found that it reached the megalopa stage 21–28 days after hatching. Laughlin et al. (1982) found that development to first crab took 28–30 days in Dromia erythropus. Knowlton (1973, table 1) characterized Alpheus dentipes, A. macrocheles, and Synalpheus fritzmuelleri as species with extended larval development. Thus these species have a larval life adequately long to allow their larvae to be transported long distances.

All of the species mentioned above and an amphi–Atlantic species of Enoplometopus occur on Ascension.

That larvae of Ranilia constricta periodically appear in swarms (Schmitt, 1956; Chace and Barnish, 1976) may help explain its wide distribution in the Atlantic. Similarly,larvae of the stomatopod Alima hyalina Leach also can occur in enormous swarms (material in USNM collections), and it, too, is a widely distributed species.

Certainly species of some genera, especially grapsids, might have reached Ascension by rafting. Dawson (1987:43) noted that “many authors referred to the habit of species of Plagusia of clinging to driftwood, floating timbers and ships hulls.” Garth (1966:447) also remarked that members of Plagusia, Planes, and Pachygrapsus are “habitually transported by drifting logs or on sea turtles as adults.”

THE FAUNA OF ASCENSION AND ST. HELENA

Distribution patterns of Ascension decapods (Table 1) can be summarized and compared with the decapod fauna of St. Helena (Table 2) as follows:

A total of 74 species is known to occur on Ascension (Table 1). Of these, 41 (55%) occur in the western Atlantic, and most of these are common there. Both Euryozius sanguineus, which outside of Ascension and St. Helena is found at St. Paul’s Rocks, and Gecarcinus lagostoma, which occurs at Fernando de Noronha and Ilha Trinidade, off Brazil, and Ascension, are included as western Atlantic species even though both of these are actually central Atlantic island species, the ranges of which do not extend to the continental mainland. Of the 41 species from Ascension also occurring in the western Atlantic, 19 (26% of total) are known only from the central and western Atlantic. In contrast, only 35 species have been reported to occur at St. Helena (Table 2), and 15 of them (43%) occur in the western Adantic. Of the 15, only 5 (14% of total) occur only in the central and western Atlantic. A larger percentage of Ascension species is found only in the central and western Atlantic.

Of the 74 species found at Ascension, 36 (49%) occur in the eastern Atlantic.and 14 of these (19% of total) are known only from the central and eastern Atlantic. Similarly, 19 of the 35 (54%) of the species known from St. Helena (Table 2) also occur in the eastern Atlantic,but 8 of these (23% of total) are known only from the central and eastern Atlantic. Eastern Atlantic components of the fauna are therefore slightly higher at St. Helena than at Ascension.

Twenty-two of the Ascension species (30%) are amphi-Atlantic, and 15 of these (19% of total) do not occur outside of the Atlantic. Members of the other 8 species, Rhynchocinetes rigens, Brachycarpus biunguiculatus, Alpheus bouvieri, Alpheus paracrinitus, Metalpheus rostratipes, Lysmata intermedia, Sympagurus dimorphus, and Percnon gibbesi, occur in the Indo-West Pacific and/or the eastern Pacific. Ten (29%) of the species from St. Helena are amphi-Atlantic. Thus amphi-Atlantic components of the faunas of these two islands are slightly greater on Saint Helena.

That the distance across the Atlantic is too great for most species to cross is supported by the fact that components of the Ascension decapod fauna with ties to one side or the other, 31 species or 42% of the total, are larger than amphi-Atlantic components, 22 species or 30% of the total. Cases of amphi-Atlantic distribution patterns are rare in the decapods, possibly reflecting their limited long distance dispersal potential. For example, of more than 200 species of crabs from West Africa treated by Manning and Holthuis (1981), only 20 or about 10% show amphi-Atlantic distributions. Fourteen of these are shallow-water tropical species, and 8 of the 14, or the majority of shallow-water, tropical species, Apiomithrax violaceus, Calappa galloides, Microcassiope minor, Percnon gibbesi, Plagusia depressa, Laleonectes vocans, Ranilia constricta, and Xanthodius denticulatus, are known to occur on Ascension.

Of these, Apiomithrax violaceus is the only majid, Calappa galloides is the only calappid, Ranilia constricta is the only raninid, and Laleonectes vocans is one of only two portunids to show amphi???Atlantic distributions. Further, the only amphi-Atlantic palinurid, Panulirus echinatus, also lives at Ascension.

So far as we can tell, the following species are the only shallow-water tropical amphi-Atlantic species (including pantropical but excluding introduced species or pelagic species such as Portunus sayi (Gibbes); an asterisk identifies the species found on Ascension (ASC) or St. Helena (SH)):

PENAHDAE

Penaem duorarum notialis Pérez Farfante