SUBTIDAL ENVIRONMENTS AND ECOLOGY 



135 



Atolls. At the last atoll, Fosburg (1955) reported "a rather 

 extensive strip" of T hemprichii along the lagoon shore of 

 Ujilang Island. 



Thalassia hemprichii was found by the author only 

 along the lagoon shore of Ujilang Island in July 1982. 

 None was seen on several lagoon shores on the windward 

 side visited or on two coral pinnacles with extensive sandy 

 areas above 15 to 20 m depth. The strip along Ujilang 

 Island occurred only at depths <1 m. Reliable information 

 exists that 7 hemprichii (or any other sea grass) does not 

 occur at Bikini, Rongelap, or Rongerik (Emery ct al., 

 1954). Fosburg (1955) reported on visits to many north 

 Marshall atolls (Kwajalein, Lae, Ujae, Wotho, Likiep, 

 Aihik, Bikar, Pokak. and Ujilang) with Ujilang the only of 

 these where sea grasses were noted. At Kwajalein, signifi- 

 cant diving and collecting activities by knowledgeable 

 marine biologists over large portions of the atoll have 

 failed to reveal 7. hemprichii. Tsuda (Chapter 1, Volume 

 II) documents that no record of sea grasses from Enewetak 

 exists. Because of the large amount of marine work carried 

 out at this atoll, it is reasonable to say they do not occur 

 here. 



The presence of T. hemprichii at Ujilang, only 200 km 

 away, is intriguing. The decline, however, in the numbers 

 of sea grass species eastward through Micronesia (Tsuda et 

 al., 1977) and the probable absence of T. hemprichii at 

 most — if not all — other Marshall atolls, indicate that per- 

 haps there has been no opportunity for transport of 

 T. hemprichii to Enewetak. The areas upcurrent of the 

 atoll are similar atolls without sea grasses. A similar condi- 

 tion has been noted for Sargassum (Tsuda, 1976) with no 

 records from any northern Marshall atoll, including 

 Enewetak. The means of dispersal of 7. hemprichii are lim- 

 ited. Potentially it could be transported as drift material 

 torn loose during storms or as drifting seeds or seed cap- 

 sules. Both potential mechanisms are current dependent, 

 which would work against transport to Enewetak. Atolls 

 farther south, in the influence of the Equatorial Counter- 

 current, may have received their populations via this cur- 

 rent. 



Zoogeographic Considerations 



One interesting phenomenon is that many marine 

 animals that arc relatively scarce at Enewetak are much 

 more common elsewhere. This seems true even within the 

 Marshall Islands where disparity exists between Enewetak 

 and the more southerly Marshall Island atolls— such as 

 Kwajalein, Majuro, and Arno — which have had a signifi- 

 cant collecting effort. 



This scarcity is true among fishes. For example, Allen 

 (1972a) commented that most anemone species were rela- 

 tively scarce at Enewetak as compared to his personal 

 observations in Tahiti and literature from the Nicobor 

 Islands. "Several hours of intensive searching may at best 

 result in finding four or five widely scattered sp>ecimens (of 

 anemones) of the variety which harbor Amphiprion." As 

 has been pointed out elsewhere, even when present, the 

 occurrence of anemones may be transitory. 



Hiatt and Strasburg (1960) indicate that a number of 

 fish species are uncommon or rare at Enewetak (and often 

 Bikini) compared to Arno Atoll. They felt that Arno was a 

 more productive atoll than either Enewetak or Bikini 

 because it is located in an area of upwelling where the 

 North Equatorial Current and Equatorial Countercurrent 

 meet and because it has a higher rainfall than the other 

 two atolls. Whether this has an effect on the abundance of 

 reef fishes or whether the differences observed are pro- 

 duced by sources of larval recruits, etc., is not known. 

 Species of Plesiops and Pseudogramma are among fishes 

 that are less common at Enewetak. Randall (1986) lists a 

 number of species from Kwajalein Atoll which, in spite of 

 comparable collecting effort, are not known from 

 Enewetak. 



Within the overall picture of Indo-West Pacific shore 

 fish distribution, the Enewetak fauna is less diverse than 

 the "core" areas of the Indo-Malayan Archipelago. This is 

 well known for individual families (Allen, 1975), but the 

 Enewetak fish fauna is at a level of diversity "expected" 

 when compared to adjacent areas. The differences exist 

 with respect to abundance of quite a number of species. 



Why Are There No Mangroves at Enewetak? ACKNOWLEDGMENTS 



Wiens (1962) has summarized much of the information 

 on mangroves on atolls. He cites records of "mangroves" 

 on several southern Marshall Island atolls. Hatheway 

 (1953) described stands of Sonrieratia caseolahs and 

 Bruguiera conjugata on Arno Atoll. Wiens (1956) observed 

 a tidal inlet on Ailinglapalap Atoll with three species of 

 mangroves. Fosburg (1955) reports B. conjugata on 

 Utirik, Ailuk, and Lae to be rare and limited to "wet 

 depressions." Otherwise, he does not record any 

 "mangrove" species, particularly those of the Rhizophori- 

 dae from the northern Marshall Islands. 



Again the situation is similar to sea grasses. Mangroves 

 can certainly survive at atolls like Enewetak, but it is likely 

 their transport mechanisms have never allowed their intro- 

 duction. 



The staff of the Mid-Pacific Research Laboratory 

 (MPRL) and its predecessor institutions made possible the 

 vast majority of the marine research undertaken at 

 Enewetak Atoll since 1954. Many of the people involved 

 in this work have been cited in the preface to this volume. 

 I would like to thank in particular the following MPRL staff 

 members for their help in my own fieldwork and that of 

 others: L. J. Bell, L. M. Boucher, V. S. Frey, S. Johnson, 

 J. T. Harrison, III, and R. M. Richmond. The operation of 

 MPRL would not have been possible without their dedica- 

 tion and perseverance under extremely difficult cir- 

 cumstances. 



I would like to thank the following for their comments 

 on the manuscript: L. J. Bell, J. T. Harrison, III, A. Kohn, 

 and J. E. Randall. 



