Size-frequency and population structure of brachiopods

Palaeogeography, Palaeoclimatology, Palaeoecology , 17( 1975): 139--148 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

SIZE~FREQUENCY AND POPULATION STRUCTURE OF BRACHIOPODS

CHARLES W. THAYER Department of Geology, University of Pennsylvania, Philadelphia, Pa. (U.S.A.) (Received March 20, 1974; accepted for publication November 11, 1974)

ABSTRACT Thayer, C. W., 1975. Size-frequency and population structure of brachiopods. Palaeogeogr., Palaeoclimatol., Palaeoecol., 17 : 139--148. The living terebratulids, Terebralulina unguicula, Terebralalia transversa, Laqueus vancouverensis, and the rhynchonellid Hemithiris psittacea were studied in the San Juan Islands, Washington, U.S.A. Those results and a review of the literature lead to the conelusion that most brachiopod populations experience episodic recruitment at intervals which may be irregular. The occurrence of juveniles attached to adults, brooding, and hior multimodal size-frequency distributions demonstrate that, contrary to a previously suggested hypothesis, adult brachiopods do not generally exelude juveniles from the same area. The commonly observed rarity of small individuals is regarded as a product of local recruitment failure due to patchy distribution of larvae; it does not justify the assumption that braehiopods are unaffected by high post-larval juvenile mortality. However, the frequent rarity of small individuals confirms that this cannot be used as a criterion of transport in assemblages of fossil braehiopods. INTRODUCTION F o l l o w i n g t h e s u g g e s t i o n of B o u c o t ( 1 9 5 3 ) , s i z e - f r e q u e n c y d i s t r i b u t i o n s have b e e n e m p l o y e d t o r e c o g n i z e f o s s i l i z e d in situ life a s s e m b l a g e s (e.g., H a l l a m , 1 9 6 1 ; F a g e r s t r o m , 1 9 6 4 ) . In t h e o r y , high j u v e n i l e m o r t a l i t y ( o r m o r t a l i t y c o n s t a n t w i t h age) s h o u l d p r o d u c e a r i g h t - s k e w e d d i s t r i b u t i o n (in w h i c h m o s t i n d i v i d u a l s are small). T h e s e small i n d i v i d u a l s w o u l d be r e m o v e d f r o m a s s e m b l a g e d s u b j e c t e d to c u r r e n t t r a n s p o r t , r e s u l t i n g in a m o r e symm e t r i c a l s i z e - f r e q u e n c y d i s t r i b u t i o n . H o w e v e r , t h i s a p p r o a c h is valid o n l y if t h e o r g a n i s m in q u e s t i o n e x p e r i e n c e s s u b s t a n t i a l j u v e n i l e m o r t a l i t y a f t e r t h e acquisition of preservable hard parts. B e c a u s e b r a c h i o p o d s are a m o n g t h e m o s t a b u n d a n t m a c r o i n v e r t e b r a t e s in P a l e o z o i c s t r a t a , it is i m p o r t a n t to d e t e r m i n e w h e t h e r or n o t t h e y m e e t this c r i t e r i o n . B r o o k f i e l d ( 1 9 7 3 ) s u g g e s t e d t h a t t h e y do n o t , since m a n y R e c e n t b r a c h i o p o d p o p u l a t i o n s c o n t a i n f e w j u v e n i l e s a n d are d o m i n a t e d b y large i n d i v i d u a l s ( R u d w i e k , 1 9 7 0 ) . He a r g u e d t h a t " o n e s p a t f a l l o f b r a c h i o p o d s c o u l d c o l o n i z e an a r e a a n d p r e v e n t t h e d e v e l o p m e n t o f a n y l a t e r s p a t f a l l on t h e s a m e a r e a . . . " A s i m i l a r a r g u m e n t was a d v a n c e d b y Neall ( 1 9 7 0 ) . If j u v e n i l e

140 brachiopods are excluded from mature populations, the following should be true: (1) Young brachiopods should not attach to older ones. (2) Prolonged brooding should be absent. In brooding aquatic invertebrates, the initial part of the otherwise free-swimming larval stage is spent within the parent, and the larvae are released before metamorphosis and settlement. The dispersal phase of the life cycle is thus reduced or eliminated, and juveniles often remain within the parent population. (3) Populations should be of a single age-class, i.e., size-frequency distributions should be unimodal. All of these corollaries can be tested by examination of living brachiopods.

LIVING BRACHIOPODS The following discussion utilizes data which I gathered during a study of the ecology of the articulates Terebratulina unguicula, Terebratalia transversa, Laqueus vancouverensis, and Hemithiris psittacea in the San Juan Islands, Washington.

Attachment of juveniles to adults Small brachiopods were c o m m o n l y found attached to large ones. Terebratalia was found on other Terebratalia, Hemithiris, and Terebratulina. In some cases, Terebratalia formed clusters with as many as four individuals sequentially attached to one another. Hemithiris attached to others of the same genus and Terebratalia. Laqueus was found on Terebratalia. Terebratulina occurred on all four genera. (A detailed discussion of the substrata utilized by these brachiopods is planned for a future publication.) Paine (1969) observed small Terebratalia attached to larger individuals, but found no clusters. Mattox (1955) reported the attachment of Laqueus californianus to others of the same species and to Terebratalia. Macandrevia cranium (Bromley and Surlyk, 1973), Magellania venosa and Liothyrella uva (McCammon, 1973) all attach to others of the same species. Neall (1970) found that juveniles of Magasella sanguinia often attached to the adults, but Neothyris rarely did so. Since the adults of Neothyris outgrow the pedicle and roll along the b o t t o m in the currents (Bowen, 1968) it is not surprising that the juveniles shun such an unstable substratum. If larvae did settle on the adults, they would probably be removed during transport. Similar occurrences are known among fossil brachiopods. G. Baird (personal communication, 1973) reports the attachment of juveniles to conspecific adults in Ordovician atrypids and Devonian rhynchonellids, spiriferids and craniids.

141

Brooding Several living brachiopods are k n o ~ to brood their larvae within the mantle cavity. These include Pumilus antiquatus (Rickwood, 1968), Magellania venosa (McCammon and Buchsbaum, 1968), Liothyrella antarctica (Blochmann, 1906), Terebratella inconspicua (Percival, 1944), Argyrotheca (Atkins, 1960), Lacazella (Lacaze-Duthiers, 1861), Terebratulina septentrionalis (A. Logan, personal communication, 1974), Terebratulina unguicula and Hemithiris (Long, 1964).

Size-frequency distributions Size-frequency data rarely provide a reliable basis for detailed inferences concerning population dynamics (see Paine, 1969). One of the major obstacles is the need for an independent determination of individual ages. Size alone is a crude and sometimes misleading indicator of age. In the case of the San Juan Island brachiopods, the basal individual in a cluster was often considerably smaller than the younger brachiopods of the same species attached to it. Despite such difficulties, size-frequency data can be used to resolve the present questions. Because growth is initially rapid, it can be assumed that very small shells represent recent recruitment. If variations in growth rate are random, polymodal distributions record multiple episodes of recruitment. Three subtidal stations in the San Juan Islands were sampled by dredging during the summer of 1973. The localities and depths are: (1) South of Iceberg Point, Lopez Island; 48 ° 25' N 122°53'W; 80--90 m; June 30, 1973. (2) Near Reid Rock, San Juan Channel; 48°33'N 122° 59'W; 35--55 m; July 5 and July 21, 1973. (3) North of Skipjack Island; 48°44.5'N 123°02'W; 65--90 m: July 5, 1973. The m a x i m u m dimensions of openings in the dredge mesh were approximately 3.2 cm (station 1) and 1.3 cm (stations 2 and 3). Few brachiopods would have been collected had they not been attached to larger substrata. Thus, there was minimal sampling bias against small individuals. Juveniles were f o u n d by examining substrata (especially adult brachiopods) under a dissecting microscope. They would have escaped detection otherwise. The area sampled cannot be determined, because the b o t t o m time of the unfilled dredge is not known. It almost certainly exceeds 10 m 2 in all cases. Measurements were made with calipers and microscope micrometer. Only living individuals were utilized. The results are presented in Figs.l--4. Where similar distributions were obtained at different stations, the data are plotted on the same histogram. They are distinguished by different patterns. Terebratalia transversa (Fig.l) has a distinctly bimodal distribution, with the major peak at a length of 2.25 cm and a secondary peak at 0--0.1 cm. A possible tertiary peak appears between 0.5 and 0.8 cm. These data suggest a

142 8O 09 ..J '~ 6C a

ii O e~ 2G

Z

G 0

1 LENGTH

2 (cm)

3

Fig. 1. Size f r e q u e n c y d i s t r i b u t i o n o f live Terebratalia transversa, S a n J u a n I s l a n d s . C u r v e r e p r e s e n t s t h r e e - p o i n t m o v i n g a v e r a g e . S t a t i o n s a r e l i s t e d b y n u m b e r in t e x t . N : n u m b e r

of individuals in sample. life span of 4 years or more with an initial growth rate of approximately 0.6 cm/yr. Studying the same species near Seattle, Paine (1969) also found polymodal distributions. Several of his samples show peaks in the positions of my major and tertiary peaks. There are two major differences. Paine's distributions do not include the first-year class, but he assumed that they were present and overlooked during collection. My data support this supposition. Paine, also obtained several more modes at larger sizes. My largest individuals were slightly more than 3.0 cm long, whereas his occasionally exceeded 5 cm (average of length and width). Paine inferred a life span of up to 13 years with a growth of about 1.5 cm during the first year. C. Birkeland (personal communication, 1974) has photographically recorded changes in subtidal benthic communities in Shady Cove, San Juan Island. Some T. transversa individuals which were initially present persisted throughout the 6-year study period. T. transversa which settled during the study attained " m e d i u m " size in 4 years. Mediumsized individuals grew to "large" size in four years, this indicates a total life span of about 8 years. R. Best (personal communication, 1974) measured four intertidal populations of T. transversa from Long Harbor, Salt Spring Island, British Columbia (N = 123--411). All his distributions were polymodal, with as many as 3--4 peaks. In August, the first of these were at 0.5 and 1.2 cm. The findings of Best and Birkeland are consistent with an initial growth rate of about 0.6 cm/yr. H e m i t h i r i s (Fig.2) has a markedly bimodal distribution. The juvenile peak (0.1--0.2 cm) is nearly as large as the major peak (1.8--1.9 cm). The relatively good representation of the most recent age-class is probably due to retention of larvae by brooding. The distribution suggests a 2-year life span. However, the larger individuals c o m m o n l y have thick valves and many closely spaced growth lines near the commissure. It is likely that there is little or no increase in length after age two, while deposition of secondary shell material continues.

143

,o

[:]

C

0

,78

/i

1 LENGTH (era)

2

Fig.2. S i z e - f r e q u e n c y d i s t r i b u t i o n of live Hemithiris psittacea, San J u a n Islands. Legend as in Fig.1.

Laqueus vancouverensis (Fig.3) shows an apparent m a x i m u m at 2.6 cm and a hint of another at about 1.25 cm. While the n u m b e r of individuals measured was to o small to pr oduc e definite peaks, the distribution is apparently p o l y m o d a l with rare juveniles. Terebratulina unguicula displays t w o different distributions. In the first (Fig.4A), small individuals are rare or absent, a situation considered typical of many brachiopods (e.g. Rudwick, 1965). However, there is a hint of trimodality, suggesting successive recruitment. The second t ype of distribution (Fig.4B) has a p r o n o u n c e d m a x i m u m among the smallest individuals, possibly due to " n o r m a l " high post-larval juvenile mortality.

O2o! ~.09

Z

Z

0

58 N__

1

2

3

LENGTH (cm) F i g . 3 . S i z e - f r e q u e n c y d i s t r i b u t i o n o f live Laqueus vancouverensis, S a n J u a n I s l a n d s . L e g e n d as in F i g . 1 .

A limited n u m b e r of the San Juan brachiopods were examined to determine sexual matu r ity in August, 1974. These results are in accord with the generalization that branchiopods mature when one-third to one-half of the " a d u l t " size (Williams and Rowell, 1965). The following valve lengths bracket the appearance of mature gonads (values given are for largest i m m at ure and smallest mature specimens, respectively):

Terebratulina unguicula, Terebratalia transversa, Laqueus californianus, Hemithiris psittacea,

0.3--0.9 0.6--1.1 1.3--1.5 0.7--1.5

cm cm cm cm

(N (N (N (N

= 37); = 95); = 43); = 16).

Although it is difficult to assign year-classes to the peaks in Figs.l- -4, it appears that maturity is attained at an age of a b o u t one year.

144

501

_

,3

ii

1

0 ~

40

2

..........

S

/ .......

N

~\ 2 ~_1\

[~ m

99 121

G 0

1

2

LENGTH (cm)

Fig.4. Size-frequency distribution of live Terebratulina unguicula, San Juan Islands. Legend as in Fig.1. A. Right-skewed distribution. B. Left-skewed distribution.

DISCUSSION The fact that Terebratulina unguicula has such different distributions during the same season indicates t hat r e c r u i t m e n t occurs irregularly at any given location. Because brooding shortens the dispersal time of the larvae, distribution by currents could easily pr oduc e a " p a t c h y " settlement pattern. Absence of a juvenile m a x i m u m in a particular sample would not mean absence of postlarval juvenile mortality, but rather a local failure of recruitment. Examination o f o t h e r size-frequency data supports this conclusion. Distributions of Terebratulina septentrionalis are frequently right-skewed, from which high juvenile m or t al i t y has been inferred (Logan and Noble, 1971). However, o th er stations sampled at about the same time have a more symmetrical distribution with relatively few small individuals. Thus, Terebratulina septentrionalis appears to show the two distribution types f o u n d in Terebra-

tulina unguicula. This is also a possible explanation for the different distributions obtained for Terebratella (Waltonia) inconspicua by Rudwick (1962) and Percival (1944). Rudwick f o u n d more-or-less symmetric bimodal distributions whereas Percival obtained a right-skewed one.

Polymodality Table I summarizes the available information on brachiopod size-frequency distributions. It confirms Rudwick's (1965) conclusion that " t h e distribution is c o m m o n l y bimodal or multi-modal". Of 18 species studied, only Argyrotheca johnsoni, Terebratella dorsata, Neothyris lenticularis, and Neorhynchia strebeli

145 lack bimodal or polymodal distributions. In the case of Argyrotheca, continuous recruitment was inferred (Jackson et al., 1971). If the adults of Neothyris are subject to current transport, as previously noted, this fact alone could explain the absence of small individuals in the same population. The sampling procedure used for Neorhynchia strebeli omitted small specimens, so its unimodal distribution may be an artifact (McCammon and Buchsbaum, 1968). Brookfield (1973) was surprised at the marked bimodality which Jackson et al. (1971) observed in a stable tropical environment. However, a growing body of evidence indicates that periodic, non-continuous reproduction may occur in stable environments (e.g., George and Menzies, 1967; Schoener, 1968; Turner, 1973). It may be timed to utilize a seasonal food source. In addition, synchronized spawning favors fertilization and minimizes wastage of gametes. However, Rokop (1974) indicates t h a t most deep-sea species have continuous reproduction.

Effect op sample size Rudwick (1962) suggested that Percival (1944) obtained a right-skewed distribution because the area sampled was too small to be representative of the patchily distributed population. Percival's 0.023 m 2 included few of the large individuals which dominated Rudwick's 0.25 m 2 samples. However, this is evidently not the explanation for other right-skewed distributions. Logan and Noble (1971), for example, obtained right-skewed distributions from samples which were all greater than 0.5 m 2 . Any study utilizing dredged specimens is likely to have sampled a more than adequate area, yet my own work and that of Rowell (1960) shows that right-skewed curves may result. The small-sample argument can also be applied to the adult-dominated distributions considered " t y p i c a l " of brachiopods; a small sample could have omitted the more abundant first age-class. Dredged samples are subject to another uncertainty. Because large areas are covered, a polymodal curve might result from mixing of separate unimodal populations. This effects was probably minimal in my own samples: separate size-classes could be clearly distinguished by visual inspection of a single piece of substratum. Coexistence of separate age-classes is also proven by the frequent attachment of juveniles to adults.

Presence of juveniles Although the d o m i n a n t size-class in brachiopod populations often falls in the adult range, most distributions do show a juvenile peak. It is significant that many workers obtaining juvenile peaks (e.g., Jackson et al., 1971; Logan and Noble, 1971) also reported that substrata were removed for laboratory examination. It is probable that many juveniles are otherwise overlooked, as Paine (1969) suggested in the case of Terebratalia. Even with detailed study, small individuals are more likely to be overlooked than large ones. Therefore the relative rarity of juveniles may in some cases be an artifact of collection and study techniques.

septentrionalis

lenticularis

congregata

Thecidellina

* Rare.

Glottidia pyramidata Discinisca strigata Crania anomala

INARTICULATES

Hemithiris psittacea Neorhynchia strebeli

Rhynchonellida

barretti

Thecidellina

Theceidina

Neothyris

Terebratella (Waltonia) inconspicua Magellania venosa

Laqueus vancouverensis Terebratella dorsata

Argyrotheco bermudona Pumilus antiquatus Terebratalia transuersa

Terebratulina unguicula Argyrotheca johnsoni

Terebratulina

Terebratulida

ARTICULATES

x x

x

unimodal

X

X

X

X

Xc.d

x

bimodal

X

xa

x

x

intermediate

size-frequency distributions

Modality*

Summary of Recent brachiopod

TABLE I

X

X

x”

x*

polymodal

X

X

X

X

X

Xd

X

x

x

x

rightskewed

Symmetry

X

X

X

Xb

x

x

symmetrical

XC

intermediate

X

X

X

X

X

XaXb

x

x

leftskewed

X

x

X

x

Xc.d

x

Xa

x

x*

X

x

?zninile present

Paine (1963) Paine (1969) Rowe11 (1960)

this paper McCammon and Buchsbaum (1968)

Jackson et al. (1971) Jackson et al. (1971)

Logan and Noble (1971) this paper Jackson et al. (1971) Logan (1975) Rickwood (1962) a = this paper; b = Paine (1969) this paper McCammon and Buchsbaum (1968) c = Rudwick (1962) d = Percival (1944) McCammon and Buchsbuam (1968) Neal1 (1970)

References

147 CONCLUSION A l t h o u g h s o m e living b r a c h i o p o d s have r i g h t - s k e w e d s i z e - f r e q u e n c y histograms, o t h e r s do n o t . T h u s t h e a b s e n c e of a r i g h t - s k e w e d d i s t r i b u t i o n in fossil b r a c h i o p o d s is n o t a reliable i n d i c a t o r o f t r a n s p o r t . H o w e v e r , the m e c h a n i s m p r o p o s e d b y Neall ( 1 9 7 0 ) a n d B r o o k f i e l d ( 1 9 7 3 ) t o explain t h e a b s e n c e o f r i g h t - s k e w e d d i s t r i b u t i o n s c a n n o t be r e g a r d e d as a general one. Juvenile peaks, bi- and p o l y m o d a l d i s t r i b u t i o n s , a t t a c h m e n t to adults, a n d b r o o d i n g b e h a v i o r are all e v i d e n c e f o r the a d d i t i o n o f juveniles to e s t a b l i s h e d p o p u l a t i o n s . In m o s t cases, it a p p e a r s t h a t u n i m o d a l sizef r e q u e n c y curves result f r o m local a b s e n c e o f r e c r u i t m e n t ( p e r h a p s due to p a t c h y dispersal of larvae) r a t h e r t h a n e x c l u s i o n o f juveniles b y adults. P a t c h y r e c r u i t m e n t also explains the f r e q u e n t l y o b s e r v e d rarity o f small individuals w i t h o u t r e c o u r s e to the h y p o t h e s i s t h a t b r a c h i o p o d s are r e m a r k a b l y i m m u n e t o post-larval juvenile m o r t a l i t y . ACKNOWLEDGEMENTS My w o r k o n living b r a c h i o p o d s was d o n e at F r i d a y H a r b o r L a b o r a t o r i e s , A. O. D. Willows, D i r e c t o r . I a m i n d e b t e d to m a n y o f t h e staff, investigators, a n d s t u d e n t s f o r their c o o p e r a t i o n . Special t h a n k s are d u e R. Best and C. B i r k e l a n d f o r p r o v i d i n g d a t a on Terebratalia. T h e c o o p e r a t i o n of R. R. S t r a t h m a n n , Associate D i r e c t o r , a n d C. C. V a n d e r s l u y s , C a p t a i n of the R. V. " H y d a h " , is greatly a p p r e c i a t e d . G o r d o n Baird g e n e r o u s l y m a d e available his u n p u b l i s h e d d a t a on the a t t a c h m e n t o f fossil b r a c h i o p o d s to o n e a n o t h e r . I t h a n k J. B. C. J a c k s o n , J. S. L e v i n t o n , a n d R. R. S t r a t h m a n n f o r t h e i r c o m m e n t s on the m a n u s c r i p t . REFERENCES Atkins, D., 1960. The ciliary feeding mechanism of the Megathyridae (Brachiopoda), and the growth stages of the lophophore. J. Mar° Biol. Assoc. U.K., 39: 459--479. Blochmann, F., 1906. Neue Brachiopoden der Valdivia - and Gaussexpeditionen. Zool. Ariz., 30: 690--702,824. Boucot, A. J., 1953. Life and death assemblages among fossils. Am. J. Sci., 251: 25--40. Bowen, Z. P., 1968. A guide to New Zealand Recent brachiopods. Tuatara, 16: 127--150. Bromley, R. G. and Surlyk, F., 1973. Borings produced by brachiopod pedicles. Lethaia, 6: 349--367. Brookfield, M. E., 1973. The life and death of Torquirhyl~chia inconstans (Brachiopoda, Upper Jurassic) in England. Palaeogeogr., Palaeoclimatol., Palaeoecol., 13: 241- 259. Fagerstrom, J. A., 1964. Fossil communities in paleoecology: their recognition and significance. Geol. Soc. Am. Bull., 75: 1197--1216. George, R. Y. and Menzies, R. J., 1967. Indication of cyclic reproductive activity in abyssal organisms. Nature, 215: 878. Hallam, A., 1961. Brachiopod life assemblages from the Marlstone Rock-bed of Leicestershire. Palaeontology, 4: 653--659. Jackson, J. B. C., Goreau, T. F. and Hartman, W. D., 1971. Recent brachiopod--coralline sponge communities and their paleoecological significance. Science, 173: 623-625.

148 Lacaze-Duthiers, H. de, 1861. Histoire naturelle des braehiopodes vivants de la M~diterran6e. Ann. Sci. Nat., Ser. 4, 15: 259--330. Logan, A., 1975. Ecological observations on the Recent articulate brachiopod Argyrotheca bermudana Dall from the Bermuda platform. (In preparation.) Logan, A. and Noble, J. P. A., 1971. A Recent shallow-water brachiopod community from the Bay of Fundy. Marit. Sed., 7: 85--91. Long, J. A., 1964. The Embryology of Three Species Representing Three Superfamilies of Articulate Brachiopoda. Thesis, University of Washington, Seattle, Wash. McCammon, H. M., 1973. The ecology of Magellania venosa, an articulate brachiopod. J. Paleontol.. 47: 266--278. McCammon, H. M. and Buchsbaum, R., 1968. Size and shape variation of three Recent brachiopods from the strait of Magellan. In: W. L. Schmitt and G. A. Llano (Editors), Biology of the Antarctic Seas, 3. Antarctic Res. Ser. 11, pp.215--225. Mattox, N. T., 1955. Observations on the brachiopod communities near Santa Catalina Island. In: Essays in the Natural Sciences in Honor of Captain Allan Hancock. University of Southern California Press, Los Angeles, Calif., pp.73--86. Neall, V. E., 1970. Notes on the ecology and paleoecolog:~ of Neothyris, an endemic New Zealand brachiopod. N.Z.J. Mar. Freshwater Res., 4: 117--125. Paine, R. T., 1963. Ecology of the brachiopod Glottidia pyramidala. Ecol. Monogr., 33: 255--280. Paine, R. T., 1969. Growth and size distribution of the brachiopod Terebratalia transversa Sowerby. Pac. Sci., 23: 337--343. Percival, E., 1944. A contribution to the life-history of the brachiopod Terebratella inconspicua Sowerby. Trans. R. Soc. N.Z., 74: 1--23. Rickwood, A. E., 1968. A contribution to the life history and biology of the brachiopod Pumilus antiquatus Atkins. Trans. R. Soc. N.Z., 10: 163--182. Rokop, F. J., 1974. Reproductive patterns in deep-sea benthos. Science, 186:743--745. Rowell, A. J., 1960, Some early stages in the development of the brachiopod Crania anomala (Mi]ller). Ann. Mag. Nat. Hist., Set. 13, 3: 35--52. Rudwick, M. J. S., 1962. Notes on the ecology of brachiopods in New Zealand. Trans. R. Soc. N.Z. (Zool.), 1: 327--335. Rudwick, M. J. S., 1965. Ecology and paleoecology. In: R. C. Moore (Editor), Treatise on Invertebrate Paleontology, Part H, Brachiopoda. Geol. Soc. Am. and Univ. of Kansas Press, Lawrence, Kansas, pp.H199--H214. Rudwick, M. J. S., 1970. Living and Fossil Brachiopods. Hutchinson, London, 199 pp. Schoener, A., 1968. Evidence for reproductive periodicity in the deep sea. Ecology, 49: 81--87. Turner, R. D., 1973. Wood-boring bivalves, opportunistic species in the deep sea. Science, 180: 1377--1379. Williams, A. and Rowell, A. J., 1965. Brachiopod anatomy. In: R. C. Moore (Editor), Treatise on Invertebrate Paleontology, Part H, Brachiopoda. Geol. Soc. Am. and Univ. Kansas Press, Lawrence, Kansas, pp.H6--H57.