REEF-DWELLING BRYOZOANS 



91 



limestone cap atop Enewetak's volcanic pedestal yielded 

 fossil remnants of encrusting cheilostomes (Escfiaroides? 

 sp. and possibly Cosciniopsis aff. coehtus), a tuft-like 

 cheilostome (Nellia aff. oculata), an unidentified rcteporid 

 cheilostome, and a tubuliporine cyclostome {Diastopora? 

 sp.) (Brown, 1964). 



BIOGEOGRAPHIC IMPLICATIONS 



Tficse bryozoan records from Enewetak (Table 1) 

 extend the published ranges of all the species onto that 

 atoll and probably also into the Marshall Islands and 

 Micronesia overall. [A small Enewetak collection years ago 

 was not published (D. Soule and J. Soule, 1971, personal 

 communication)]. Most of the Enewetak species have been 

 recorded previously from elsewhere in the Indian Ocean or 

 western Pacific; therefore, this atoll's bryozoans do app>ear 

 typically Indo-Pacific. 



Several of the Enewetak species range more widely 

 than Indo-Pacific. A few are even circumtropical or nearly 

 cosmopolitan, most noticeably Aetea truncata, Cleido- 

 chasma porcellanum. Crepidacantha poissonii, Cribhlaria 

 radiata, Crisia elongata. Escharina pesansehs, Fenestruhna 

 malusii, Hippopodina feegeensis, Sauigni/eUa lafontii, and 

 Tr\;postega venusta. 



Two Enewetak forms, Rh^/nchozoon solidum and 

 Thalamoporella distorta, represent substantial range exten- 

 sions from the Caribbean region. Moreover, three others, 

 Cribrilaria calamorpha, C. simulator, and Rhi;nchozoon 

 curtum, were previously known only as lower and mid- 

 Tertiary fossils from lands near the Gulf of Mexico. 



It is interesting to compare the Enewetak fauna with 

 other reefal bryozoans investigated by similar methods. 

 Several species (25), more than a quarter of the Enewetak 

 fauna, also occur on Caribbean or western Atlantic reefs. If 

 genera are considered, even more (34), about half the 

 fauna, are common to Enewetak and the Bermuda- 

 Bahamas reefs (Fonda and Cuffey, 1976; Cuffey and 

 Fonda, 1977, 1979). 



Bryozoan endemism is difficult to establish with any 

 confidence at present because so few reef sites have been 

 studied comprehensively for these animals. However, it is 

 noteworthy that among the 62 encrusting cheilostome 

 species identified from Enewetak (Table 1; Cox, 1983), six 

 are new to science. These six species may be narrowly 

 endemic to Enewetak or instead may be more widely dis- 

 tributed in the surrounding region (and were discovered 

 here simply because this atoll was the first investigated 

 bryozoologically). Comparable reefal-bryozoan surveys on 

 several other atolls will be required before any degrees of 

 endemism can be determined. 



ECOLOGIC DISTRIBUTIONS 



Like other Pacific atolls, Enewetak exhibits distinct eco- 

 zones developed across its prevailing windward-leeward 

 gradient (northeast-southwest, respectively; Cuffey, 1973, 

 pp. 35-40). Recognition of similar zones in fossil reefs is 

 essential for interpreting those deposits; hence, ecozonal 



distributions of species in modern reefs like Enewetak Atoll 

 are of the greatest interest for comparative paleoecologic 

 studies. 



Unlike certain Caribbean coral species, the Enewetak 

 bryozoans do not fall into sharply defined, ecozonally re- 

 stricted assemblages. Instead, the conspicuous species 

 (Figs. 1 and 2) characterize somewhat different parts of 

 the atoll in a more generalized fashion. Consequently, it is 

 sufficient here to contrast oceanward reefs with lagoonal, 

 and deep reefs (10 to 40 m or 30 to 135 ft), shallow reefs 

 (0 to 10 m or to 30 ft), and reef flats (intertidal). 

 The oceanward reefs that were examined for bryozoans 

 are nearly vertical coral-rock cliffs on the leeward side of 

 the atoll and sloping coral pavements on its windward side. 

 The lagoonal reefs, all surrounded by barren sand bottoms, 

 range from low coral knolls to platform-like patch reefs to 

 tall steep-sided pinnacles. The rarer Enewetak bryozoan 

 species are sparsely scattered in a few ecozones but are 

 without immediately obvious patterns (clearly a subject for 

 further research). 



Overall, bryozoan colonies appear most abundant on 

 the deeper Enewetak reefs (below 10 m or 30 ft) and on 

 the Iceward-oceanward, and lagoonal reefs. 



Cranosina coronata, recognized by its large open op>esia 

 and crenulated mural rims (Fig. la), forms by far the 

 greatest number of colonies, particularly large ones, in our 

 collections. It thus appears as the most conspicuous 

 Enewetak bryozoan species, occurring almost ubiquitously 

 on deep and shallow oceanward (leeward and windward) 

 and lagoonal reefs, and locally also on the windward reef 

 flat. Arachnopusia spathulata, much less numerous though 

 still common, exhibits the same distribution; it has a cal- 

 careous frontal perforated by large pores (Fig. 2a). Simi- 

 larly distributed, except for not inhabiting reef flats, are the 

 next most important Enewetak bryozoans in terms of 

 colony numbers, Cosciniopsis n. sp. ("enewetakensis" ) with 

 large peristomes and granulate frontals (Fig. lb) and 

 Chorizopora ventricosa with transversely striate frontals 

 (Fig. Ic). 



Two species each encrust substantially larger total sub- 

 strate areas than any others and therefore app)ear espe- 

 cially conspicuous in the Enewetak fauna. Thala- 

 moporella expansa, distinguished by opesiules and 

 opesium-flank swellings (Fig. Id), is abundant on the deep 

 oceanward and lagoonal reefs. Reptadeor\ella joloerisis, 

 with coarse areolar p)ores, conspicuous peristomes, and 

 violet-stained frontals (Fig. le), is especially abundant on 

 deep and shallow lagoonal reefs but is also found on lee- 

 ward oceanward reefs. 



Two common species are restricted to lagoonal reefs, 

 Hippopodina feegeensis (convex granulate frontals, nearly 

 circular apertures. Fig. 2b) on shallow reefs, and 

 Rhi/nchozoon curtum (large areolar fxjres, slit-like apertural 

 sinus. Fig. 2c) on deep reefs. 



Four sjDecies — Smittipora americana (opesiular notches, 

 Fig. 2d), Caltjptotheca impar (raised interzooecial threads, 

 Fig. 2e), Trypostega uenusta (zooeciules and minute frontal 

 pores. Fig. 2f), Cleidochasma porcellanum (keyhole-shaped 



