BULLETIN OF MARINE SCIENCE, 25(2): 186-204,1975
ECOLOGICAL OBSERVATIONS ON THE RECENT ARTICULATE BRACHIOPOD ARGYROTHECA BERMUDANA DALL FROM THE BERMUDA PLATFORM
Alan Logan
ABSTRACT The occurrence and life habits of Ihe diminutive articulate brachiopod Argyro/Izeca bermu- dalla have been investigated along a general NW-SE traverse across the Bermuda Platform, in- corporating all seven biotopes recognized by Upchurch (1970). Sediment samples indicate that brachiopod tests comprise a consistent though minor « 1%) fraction in reefal shoal sediments, but are totally absent from lagoonal sediments. Preliminary transects reveal that in the reef-front terrace, north reef and central and south reef biotopes, A. bermudalla is cryptic in habit, occurring on the undersides of the foliaceous hermatypic corals MOil/as/rea cavernosa (L.), M. annularis (Ellis and Solander), and Agaricia fragilis Dana. The main host coral for brachiopods on the south shore reefs and associated reef-front terrace is M. cavemosa, on the north reef-front terrace, north and central reefs it is M. anllularis, while A. fragilis is more important on the lagoonal and near-shore reefs. Brachiopod densities generally decrease shorewards from the edge of the Platform. The distribution and density of brachiopods appears to be controlled primarily by the relative abundance of the host corals and their growth form, although other limiting factors may be at work. Clumped distributions are common, probably the result of brooded larvae and in- tense competition for living space. Inter-island areas such as Castle Harbor and Harrington Sound are virtually devoid of brachiopods, although a relict population exists marginally in an unusual sub-boulder habitat in land-locked Walsingham Pond.
Following early work by Rudwick (1962), vation of coral reefs, particularly their cryp- there has been a recent resurgence of interest tic habitats, and articulate brachiopods, such in the ecology of modern articulate brachio- as megathyrids and thecidioids, have been pods, mainly cool and cold-water forms. shown by Jackson, Goreau and Hartman Thus examples of laboratory and/or field- (1971) to be important members of a cryp- oriented studies include those by Rickwood tic brachiopod-coralline sponge community (1968), Foster (1969), Neall (1970), Mc- in the Caribbean (J amaica, Cura~ao), Red Cammon (1971, 1973), and Logan and Sea (Ros Muhammad), and Pacific (Guam, Noble (1971). Some of the studies men- Saipan) areas. These workers have noted tioned above have combined ecological ob- high densities of brachiopods in intimate as- servations with growth and behavioral stud- sociation with coralline sponges in two cryp- ies and have greatly advanced our knowledge tic habitats in the coral reefs: on the under- of this once-prolific phylum. sides of foliaceous corals or overhangs, and Studies on warm-water extant brachio- in the interiors of crevices and caves. Grant (1971 and personal communication, 1973) pods, especially those forms existing on has observed species of Frenulina and The- shallow-water coral reefs, have been some- cidellina in similar habitats on the reefs of what neglected by comparison, presumably Eniwetok AtolI, MarshalI Islands, Pacific. because brachiopods have traditionally been The present study records observations regarded as rare in such environments (El- on the life habits of the diminutive megathy- liot, 1950; Rudwick, 1965). The advent of rid Argyrotheca bermudana from the Ber- SCUBA, however, has allowed close obser- muda Platform. Data on the contribution of
186 LOGAN: ECOLOGY OF ARGYROTHECA BERMUDANA ]87 this species to the sediments of the area are Harrington Sound, just below low-tide also included. Such documentation of an mark." additional reef brachiopod may hopefully Gautier (1965, unpublished report, Ber- stimulate further search for brachiopods in muda Biological Station; personal communi- cryptic reef habitats elsewhere and help dis- cation, 1972) recorded A. bermudana from pel the impression that modern coral reefs sediment samples across the Bermuda Plat- arc almost devoid of brachiopods. form but was able to collect live specimens For discussion on the role of brachiopods only from Walsingham Pond, where they oc- in fossil coral reefs, particularly those of the curred in shallow water beneath rocks on a post-Paleozoic, the reader is referred to El- talus slope. Lowenstam (1961; personal liot (1950), while Grant (1971) has stressed communication, 1972) has obtained this the importance of brachiopods in the es- species live from the undersides of the coral sentially non-coral-bearing Permian "reefs" Agaricia fragilis within undercuts in Harring- of west Texas. ton Sound in 1 m of water and also on under- cuts in the vicinity of North Rock at depths GENERAL REMARKS ON ARGYROTHECA of 12 m. The genus Argyrotheca Dall, which be- Prior to the present study, however, no longs to the family Megathyrididae, extends systematic search has successfully been made as far back as the Late Cretaceous and is for living populations of this species from widely distributed in modern seas, being Bermuda, in spite of the fact that it forms a represented by several species, most of which consistent though minor element in most are continental shelf dwellers (Elliot, 1951). sediment samples from reefal shoal areas In the Caribbean faunal province, at least across the Platform. six species of Argyrotheca occur, according Various aspects of the embryology and to Dall (1920) and Cooper (1934, 1954). development of Argyrotheca and related Of these Caribbean species, only A. ber- megathyrids have been described by Shipley mudana occurs in Bermudian waters but is (1883), Schulgin (1884), Plenk (1913) not confined to the region. and Atkins (1960). Studies by Senn (1934) Argyrotheca bermudana was first formally and earlier workers have shown that the described by Dall in 1911, presumably from species A. cordata and A. cuneata are her- seven specimens collected by Haycock from maphroditic and possess paired brood Harrington Sound and now in the U.S. Na- pouches in which free-swimming larvae are tional Museum (Dall, 1920). Verrill (1903) retained until a relatively late stage of devel- had previously illustrated this form and iden- opment. In addition, Atkins (1960) has tified it, on the advice of C. E. Beecher, as a described the escape of such larvae from variety of the European Cistella cistellula brood pouches in A. cordata. Brood pouches (Searles Wood). Verrill (op. cit., p. 592) containing larvae in various developmental provided the first and only published refer- stages are clearly seen in A. bermudana; pos- ence to the life habits of this species when session of such pouches appears to be char- he remarked: "By examining carefully the acteristic of most members of the genus. underside of unbleached specimens of the PHYSIOGRAPHY AND OCEANOGRAPHY delicate foliaceous coral, Mycidium fragile', OF THE BERMUDA PLATFORM I found a number of small specimens, mostly immature, of a reddish species of Cistella. The physiography, oceanography and meteorology of the Bermuda Platform is rela- A few were also found on the underside of tively well known and recent detailed ac- Tsophyllia dipsacea and on the base of Ocu- counts are available by Stanley and Swift lina. Most of these, if not all, were taken in (1968), Upchurch (1970) and Garrett et
1 Now Agarida fragilis Dana al. (1971). Suffice to say here that, because 188 BULLETIN OF MARINE SCIENCE, VOL. 25, NO.2, 1975
...... ;',;: :..::.~. ' •....::.~... 32'30 Reefal shoals "';" « 7m in depth) •.<:.<:'~~.
"',". \\ ... , : A , . '.'
INSET I (/
" " f~" .'
'@l-. ..:/'
,',
" j/ l ,..:',' :,:', , ..:''
\:,\ A
:1, '" "\::.\ ".~,
Figure 1. Map of Bermuda Platform, showing collecting and sediment sample localities.
Table 1. Numbers of reef coral colonies (or parts thereof) obtained, and approximate coral surface areas examined, in square meters (parentheses), in preliminary traverses at various localities along a gen- eral NW-SE transect across the Bermuda Platform. [fl and ~ denote presence or absence, respectively, of the brachiopod A. bermudana on these coral colonies.
1l ~ ~ ~ c ~ ~ Sampling locality: "" " ... .~ "::l~Q.•.. " '" :; ~ description and approximate ",,°0 ""~ ~ c ~'" position (see fig. 1 and ,,<-- 'O.t) .D -E ';:: "'S ::l ::l ~ ~ci. c Admiralty Charts 334 and 868) b"~~ O~ til f-< l:l ~::l Q., Upper reef-front terrace, Coralgal veneer 19 25 13 N. of North Rock A 18-19 on aeolianite; Medium ~ [fl ~ (32°29'2"N, 64°46'18"W) sand channels (0.9) (0.6) (0.4) ___ . North Ledge Flats, south Coralgal veneer 21 27 25 of Hog Breaker B 9-12 on aeolianite; Medium- ~ [fl ~ (32°27'18"N,64°49'24"W) sand channels high (2.3 ) ( 1.0) (0.8) Three Hill Shoals, Coralgal veneer 20 25 27 patch reef C 5-7 on aeolianite; Low- ~ [fl ~ (32°25'6"N,64°44'0"W) sand channels medium (2.2) (0.8) (0.9) Reefs outside entrance to Poorly-de vel- 11 11 3 Whalebone Bay F 5-8 oped coralgal Usually ~ [fl ~ (32°21'53"N,64°42'55"W) reef low (0.5) (0.3 ) (0.1 ) Shark Hole, Harrington Aeolianite cliff; Sound F 1-5 sands and low (32°20'10"N,64°42'11"W) sparse corals at base Abbot's Cliff, Harrington Cliff; sands, Sound F 1-4 blocks and low (32°20'23"N,64°43'21"W) sparse corals at base Patton's Point, Cliff; sands, Harrington Sound F 2-4 blocks and low (32°19'21"N,64°43'10"W) sparse corals at base N. side FlaUs Inlet, Aeolianite cliff; Harrington Sound F 2-4 sands and low- (32° 19'22"N, 64°44'7"W) sparse corals medium at base N. side, Walsingham Pond Steeply-dipping (32°20'49"N,64°42'30"W) G 1-3 aeolianite talus low slope above mud "Red Stake" pinnacle reef, Coralgal pin- 11 17 Castle Harbor F 1-5 nacle reef usually ~ ~ (32°20'35"N,64°41'20"W) low (0.3 ) (O.?_) __ "Meandrina" pinnacle reef, Coralgal pin- 8 2 21 Castle Harbor F 2-6 nacle reef usually ~ ~ ~ (32°20'47"N,64°40'52"W) low (0.2) (.05 )___ (O.D-_ Inshore shoal reef, Poorly-devel- 2 3 Causeway, Castle Harbor F 1-3 oped coralgal usually ~ ~ (32°21'16"N,64°42'20"W) inshore shoal low (.05) (0.1 ) reef ------South reefs, SE of Coralgal veneer 22 18 11 Nonsuch Island D 3-12 on aeolianite medium- ~ [fl ~ (32°20'24"N,64°39'23"W) high (1.1) (0.5) (~~-)- Upper reef-front terrace, Coralgal veneer IE 21 14 ENE of S1. David's Head A 9-18 on aeolianite medium- ~ [f] ~ (32°22'12"N,64°38'43"W) high CO: 8 ) (0.6) (0.5) Upper reef-front terrace, Coral veneer 16 22 8 SE of Gurnet Rock A 18-22 on aeolianite; medium ~ ~ ~ (32°20'5"N,64°39'32"W) sand channels (0.8) (0.7) (0.3 ) LOGAN: ECOLOGY OF ARCYROTHECA BERMUDANA 191 192 BULLETIN OF MARINE SCIENCE, VOL. 25, NO.2, 1975
0\ ..., ..., ..., o lrl lrl lrl r-- - r--r-- 00 r-- \C lrl 0\ NOOO "":0 00 o 00 O~N II II II II II II II II II II II ,<,< ,<,< ,<,< ,<,<,<
00 lrl I I " " o . v: 0\. '". Z I I ~ . ..., O' \C. CI.l • .CI.l .CI.l CI.l'ON•• o • 00 ..., O' ...,' 'lrllrl Zoor-- \CZ " ~O o lrl_ I I ~ NO\N 000 --N \C LOGAN: ECOLOGY OF ARGYROTHECA BERMUDANA 193 to be patchy and rarely did more than 50 percent of the corals collected yield brachio- pods. Nevertheless, to test whether distribu- tion was random or truly aggregated the vari- ance/mean ratio of brachiopod abundance for a standard sample area (I OOcm~) for each biotope was calculated, following the method outline by Pielou (] 969). In all cases involving sizeable numbers of brachio- pods the ratio exceeds unity significantly (Table 2), suggesting a large measure of aggregation, such gregariousness probably resulting from late release of brooded larvae, rapid settlement, and post-settlement compe- tition with other organisms. The following biotopes recognized by Up- church (1970) were sampled across the Platform: Reef-front Terrace Biotope The reef-front terrace biotope was defined by Upchurch (op, cit.) as extending from the edge of the Platform at 10-15 fathoms (18-27 01) inward to the reef tract. Its physiographic equivalent has been described by Bigelow (1905), Stanley and Swift (1967, ] 968) and Stanley (1970), Skeletal frag- Figure 2 (top). A rgyrotheca ball/udana on under- ments from samples from this biotope have side of Agaricia tragi/is, south reefs south-east of been examined by Upchurch (1970) and Nonsuch Island, 12 m depth, X 6, Inset, pedicle Waller (1973), both of whom recorded Ar- and brachial valve, showing coarse punctation and gyrotheca bermudana tests from their sam- color banding, specimen from sediments within sand channel, Three Hill Shoals patch reef, 5 m ples, Waller (op. cit., p. 40) suggesting that depth, X 6; (bottom) Group of A, bermudana the tests were derived from "living sites (arrowed) around base of pedestal of Agaricia among and beneath small corals". tragi/is, reefs outside Whalebone Bay, 5 m depth, X 4, Southern Development.-The reef-front ter- race biotope on Bermuda's south shore was examined by the author at a locality one of attachment by the pedicle to the un- about 150 meters south-east of Gurnet Rock, derside of foliaceous or encrusting corals in (tl-Je reef edge) at a depth of 22 meters. The coral-dominated macrohabitats; occasional effects of the strong surge are felt at this attachment to the underside of boulders in depth and energy conditions may be de- bedrock-dominated macrohabitats is also scribed as medium, Long ridges and mounds known. The shell is normally orientated in of coral-encrusted aeolianite, many orien- an almost vertical position (Fig. 2), with the tated almost at right angles to the trend of anterior margin thrust well clear of all asso- ciated encrusting organisms, such as algae, the coastline, are separated by 9-12 01 chan- sponges, bryozoans, and foraminifera. nels of coarse, rippled sand, These win- At all localities sampled, the distribution nowed sands contain mostly Homotrema of individuals upon the host corals appeared rubrWI1 and shell fragments, including tests 194 BULLETIN OF MARINE SCIENCE, VOL. 25, NO.2, 1975 of A. bermudana. It is believed that these tests are mainly derived from local spurs of aeolianite reef rock encrusted with large heads of Diploria ssp. and Montastrea ssp., mainly M. cavernosa (L.). H. rubrum oc- curs in great abundance beneath both these corals but is associated with brachiopods only beneath M. cavernosa and, to a lesser extent, beneath M. annularis (Ellis and So- lander). The latter species is less common than M. cavernosa here and, in contrast to the heavy sheet-like overhangs produced by M. cavernosa (Fig. 3), usually forms pockets of small, shingle-or plate-like colonies with little overhang for possible attachment sites (Fig. 3, bottom). Brachiopod yields from this species are correspondingly low (Table 2), A second locality about 100 m off the reef front at St. David's Head was also ex- amined and yielded a sizeable brachiopod population, again mostly from M. cavernosa. Hydrographic conditions are similar here to those encountered at the previous locality. Of the brachiopods obtained from the two localities in the south shore reef-front terrace biotope, most came from M. cavernosa. Distribution is nevertheless patchy (Table Figure 3 (top). MOil/as/rea cavenlOsa on aeolian- 2) and some colonies are totally barren, ite, upper reef-front terrace, south-east of Gurnet mainly because of heavy organic growth on Rock, 20 m depth. Arrow indicates flattened col- ony of A. tragi/is, A shows typical overhang fa- the underside of the host corals. The coral vored by brachiopods. Width of white bar repre- Agaricia fragilis is rare at both localities; sents 10 em; (bottom) Platelike growth of M. where present it occurs in sheltered areas allllu/aris between massive M. cal'ernosa on aeolian- and is "molded" to the aeolianite rock (Fig. ite, same locality as above, 17 m depth. Width of white bar represents 10 em. 3, top), supporting little or no cryptofauna on its undersurface. The brachiopod tests encountered by Waller (1973) from sedi- the Platform is similar to that in the south. ment samples taken at depths of 34 m and From the outer reef edge coral-encrusted 51 m south of Tucker's Town (Fig. 1) may ridges, separated by valleys of rippled sand, be, in part at least, locally derived from grade seawards into sand wedges. The most coral-algal encrusted patches which Waller important bio-constructional agents are Di- states are present at that depth, but most ploria ssp., Montastrea annularis and Porites are probably derived from the reef and up- astreoides, with M, cavernosa here reduced per reef-front terrace by downward move- to a minor role. Medium energy conditions ment of sediment along the sand channels again prevail and M. annularis produces which transect this zone. mushroom-like heads, ideal for brachiopod Northern Development.-The topography of attachment (Fig. 4), whereas M. cavernosa the reef-front terrace in the northern part of has a mamillated or bulbous growth form LOGAN: ECOLOGY OF ARGYROTHECA BERMUDANA 195 pecially at the seaward edge of the reefs (e.g. North Rock) and large, flat coral encrusta- tions, poor in brachiopods, prevail. Lagoon- wards, away from the surf zone, energy con- ditions for coral growth are similar to those on the reef-front terrace and very large, sheet-like or mushroom-like encrustations of Mantastrea ssp. occur, together with large heads of Diplaria ssp. The north reef biotope was sampled on North Ledge Flats about 1.3 km almost due south of Hog Breaker, in waters of up to 12 m in depth. Most brachiopods were taken from the underside of M. annularis, the com- Figure 4. Mushroom-like heads of M. al1llu/aris monest of the framework-building corals at (A) and Dip/oria ssp., upper reef-front terrace, this locality. The growth form of this species north of North Rock, 18 m depth. Width of white is similar to that seen on the northern reef- bar represents approximately 30 em. front terrace, where medium energy condi- tions also prevail. Common associates of enclosing little or no cryptic habitat. At a the brachiopods include H. rubrum in great locality 1.2 km north of North Rock, most abundance, together with ectoproct bryo- brachiopods were obtained from the former zoans and coralline algae. species. H. rubrul11 is again an important faunal associate. Agaricia fragilis colonies Central and South Reef Biotope arc rare and support few cryptic organisms. Much of the reef tract on the eastern, Upchurch (1970) has recorded brachiopod western and southwestern margins of the tcsts from greater than 2 mm fractions from Platform is included within this biotope by his sediment sample 96 locatcd on the reef- Upchurch (1970). Here energy conditions front terrace about 0.5 mile west of Hog are lower than in the previous two biotopes, Breaker at the outer edge of the north reefs particularly in the east-west trending tract (water depth estimated at 22 m). Local of patch reefs known at Three Hill Shoals, reefal mounds may contribute some tests which lie approximately midway between here, the others presumably being derived North Rock and the northern shore of the from the main reef tract. eastern part of the Islands. In summary, the medium energy reef- An unnamed patch reef in Three Hill front terrace biotope, with its prolific growth Shoals was sampled in shallow water not ex- of large coral heads, provides a favorable ceeding 7 m in depth. The main reef-build- environment for attachment of Argyrotheca ing corals present are Millepara alcicornis, bermudana, which is a common constituent Porites astreaides, Mantas/rea annularis and of the eryptofauna on the underside of sheet- species of Diplaria. Preliminary investiga- like overhangs of species of Mantastrea. tions revealed that brachiopods were rare at this locality and this was confirmed after North Reef Biotope detailed collecting, A. bermudana being re- Upchurch (1970) restricts this biotope to corded mainly from the undersides of col- the northern part of the reef tract. It shares onies of Agaricia fragilis in deep, shaded thc rich coral growth of the reef-front terrace clefts in the reef (Fig. 5). M. cavernasa is biotope but because framework organisms less common than M. annularis here, the grow upwards to the intertidal zone, higher latter usually exhibiting a shingle-like, over- cncrgy conditions are often encountered, es- lapping growth form (see Garrett et ai., 196 BULLETIN OF MARINE SCIENCE, VOL. 25, NO.2, 1975 1971, Fig. 5) which might be expe~ted to provide a suitable niche for br~chlOpod~, However the undersides of colomes of thIs species a~e profusely overgrown with purple and green algal encrustations which cover almost the entire available surface; other notable epibionts include red and black sponges, Halimeda and encrusting cheilo- stomatous bryozoans. Such profusion of growth on Montastrea has rarely been seen elsewhere on the Platform. Homotrema rubrul11 is much reduced in numbers and ••.... confined to the inner areas of the host col- ;....~ ..• .. onies. The larvae of both this species and Figure 5. Typical Agaricia fragilis colony in A. bermudana, with which it is often associ- shaded cleft, Three Hill Shoals patch reef, 6 m. ated, may have great difficulty in fin~ing a suitably firm substrate for settlement, III ~he none of whom note the presence of brachio- face of this prolific growth. Isolated high pods. However, Schroeder (personal. com- densities of brachiopods have been recorded munication, 1972) remarks that specImens (Table 2) but such occurrences are ex- possibly belonging to A. bermudana hav.e tremely rare and the overall yield per colony been noticed by him within internal sedi- is low. Many of the brachiopods collected in ments in some Bermudian cup reefs. July 1972 were very small and obviously Boilers were encountered along transects immature, suggesting a recent spatfa~l. run transverse to the shoreline at Horseshoe Although brachiopod tests are consistently Bay, Warwick Long Bay, Elbow Ba~, and present in sediment samples from sand pock- John Smith's Bay (Fig. 1). Brachiopods ets sand channels and reef flanks in the were found in several sediment samples, in- Th'ree Hill Shoals patch reef, they are ab- cluding one taken at the base of a boiler at sent from the nearby lagoonal sediments, Warwick Long Bay, but they may have been confirming the observation of Jordan (1973) derived from the outer reefs in this case. that movement of reef-derived sediments la- Seaward of the boiler zone on the south goon wards from patch reefs through reef face shore are isolated coral developments simi- channels in Bermuda is in the order of only lar to those seen in the north reef and cen- tens of feet. tral and south reef biotopes and it is possible Boiler Biotope that both biotopes are represented in a dis- continuous and attenuated form along the Upchurch (op. cit.) restricts this biotope south shore. Most of the common reef- to the innermost reefs along the southeastern building corals, again occurring on aeolian- shore of the Islands and to a few outer reefs ites under medium energy conditions, were along the northern part of the reef tract east sampled at a locality south-east of Nonsuch of North Rock. The boilers themselves are Island (see inset 2 of Fig. I). Significant small, cupshaped, algal-vermetid gastropod numbers of brachiopods were obtained only reefs and their main characteristics have been from the undersides of large encrustations of described by lams (1970) and Ginsburg, Montastrea cavernosa, which is the dominant Schroeder, and Shinn (1971). The fauna coral at this locality. and cements of the internal cavities within south shore cup reefs have been investigated Basin Center Biotope by Ginsburg, Marszalek, and Schneidermann This biotope occurs in the deeper parts (1971) and Ginsburg and Schroeder (1973), of the lagoon, is largely devoid of foliaceous LOGAN: ECOLOGY OF ARGYROTHECA BERMUDANA 197 reef-building corals and therefore does not bor. Brachiopods were not recorded from support any brachiopod populations. In ad- either reefs or sediments within the Harbor dition sediment samples obtained from North by Frazier (1970), Robelen (1970) or my- and West Lagoon contained no brachiopod self, lending support to the premise by Mack- tests. enzie et al. ( 1965) that little sand-sized sediment is transported into Castle Harbor Nearshore-Sandy Substrate Biotope from open-water areas. Three localities, representing different as- It is conceivable that brachiopods inhab- pects of this biotope, were sampled: ited Castle Harbor reefs before dredging operations for Kindley Field Air Base (1941- Whalebone Bay.-Whalebone Bay is ex- 45) began and two cores taken near the posed to the open ocean and intensive wave pinnacle reefs revealed isolated and worn action; nevertheless it is representative of brachiopod tests at depths about 4 em be- the more open-water part of this biotope, neath the present surface of the sediment. and therefore the poorly-developed coralgal However, no systematic faunal surveys reefs off the mouth of this bay were sampled were ever carried out by any investigators for brachiopods. A preliminary traverse re- prior to the dredging-in addition, evidence vealed that most of the common reef-build- from Bigelow (1905) suggests that the wa- ing corals were present here, though much ters of the Harbor have always been silty. reduced in areal coverage. Small numbers A cause and effect relationship would there- of brachiopods were obtained mainly from fore be difficult to prove. Agaricia fragilis. As at Three Hill Shoals, M. annularis colonies, although often shingle- Harrington Sound.-Harrington Sound rep- like in growth form, were heavily encrusted resents a shallow restricted basin (inset with algae, bryozoans, and sponges on the 1, Fig. 1) with access to North Lagoon underside and brachiopods were conse- through only a narrow tidal inlet at Flatt's quently rare. Village. The hydrography, ecological zo- nation and sedimentology of Harrington Castle Harbor.-Castle Harbor is a shallow, Sound have been described by Neumann semi-enclosed basin (inset 2, Fig. 1) with ( 1965). Many of the corals abundant on east-west connections to the open ocean and the reefal shoals of the Platform, such as an essentially sandy substrate. The reefs of M. annularis and M. cavernosa, are absent Castle Harbor have been described by Fra- here; on the other hand Agaricia fragilis is zier (1970) and the composition and distri- the most abundant coral, especially in the bution of sediments by Robelen (1970). Rocky Zone (Neumann, 1965). Neverthe- Three areas were sampled in Castle Har- less, I did not find living brachiopods at any bor; an inshore shoal reef near the Cause- of the five localities sampled (Table 2), al- way, and two pinnacle reefs (named "Red though Lowenstam (personal communica- Stake" and "Meandrina" reefs by the au- tion, 1972) reported finding A. bermudana thor) in the central part of the Harbor. Spe- behind colonies of Agaricia fragilis in shal- cies of Montastrea are rare in Castle Harbor, low water near Flatt's Inlet. Traces of rather while colonies of Agaricia fragifis, though worn brachiopod tests occurred at five sta- relatively common, have only a thin algal- tion in the Sandy Zone, along Neumann's sediment slime on their undersides; conse- transect east of Devil's Hole, at depths be- quently no living brachiopods were obtained tween 3-4 m, and 5.5-7 m approximately, from any of these reefs. Sediment samples but it is not thought that significant living from the eastern and western ends of the populations of A. bermudana now exist in Harbor yielded a small number of brachio- Harrington Sound. Their absence is prob- pod tests, but they are believed to have been lematical, for suitable substrates are present, derived from reefal areas outside the Har- while Beers and Herman (1969) have shown 198 BULLETIN OF MARINE SCIENCE, VOL. 25, NO.2, 1975 that Harrington Sound has above-average singham Pond is even more unexpected when levels of nutrients and phytoplankton follow- one considers their absence from most of ing the breakdown of the seasonal thermo- Castle Harbor, with which the Pond is con- cline in late summer. Bretsky (1967) has nected. It is likely that a relict population attributed the scarcity of the arcid bivalve survives here, protected from the silty waters Barbatia domingensis, a common associate of Castle Harbor. of A. bermudana, partly to the absence of large corals for attachment and partly to the POSSIBLE LIMITING FACTORS AFFECTING sheltered and almost enclosed situation of BRACHIOPOD DISTRIBUTION ACROSS the Sound which restricts larval intake from THE BERMUDA PLATFORM open waters and limits wave and current ac- From the preliminary traverses (Table 1) tivity necessary for their distribution within it is cIear that A. bermudana occurs almost the Sound. exclusively on the undersides of the folia- ceous hermatypic corals Mantastrea caver- Nearshore-Muddy Substrate Biotope nosa, M. annularis and Agaricia fragilis. Ta- Typically this biotope is characterised ble 2 indicates that, notwithstanding the by mangrove roots and a mud substrate, with clumped distributions, mean brachiopod hard corals invariably absent. Such a bio- densities per square meter generally decrease tope would thus not normally be expected to shorewards from the edge of the Platform, support brachiopods unless some other kind with brachiopods virtually absent from most of firm substrate was present. of Castle Harbor and Harrington Sound, Walsingham Pond is a shallow, land- surviving marginally only in land-locked locked, marine pond east of Harrington Walsingham Pond in an unusual sub- Sound with vertical rock walls, mangroves boulder, talus slope environment. This gen- and a muddy substrate. Because of tidal ex- eral distribution of A. bermudana parallels change with Castle Harbor through caves quite closely that of its common associates, and fissures, salinity values in the Pond are the encrusting foraminifer Homotrema ru- almost normal. The pond has been studied brum (Rooney, 1970) and the small arcid by many investigators, notably Gould (1968), bivalve Barbatia domingensis (Bretsky, Walton (1969) and Waller (1973). On 1967) and suggests that similar limiting fac- the northeastern side of the pond a rock fall tors might be operating to control the distri- has produced a talus slope which extends bution of these and other associated filter- from above the surface of the water to about feeding species. Some possible limiting 4.5 m in depth. Brachiopods were first re- factors are here reviewed: corded from this locality, living on the under- Salinity sides of boulders on the talus slope, by Gau- Recent experimental data obtained by tier (1965, unpublished report, Bermuda Thayer (1974) on salinity tolerances of four Biological Station). I have found them in Recent cool-water species of articulate the same situation; distribution is patchy brachiopods show a remarkable tolerance of with only about 40 percent of boulders from salinity extremes by these species. It there- 0-3 m in depth yielding brachiopods. Up to fore does not seem reasonable to assume 34 individuals on a single boulder have been that the slight seasonal variation in salinity found for a maximum density of 365/m2• encountered on the Platform could affect Such bouldery substrates are unusual in Ber- distribution of the argyrothccid brachiopod muda but is should be remembered that the populations. sub-boulder habitat is commonly colonized by cool-water brachiopods living at shallow Temperature depths elsewhere (Logan and Noble, 1971). Under natural conditions, the amount of The occurrence of brachiopods in Wal- tolerance of A. bermudana to wide ranges in LOGAN: ECOLOGY OF ARGYROTHECA BERMUDANA 199 temperature is not known. Nevertheless the in a mixture of phytoplankton and dissolved lagoonal temperature range of 12.7 C in nutrients as food supply (McCammon, 1969; Bermuda is more than that normally en- Suchanek and Levinton, 1974), then the countered for other articulate brachiopods distribution of brachiopods (and other filter (McCammon, 1973) and may be a subsid- feeders) in Bermuda may be in part con- iary limiting factor responsible for the com- trolled by the availability of seasonal phyto- parative scarcity of brachiopods in lagoonal plankton supplies supported by nutrients and near-shore regions. The low winter brought in by convergence of major water water temperatures across the Platform have masses. been invoked to account for a faunal and Boden (1952) has shown that plankton floral diversity considerably lower than that are conserved and concentrated in the area of the Caribbean (Upchurch, 1970). This of a convergence of water masses in summer; may also, in part at least, account for the thus the water flowing lagoonwards over the lower brachiopod diversity in Bermuda and reefs is rich in both plankton and nutrients the complete absence of sclerosponges, com- from the convergence and supports a rich mon associates of brachiopods in the Carib- and diverse flora and fauna in these areas. bean and elsewhere (Jackson et al., 1971). Both nutrients and plankton are gradually depleted as the water passes into the lagoon. Depth and light Brachiopods have been recorded by the Biotic Associations author from low water mark to 28 m in Ber- A. bermudana is commonly associated muda, which should be regarded as the limit with sponges, tunicates, Foraminifera, bryo- of observations rather than the lower limit zoans, bivalves and calcareous algae. Im- of the depth range of brachiopods. Because portant and distinctive forms include the red coral growth is sparse at the lower limit of foraminifer Homotrema rubrum and the bi- this range, it has not been possible to ascer- valves Barbatia domingensis, Lima hians, tain whether brachiopod densitics increase Spondylus americanus, and Lithophaga nigra, with depth off the reef front in Bermuda. as well as chamid species. Areal coverage of However Jackson et aI., (1971) note that the undersides of corals by encrusting chei- Argyrotheca johnsoni from Jamaican fore- lostomatous bryozoans, sponges, and algae, reef slopes increases in density with depth in particular, is often high and brachiopods and suggest that density can be correlated are consequently rare at these localities, per- with low light intensity. Similarly, Grant haps arriving at a relatively late stage in the (personal communication, 1973) reports succession of such communities. Cuffey finding Frenulina sp. from 17-130 feet on ( 1973) has shown that encrusting cheilo- Eniwetok reefs but notes an increase in den- stomatous bryozoans are most abundant in sity with increasing depth. lagoonal reefs, such as Three Hill Shoals, If light intensity is a factor in brachiopod and it is at such localities that the undersides distribution, then it probably operates in the of colonies of Montastrea annularis, for in- larval periods of the species in question. stance, are most profusely overgrown, to the Mano (1960) has shown that the articulate detriment of brachiopod and Homotrema brachiopod Frenulina sanguinolenta (Gme- populations. lin) is initially photopositive but becomes Thus, although A. bermudana has no photonegative in its latest larval stage, seek- known predator, competition for living space ing out shaded areas of low light intensity may be an important limiting factor in its immediately prior to settlement. overall distribution across the Platform. Food Supply Substrate Assuming that articulate brachiopods, es- The presence or absence of a suitably firm pecially those inhabiting shallow waters, take substrate for attachment is regarded as the 200 BULLETIN OF MARINE SCIENCE, VOL. 25, NO.2. 1975 main limiting factor in the distribution and shallow. It is likely that energy conditions density of brachiopods on the Platform. The and substrate are also important in deter- commonest occurrence is one of attachment mining growth form. Where energy condi- by the pedicle to the undersides of species of tions are high, encrusting sheets of M. annu- the hermatypic coral genera Montastrea and laris and M. cavernosa generally occur (Fig. Agaricia. This somewhat restricted micro- 3); in medium and low energy environments, habitat is usually utilized by brachiopods M. annlllaris gradually becomes more shin- wherever these corals occur in coral-domi- gle-like, while M. cavernosa assumes a bul- nated macrohabitats (i.e. reefal shoals). The bous growth form. A. fragilis is flattened distribution of these host corals across the against the substrate in high and medium Platform has been outlined by Griggs and energy conditions (Fig. 3, top) and offers Atchinson (1967, unpublished report, Ber- poor attachment sites for brachiopods, yet muda Biological Station) and further infor- adopts a more emergent growth form in mation was obtained by the writer during lower energy environments. In areas where the preliminary traverses. Basically, the two coral growth forms a relatively thin veneer species of Montastrea are common on the on an aeolianite substrate (e.g. north and outer ledge flats and patch reefs within the south reefs), the growth form of the coral lagoon, but decrease to insignificance within is normally encrusting, with the overall form restricted interisland waters such as Castle of the colony closely conforming to the con- Harbor, Ferry Reach and Harrington Sound. tours of the aeolianite base (Fig. 3, top). Further, the dominant species of Montastrea The lee sides of overhangs produced by pe- on the south shore reefs is M. cavernosa, ripheral growth from encrusting or massive while M. annularis is the dominant form on growth forms of Montastrea ssp. near the the north reefs. Agaricia fragilis, on the sediment floor in medium energy environ- other hand, occurs most commonly in near- ments offer the most promising sites for shore protected waters and is of major impor- brachiopod attachment on the Bermuda Plat- tance in Harrington Sound. However, I have form. not found brachiopods on this coral in either The marked preference of argyrothecid Harrington Sound or Castle Harbor, so I brachiopods in Bermuda for three particular assume that factors additional to substrate host corals species is peculiar. Jackson et are at work in controlling brachiopod distri- aI., (1971) point out that A. johnsoni from bution. Jamaica shows no preference for anyone The factors contributing to the growth coral species as substrate, although noting form of corals are probably not yet entirely that species of the foliaceous corals Monta- understood; certainly ambient light intensity strea and Agaricia provide most of the is important (Goreau, 1959, 1963) but con- favorable undersurfaces for brachiopods. siderable overlapping of growth forms of M. Nevertheless, it is surprising that other coral annlllaris, for instance, has been recorded by species of mound or plate-like growth form Lewis (1960) and Barnes (1973), suggest- in Bermuda, such as Porites astreoides, Iso~ ing that other factors (as yet undetermined) phyllia ssp., and Diploria ssp. do not support are involved. brachiopods, especially as Homotrema ru- In Bermuda, shingle-like growth forms of brllm and Barbatia domingensis are common M. annularis are common on all parts of beneath these corals. This anomaly is, as patch reefs in North Lagoon (Garrett et aI., yet, unexplained. 1971, Fig. 5), while lobate or cluh-shaped growth forms of M. cavernosa occur on the POPULATION STRUCTURE marginal ridge and reef face of such patch Width-frequency histograms of living spec- reefs (Garrett et al., 1971, Fig. 9). In both imens of A. bermlldana collected from five cases light intensity is high and depths are localities across the Platform during July LOGAN: ECOLOGY OF ARGYROTHECA BERMUDANA 201 50 a AO AO b AO C N= lAl N= lA5 30 30 30 « u 20 20 20 w I af- ~ 10 10 10 o>- «~ 0 3 A 0 0 3 ou.. 30 d 20 20 N= 75 e N=95 10 10 o 3 o 3 WIDTH (mm) Figure 6. Width-frequency histograms of A. bermudunu. a. North Ledge Flats, south of Hog Breaker, collected July 26th., 1972. b. North side of Walsingham Pond, collected July 6th., 1972. c. upper reef- front Terrace, ENE of St. David's Head, collected July 24th., 1972. d. south reefs, south-east of Non- such Island, collected July 24th., 1972. e. Three Hill Shoals patch reef, collected July 12th., 1972. 1972 were constructed (Fig. 6). With one observations for 1 year must necessarily be exception, all showed normal or near nor- tentative, especially as growth rate data are mal distributions; only the sample from lacking. However, the dominance of large Three Hill Shoals (Fig. 6e) showed a rela- individuals at four out of five localities sam- tively high proportion of juveniles. Similar pled on the Bermuda Platform suggests the distributions for living brachiopods from following possibilities: other areas have been obtained by Rudwick (a) A high juvenile mortality caused by (1962), Paine (1969), and Neall (1970), selective destruction of small shells by pred- although Percival (1944) and Logan and ators. Paine (1969) has suggested this ex- Noble (1971) obtained positively-skewed planation for Terebratalia transversa popula- curves for their brachiopod populations. tions from the west coast of North America, Any conclusions based on only summer based on the high rate of incidence of damage 202 BULLETIN OF MARINE SCIENCE, VOL. 25, NO.2, 1975 caused by predators, probably crabs, on most ADDENDUM individuals in his samples, While A. bermu- Since the manuscript was prepared the author dana has no known predators, this explana- has examined coral reefs on the west coast of tion must remain a possibility until proven Barbados, W. I., where two small species of A 1'- otherwise. gyrotheca co-exist on the undersides of reef-build- ing corals of platy growth form, such as Agaricia (b) The population represents the growth tragi/is, A. agaricites, MOlltastrea annu/aris, and to maturity of a single spatfall, which, having Mycetophyllia /amarckiana. A transect run due survived the earliest settlement stages, com- east of the Bellairs Research Institute at St. James indicated that the brachiopods A rgyrotlleca her- petes successfully against later spatfalls in mudana (Dall) and A. ct. schrammi (Crosse and that area until its death. This explanation Fischer) range from the Reef Crest Zone of the has been advanced by Neall (1970) to ex- fringing reefs (Lewis, 1960) at 20 feet (6m) depth plain the relative absence of young individ- to the effective limit of prolific hermatypic coral growth at a depth of about 150 feet (46m), withm uals in populations of Neothyris lenticularis Lewis's "mixed coral community." White A. ber- from New Zealand. Neall notes that young mudana commonly outnumbers A. ct. schramm; by individuals of this species rarely settle on the about five to one, overall densities of both species at adults or in the adult colony, whereas the shallow depths are extremely low « 11m'). Num- population structure of the species Magasella bers increase gradually in deeper water, A. her- mudana, for instance, averaging 180/m" at ]50 feet sanguinea from the same area is much differ- (46m), with occasional maximum densities reach- ent, showing an intermixture of young and ing 360/m" at that depth. Patchy distributions and adult individuals to form a stable age pyra- low juvenile counts (Aug. 1974) are typical of mid. Brookfield (1973) has invoked a simi- both species and the overall life habits of reef- dwelling argyrothecids from Barbados appear to lar explanation to account for an adult-domi- be c1ose]y similar to those from Bermuda. nated distribution in the Jurassic brachiopod I thank the Bellairs Research Institute of McGill Torquirhynchia inconstans from England. University for use of facilities and Dr. G. A. Cooper of the U. S. National Museum for help in (c) Thayer (in press) has recently sug- identifying braChiopods. gested that unimodal size-frequency distribu- tions in brachiopods may indicate patchy lar- LITERATURE CITED val dispersion and local recruitment failure Atkins, D. 1960. The ciliary feeding mechanism rather than exclusion of juveniles by adults. of the Megathyridae (Brachiopoda), and the growth stages of the lophophore. J. mar. (d) The peaks may represent accumula- bioI. Ass. U.K., 39: 459-479. tions of brachiopods of many ages due to Barnes, D. J. 1973. Growth in colonial seler- near-cessation of growth at about 2%-3 mm actinians. Bull. mar. ScL, 23 (2): 280-298. width. Beers, J. R. and S. S. Herman. ]969. The ecol- ogy of inshore plankton populations in Ber- The correct interpretation of the available muda. 1. Seasonal variation in the hydrog- data cannot be resolved until growth rates raphy and nutrient chemistry. Bull. mar. Sci., are known and localities are sampled on an 19(2): 253-278. all-year round basis. Bigelow, H. B. ] 905. The shoal-water deposits of the Bermuda Banks. Proc. Am. Acad. Arts ACKNOWLEDGMENTS Sci., 40: 559-592. I am indebted to Dr. A. C. Neumann of the Boden, B. P. 1952. Natural conservation of in- University of North Carolina for splits of sedi- sular plankton. Nature, Lond., ] 69: 697-700. ment samples collected by him from a traverse Bretsky, S. S. ] 967. Environmental factors in- near Devil's Hole, Harrington Sound, and to Dr. fluencing the distribution of Barbatia dam ill- C. D. Gebelein of the Bermuda BiOlogical Station gellsis (Mollusca, Bivalvia) on the Bermuda for sediment samples from North and West La- Platform. Postilla, no. ] 08: ]4. goon. I also acknowledge the assistance of Stephen Brookfield, M. E. 1973. The life and death of Dryer and the cooperation of the staff of the Ber- TorquirhYllchia incollstans (Brachiopoda, Up- muda Biological Station. 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(Ein Beitrag zur Kenntnis der Brachiopoden) Z. wiss. Zool., 41: 116-141. ADDRESS: Department of Geology, University of Senn, E. 1934. Die Geschlechtsverhaltnisse der New Brunswick, Saint John, New Brunswick, Brachiopoden, im besonderen die Spermato- Canada.