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Journal of Marine Research, Sears Foundation for Marine Research, Yale University PO Box 208118, New Haven, CT 06520-8118 USA (203) 432-3154 fax (203) 432-5872 [email protected] www.journalofmarineresearch.org Adult-Larval interactions in dense infauna! assemblages: Patterns of abundance

by Sarah Ann Woodin 1

ABSTRACT

Dense assemblages of infauna! organisms are of three types: burrowing deposit feeders, sus- pension feeders, and tube builders. Such assemblages are discrete, often age-class dominated, and have sharp boundaries with neighboring assemblages. The hypothesis presented is that the sharp boundaries observed among assemblage types and the maintenance of these discrete dense assemblages in infauna! systems is due to interactions among the established infaunal individuals and settling larvae. Deposit feeders change the nature of the sediment by their reworking activities. They also feed in the surface sediment layers where larvae are settling. Only the larvae of infauna! burrowing polychaetes should escape these mortality factors. Sus- pension feeders filter particles (including larvae) out of the water column. Tube builders restrict the amount of infauna! space because of their tube-building activities. In addition they feed and/or defecate on the surface. Small epifaunal bivalves that brood their young should be the predominant co-occurring form. The importance of the above interactions is documented. Pre- dicted fauna! associations are the following : none with suspension feeders, burrowing poly- chaetes with deposit feeders, and small epifaunal bivalves that brood their young with tube builders. These are partially confirmed by data from the literature.

1. Introduction Experimental analyses of marine rocky intertidal communities have demonstrated that both biological and physical forces are involved in structuring these communi- ties (see Connell 1961, 1970a and 1970b; Paine 1966, 1971, 1974; Dayton 1971 and 1973; Harger 1968; and Menge 1972). Abrupt changes in species composition across physical gradients have been the foci of many of these studies. Modifications of the physical and biological structuring forces predictably determine the changes - in species composition observed (Paine 1971). Marine infaunal assemblages of soft-sediment systems also change in relation to physical gradients, such as depth (macro, Day et al. 1971, and micro, Johnson 1970). However, marine infaunal systems have rarely been analyzed experimentally

1. Department of Zoology, University of Maryland, College Park, Maryland, 20742. Present address: Department of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, Maryland 21218. 25 26 Journal of Marine Research [34, 1

and most of the available distribution data is for patterns within an assemblage type, not across assemblage boundaries (e.g., Holme 1950, Connell 1955, Treval- lion et al. 1970b, and Woodin 1974). This paper is an attempt to show how particular shifts in infauna! abundance patterns are maintained by structuring forces associated with adult-larval interac- tions. The hypothesis presented here is that the sharp boundaries observed among assemblage types and the maintenance of these discrete dense assemblages in in- fauna! systems is due to interactions among the established infauna! individuals and settling larvae. I will limit the discussion initially to areas with dense infauna! populations (see Woodin and Yorke 197 Sa and 197 Sb for a discussion of areas less densely popu- lated). Dense infaunal assemblages are not restricted to the intertidal or any one locality (Table 1). They may, however, be more common in some geographical areas than in others. The west coast of North America appears to be such an area while the east coast of North America, at least south of Cape Cod, is not. This may be due to the prevalence of such infauna! predators and disturbance agents as Callinectes sapidus and Limulus polyphemus in the latter areas and their relative absence in the former (e.g. Woodin 1974 and unpublished data). In general, areas that can be immediately excluded are physically disturbed environments (e.g., high energy gravel and sand flats and sandslide areas) and environments exposed to in- tensive predation pressures [e.g., tropical Thalassia beds with high densities of grazing and deposit-feeding sea urchins and fish (Keller pers. comm.)].

2. Infaunal assemblage types Petersen (1913 and 1918), Shelford et al. (1935), Ford (1923), and other early workers described soft-bottom assemblages on the basis of organisms that were especially conspicuous in size and/ or abundance (see Thorson 19 57 but also Day 1963). Recurrent assemblages comprised of functionally and taxonomically similar organisms were found in geographically widely separated areas within both tem- perate and tropical zones (Petersen 1918, Ford 1923, Thorson 1957, Trevallion et al. 1970a). Sanders (1958 and 1960) attributed this pattern in part to the relation- ship between sediment grain sizes and particular trophic groups. In Buzzards Bay and Long Island Sound, for example, deposit feeder densities are greater in areas with fine sediments than in areas with coarser sediments. Suspension feeder densi- ties are not so closely linked with grain size per se. Subsequent workers have con- firmed Sanders' initial observations (e.g., Bloom et al. 1972, Nichols 1970, Rhoads and Young 1970 and 1971, and Young and Rhoads 1971). In 1970, Rhoads and Young extended Sanders' animal-sediment hypothesis. In a set of elegant experiments they demonstrated that deposit-feeding bivalves change the character of the sediment by their reworking processes so that it is more easily t- Table 1. Dense infauna! assemblages. N Locality Depth Sediment Assemblage characteristics Source Washington State, USA 25m sand tube-building sedentary polychaete Lie and Kisker 1970 Co <\) (Owenia) (2300/ m') Washington State, USA 25m sand deposit-feeding bivalve (Tellina) Lie and Kisker 1970 l-c:, E: (13,158/ m') <\) Co Mitchell Bay, San Juan Island, mud-gravel tube-building sedentary polychaetes and Woodin 1974 Co inte1tidal Washington State, USA tanaids (40,000/ m') ] Elkhorn Slough, California, USA intertidal sand tube-building sedentary phoronid MacGinitie and MacGinitie ;::: (Phoronopsis) (112,400/ m') 1968 -.s Santa Barbara, California, USA 27-46m silt tube-building sedentary polychaete Barnard and Hartman 1959 <\) (Chaetopterus) (2400g/m') <\) Baltic Sea tube-building amphipod (Pontoporeia) "<::s > 20m mud Segerstrale 1962 and 1965 .s (8000/ m') :.::Co Great Britain 40m very silty sand burrowing deposit-feeding spatangoid Buchanan 1966 C ·.;::: (Echinocardium) (.) Start Bay, Great Britain 10m sand-gravel suspension-feeding bivalve (Spisula) Ford 1925 2 (1 000• /m , greater than 2.5 cm in .5 length) r Barnstable Harbor, Massachusetts, intertidal fine sand tube-building sedentary amphipod Mills 1967 USA (Ampelisca) (73 ,000/ m') ...... I Barnstable Harbor, Massachusetts, intertidal mud tube-building polychaetes (primatily Sanders et al. 1962 2 USA Streblospio) (52,400/ m ) (23g/ m') Buzzards Bay, Massachusetts, USA 13-16m mud burrowing deposit-feeding polychaetes Sanders 1958 and bivalves (Nephthys and Nucula) C (5020/ m') C Long Island Sound, USA 20m muddy burrowing deposit-feeding bivalve Sanders 1958 (Nucula) (10,000/ m') Co False Bay, South Africa 58m shell-sand tube-building amphipods and tanaids Field 1971 t- °'..... (4470/ m') 28 Journal of Marine Research [34, 1 resuspended. This sediment destabilization and the associated increase in turbidity were hypothesized to increase the rate at which the feeding mechanisms of suspen- sion feeders would clog, causing cessation or reduction in growth rates. Using the suspension-feeding bivalve, Mercenaria mercenaria they demonstrated such growth rate changes in the field (see Pratt and Campbell 1956 for a previous demonstra- tion in the same species). They also hypothesized that settling larvae of suspension feeders would be buried in such easily resuspended sediments. Rhoads (1963) had previously demonstrated that Yoldia, a deposit-feeding bivalve, quickly reworks and alters the entire top layers of sediment. Thus, the trophic amensalism hypoth- esis, Rhoads and Young's extension of the animal-sediment hypothesis, explains why suspension feeders are not found in fine sediments except in the absence of dense populations of deposit feeders. The inverse hypothesis, for which Rhoads and Young (1970) present no firm evidence, is that due to a lack of organic matter for food deposit feeders do not occur in coarser sediments where suspension feeders occur (Rhoads and Young 1970). A complication is that the trophic amensalism theory applies to the non-overlapping distributions of infauna! suspension-feeding bivalves relative to those of deposit-feeding bivalves, burrowing polychaetes, bur- rowing holothurians (Rhoads and Young 1970 and 1971), burrowing thalassinid shrimp (Aller and Dodge 1974) and other burrowing decapods (McIntyre 1968). It does not apply to the third dominant infauna! group (see Table 1), the tube- building forms, such as polychaetes, amphipods, tanaids, and phoronids. In con- rast to dense assemblages of burrowing deposit feeders that destabilize the sedi- ment, dense assemblages of tube builders stabilize the sediment regardless of their trophic type (see Barnard and Hartman 1959, Galtsoff 1964, Johnson 1959, Mills 1967 and 1969, Rhoads 1974, Richards 1969, Sanders et al. 1962, Sherk 1971, Young and Rhoads 1971). As is illustrated in Table 1 then, three distinct types of dense infauna! assem- blages have been described in soft substrata of marine systems. (1) Infaunal deposit feeder assemblages of burrowing bivalves, decapods, holo- thurians, polychaetes, and irregular echinoids; (2) lnfaunal suspension-feeding bivalve assemblages; (3) Infaunal tube builder assemblages of varying trophic types comprised of tube-building and relatively sedentary amphipods, phoronids, polychaetes, and tanaids. All have sharp boundaries that are not associated with obvious physical ecotones such as abrupt changes in temperature, depth, or sediment grain size (see for ex- ample Barnard and Hartman 1959, Burbanck et al. 1956). All may form assem- blages that cover large areas or that are in discrete patches within another assem- blage type. For descriptions of boundaries between these assemblages see Barnard and Hartman 1959, Beanland 1940, Bradley and Cooke 1959, Burbanck et al. 1956, Rhoads and Young 1970, Sanders et al. 1962, Segerstrale 1962 and 1965, and 1976] Woodin: Adult-Larval interactions in dense infauna[ assemblages 29

Vassallo 1969. As in the rocky intertidal (where experimental evidence exists) these patterns are probably indicative of the importance of biological interactions. a. Adult-Adult and Adult-Juvenile interactions. The following interactions between established infauna! individuals have been suggested: competition for space, com- petition for food, and interference via sediment destabilization. The data supporting or refuting the importance of these are summarized in Table 2. The detectable re- sults of these competitive interactions are uniform distributions, low growth rates, and high mortality rates. Except in the case of trophic amensalism, the available documentation is for interactions between closely related species and not between members of different types of assemblages. Such competitive interactions therefore should not result in sharp boundaries, except perhaps between closely related spe- cies. The trophic amensalism hypothesis predicts decreased growth rates and in- creased mortality rates of adults. These could lead to discrete assemblages if the changes occurred rapidly but as Rhoads and Young (1970) demonstrated, they do not. An alternative hypothesis is that the non-overlapping distributions of the three assemblage types result from interactions between established individuals and settling or newly settled larvae rather than between two established individuals. If the mortality of newly settled or settling individuals were high and/ or juveniles settled preferentially near adults of the same species, then discrete patches might result from differential survival and/ or settlement of particular, types of organisms. However, preferential settlement of larvae of infauna! organisms seems to be due to the presence of particular microorganisms, not adults of the larval species. In contrast, the settlement preferences of larvae in epifaunal systems often seem to relate to the presence of adults of the same species (see Gray 1974, Meadows and Campbell 1972, Newell 1970, and Wilson 1968 for reviews). The data of Connell (1955), Korringa (1941), and Muus (1973) indicating that for some bivalves the areas of settlement correspond to the presence of adults, may in fact reflect differen- tial survival, not differential settlement. Mills (1967) and Daro and Polk (1973) have shown that conspecific juveniles are unable to settle into ampeliscid amphipod and spionid polychaete mats because of the density of the tubes. Hancock (1970) has demonstrated a negative relationship between the density of O-group cockles and successful settlement of spat. Available data therefore do not consistently sup- port differential survival and/ or preferential settlement of larvae near conspecific adults. If larvae do settle into dense infauna! assemblages of a species or a func- tional type (i.e., deposit-feeding form, suspension-feeding bivalve, tube-building form) other than their own, then mortality must occur rapidly or the discreteness of the existing assemblage would not persist. The exclusion mechanisms must there- fore be ones that prevent even initial successful settlement and growth. Deposit feeders not only change the nature of the sediment by their reworking activities and thus affect suspension-feeding bivalve adults and juveniles (Rhoads and Young 1970 and 1971 and Sherk 1971) but frequently feed on the surface or l,J 0

Table 2. Competitive interactions. Interaction Source Suspension-feeding bivalves Cardium edule reduced growth rate with increased density Hancock 1970 M ercenaria mercenaria reduced growth rates in the presence of dense deposit feeders Rhoads and Young 1970 Mercenaria strialllla no change in mortality and growth rates with increased density Ansell 1961 M. striatula seasonal cessation of growth prior to period limited by low Ansell 1961 temperatures Mya arenaria increased density did not result in uniform spatial distribution Connell 1955 M. arenaria reduced growth rate with increased density Bradley and Cooke 1959, Connell 1955, Sanders et al. 1962 Petricola pholadiformis increased density did not result in uniform spatial distribution Connell 1955 Spisula solida reduced growth rate with increased density Ford 1925 Venerupis semidecussata reduced growth rate with increased density Ohaba 1956 in Ansell 1961 V. semidecussata increased mortality rate with increased density Ohaba 1956 in Ansell 1961

.... \0 --..J '-'°'

0 0 Deposit-feeding bivalves .:l.. Macoma nasuta uniform spatial dii;tribution at high density Phelps 1967 ~- Nucula proxima increased density did not result in uniform spatial distribution Levinton 1972 ;:i.,.. Scrobicularia plana increased density did not result in uniform spatial distribution Hughes 1970 .:l...... S. plana increased mortality rate with increased density Hughes 1970 I Tellina tenuis uniform spatial distribution at high density Holme 1950, Connell 1955 T. tenuis breakdown of uniform spatial distribution at higher densities Holme 1950 T. tenuis reduced growth rate with increased density Stephen 1928, Edwards et al...... s· 1970 ('I) T. tenuis seasonal cessation of growth prior to period limited by low Ansell and Trevallion 1967 i::,.., temperatures 6· T. tenuis increased growth rate with enrichment Trevallion and Ansell 1967 Tube-building forms ~·;:s Ampelisca spp. aggressive interactions with increased density Mills 1967 Corophium volutator aggressive interactions with increased density Meadows and Reid 1966 ;:s "'('I) Glycera alba aggressive interactions with increased density Ockelmann and Yahl 1970 .... Leptochelia savigni aggressive interactions with increased density Richards 1969 .a Nereis vexillosa uniform spatial distribution at high density due to aggressive Roe 1971 i::: interactions §. Phoronopsis viridis uniform spatial distribution at high density Johnson 1959 Platynereis bicanaliculata uniform spatial distribution at high density due to aggressive Woodin 1974 ('I) interactions c::r- f

-w 32 Journal of Marine Research [34, 1 within a few millimeters of the surface (Levinton 1972 and Sanders 1958 and 1960). Settling larvae are often small enough to be ingested (Loosanoff et al. 1966 and Muus 1973). Furthermore, the larvae of tube-builders, suspension-feeding bi- valves, and some deposit-feeders settle on or within a few millimeters of the sedi- ment surface. These larvae are therefore apt to be ingested or at least disturbed by spihon activities. Only those larvae that burrow down beneath the sediment surface will be less exposed to ingestion or continual disturbance. Such larvae occur among infauna! burrowing polychaetes that often cooccur with deposit-feeding bivalves (Bloom et al. 1972, Sanders 1958 and 1960, and Sanders et al. 1962). Suspension feeders filter particles out of the water column. Hancock (1970), Korringa (1941), and Thorson (1950) suggested that much of the mortality of settling larvae is due to such filtration. Glynn (1973) has shown that a coral reef under conditions of one-way water flow can reduce diatom densities by 91 % and zooplankton densities by 60%. Fitch (1965) described two populations of Pismo clam, Tivela stultorum, in which settlement occurred in 1938, 1941, 1944, 1945, 1946 and 1947 in both populations. Since 1947 settlement has occurred in one population but not in the other population. He suggests that the density of adults in the latter population was sufficient after 1947 to prevent settlement, by adults straining all of the spat out of the water column. In general, all larvae other than those that are brooded and thus do not enter the water column would be susceptible to such filtration. Even brooded larvae should be susceptible in areas where resus- pension is likely. Thus, few "alien" forms should cooccur with dense assemblages of suspension feeders except those with mobile adults that can migrate in laterally. Dense populations of suspension feeders should also be dominated by specific age classes due to failure of settlement once a large adult population has been attained. Filter feeders on hard bottoms should have a similar impact unless turbulence is high (see for example Glynn 1973). Although tube builders all build tubes in the sediment, tube-building organisms may belong to any trophic level. Daro and Polk (1973), Mills (1967), and Woodin (1974) showed that larvae were unable to penetrate the tube mat or at least could not survive in the restricted amount of space available between tubes. Sanders et al. (1962) ascribed the sharp boundary between the bivalves Mya arenaria and Gem- ma gemma to the inability of Mya spat to penetrate the surface layer of Gemma (see Bradley and Cooke 1959 for a similar interaction between Gemma and a sur- face deposit-feeding, tube-building, spionid polychaete versus Mya). The cooccur- rence of dense populations of a tube-building, deposit-feeding maldanid polychaete, was probably also important as was the presence of the spionid polychaete in the Gemma-Mya interaction described by Bradley and Cooke (1959). Burbanck et al. (1956) described a similar sharp boundary between two polychaetes, Clymenella, a sedentary deposit-feeding tube builder, and Pectinaria, a more active deposit-feeder. Pectinaria's effect on the sediment is more like that of burrowing deposit-feeding 1976] Woodin: Adult-Larval interactions in dense infauna[ assemblages 33 bivalve's than is Clymenella's which has much more of a stabilizing than a destabiliz- ing effect (see Gordon 1966 and Rhoads 1967 for turnover rates of Pectinaria and Rhoads 1967 for those of Clymenella). Tube-builders are known to feed on the surface and ingest larvae of other tube- building forms (Woodin 1974) and also of bivalves (Breese and Phibbs 1972; Daro and Polk 1973; and Segerstrale 1962, 1965, and 1973). Thus only epifaunal forms which brood should cooccur with tube builders. Other forms are excluded because of spatial competition and ingestion and/ or continual disturbance of larvae due to defecation and feeding activities of the tube builders. Some burrowing polychaetes may be present but their densities should be limited because of lack of infauna! space (Woodin 1974). Data do exist therefore to support the hypothesis that in the presence of a dense adult assemblage, successful larval recruitment will be rare. The data also indicate that larval mortality will not be differential. Larvae of tube builders will be as un- able to establish themselves in a dense tube builder assemblage as are larvae of deposit-feeding and suspension-feeding forms. Assemblages of perennial forms (deposit-feeding and suspension-feeding bivalves) should therefore frequently be dominated by single age classes. Once a dense assemblage is established, few larvae will successfully settle and establish themselves in the assemblage. Buchanan (1958), Fitch (1965), Ford (1925), and Hancock (1970) provide descriptions of dominance by age classes in infauna! assemblages. Tube-building forms are often annual or- ganisms; thus, their assemblages should be less persistent. Furthermore, assemblages of tube builders should self-destruct because of the inability of their larvae to estab- lish themselves and the short life span of the adults. See Daro and Polk (1973) and Mills (1967) for descriptions of these phenomena.

3. Predictions and tests Competition theory predicts competitive exclusion if the resource is common, necessary, and in insufficient supply. If the resource can be effectively subdivided, then coexistence is predicted (see MacArthur 1972 and MacArthur and Levins 1964). Thus, one would expect separation of functionally similar, not necessarily taxonomically close, species and coexistence of functionally dissimilar species, as- suming that the functional similarities related to a competitively important re- source. Infauna! organisms are functionally diverse in their morphologies relating to space and food utilization but this diversity does not necessarily mean that re- source partitioning has occurred. Surface deposit-feeding tube builders and deposit- feeding bivalves, for example, exploit the same sediment layers and probably take approximately the same range of particle sizes but their morphological attributes associated with their exploitation of these resources are very different. These diverse infauna! forms then are potential competitors and as predicted competitive exclusion seems to have occurred. 34 Journal of Marine Research [34, 1

No competitive mechanism has been described that would result in discrete as- semblages of such diverse forms if the interactions occur between adults (see Table 2). Mechanisms have been described, however, that would result in such discrete assemblages if the interactions occur between established individuals and settled or settling larvae. These are summarized in Table 3. Deposit feeders make the sedi- ment more easily resuspended (Rhoads and Young 1970) and ingest or disturb surface or near-surface larvae, such as those of suspension-feeding bivalves and tube builders as well as many deposit feeders. Suspension-feeding bivalves filter larvae as well as resuspended materials out of the water column. Tube-building forms because of their tubes reduce the ability of larvae to penetrate the sediment surface and, depending on the trophic type of the tube builder, ingest and/ or defe- cate on the larvae. Co-occurring epifaunal forms decrease the access of the larva to the sediment and increase the probability of trampling. All the above interactions occur as the larva is settling or soon after settlement. The requirement of speed to maintain the discreteness of the assemblage is thus met. Given these interactions, several predictions can be made for dense assemblages: 1. Suspension-feeding epifaunal bivalves which brood their young and thus re- lease them at sizes larger than those usually manageable by tube-building forms should reach their highest densities among the tube builders. Due to movement by wafting and their suspension-feeding mode on the surface the small epifaunal bivalves are susceptible to filtration by suspension-feeding bivalves and to disturbance and sediment changes associated with the deposit feeders. The densities and distribution patterns reported for Gemma gemma, a small epifaunal suspension-feeding bivalve, con.firm this prediction (see Bradley and Cooke 1959 and Sanders et al. 1962). 2. Small burrowing polychaetes should reach their highest densities among the deposit feeders. They are susceptible to larval filtration by suspension feeders and spatial limitation because of tube density among the tube-building forms. Since they immediately burrow beneath the sediment surface upon settle- ment, they are not susceptible to sediment changes or surface-associated mortality factors connected with deposit feeders. Woodin (1974) demon- strated spatial competition between a burrowing polychaete, Armandia brevis, and several tube-building polychaetes. The density of the burrowing poly- chaete increased when the tube builder densities were reduced by experi- mental manipulation. Sanders (1958) described much higher densities of Nephthys, a burrowing polychaete, in association with Nucula, a deposit- feeding bivalve, than with Ampelisca, a tube-building amphipod. 3. No infauna! forms should consistently attain their highest densities among densely packed suspension-feeding bivalves. Only individuals that migrated in laterally should be present. Even for larval forms too large to actually be ingested, deposition in an organism's pseudofeces is probably often fatal .... \C -..J O'I '-'

0

Table 3. Functional groups of infauna and types of interactions. -- ::i:.. Functional groups Ingests or Filters larvae Changes Larval type Predicted $:l..

disturbs by its feeding out of water sediment at settlement dense I t-, activities surface or prior to co-occurring l:l near surface larvae settlement forms

deposit-feeding yes no makes more may be surface burrowing --.....;::s bivalve easily or may burrow polychaetes (I> resuspended below surface ..... 0 suspension-feeding no yes no, or much surface none --;::: bivalve more slowly "'s· $:l.. tube-building yes depends on stabilizes; surface epifaunal (I> forms trophic type reduces space bivalves ;::: below surface; and tube --;::: increases epifauna l:l epifaunal -.:: space on surface §_ due to its tubes l:l (I>

c::,-

(I> "'

w VI 36 Journal of Marine Research [34, 1

(Crisp 1964 and Mileikovsky 1974). Ford (1925) described an area where Spisula solida, a suspension-feeding bivalve, attained densities of greater than 1000 per m2 and was "practically the only species represented ...." He did not, however, describe his sorting and sieving methods. 4. Since deposit-feeding and suspension-feeding bivalves are perennials and the interactions summarized in Table 3 reduce the probability of successful larval settlement by any larvae including their own, their assemblages should per- sist but be strongly age-class dominated. Fitch (1965), Ford (1925), and Hancock (1970) describe assemblages of bivalves that are both persistant and age-class dominated. 5. Since tube-building forms are usually not perennials but annuals and their larvae are prevented from recruiting, tube builder assemblages should be age- class dominated but they should not persist. This will be especially true when the assemblage is numerically dominated by one species. Daro and Polk (1973) and Mills (1967) provide evidence of such age-class domination and lack of persistence in assemblages of a tube-building polychaete, Polydora, and a tube-building amphipod, Ampelisca, respectively. Although this discussion has concentrated on dense assemblages, these interac- tions are not restricted in their occurrence to such assemblages. Larvae suffer mortality due to the feeding and defecation activities of all these forms even when only single adult individuals are present. Spatial limitation, sediment change, and larval filtration effects will, of course, be much more limited in their impact in a sparsely populated assemblage than in a dense assemblage. The sphere of influence of each adult and thus what constitutes a dense assemblage varies with the indi- vidual's size and level of activity. With the possible exception of larval filtration by suspension feeders, however, these effects may be major structuring forces in patchy, but dense, assemblages. All are major forces in large dense assemblages.

4. Summary Dense infauna! assemblages are discrete with sharp boundaries. They are of three types: burrowing deposit feeder, suspension feeder, and tube builder. The inter- actions that have been described between established individuals are either slow or restricted to functionally similar organisms and consequently cannot be responsible for the maintenance of such functionally dissimilar discrete assemblages. Adult- juvenile interactions are hypothesized as the mechanism maintaining these discrete assemblages. This hypothesis is partially confirmed by analysis of data from the literature. The larval mortality factor(s) which are fast and not restricted to organ- isms functionally similar to the protagonist differ for each assemblage type. They are as follows: deposit feeder assemblages, sediment changes and feeding and defe- cation activities in the surface layers of sediment; suspension feeder assemblages, 1976] Woodin: Adult-Larval interactions in dense infauna[ assemblages 37 filtration of larvae from the water column followed by ingestion or deposition in pseudofeces; and tube builder assemblages, spatial limitation because of tube density and defecation and feeding in the surface layers of sediment. The interactions are only described in terms of dense assemblages but with the possible exception of the suspension feeder mechanism all should be important, although not the primary structuring force, in less densely populated infauna! assemblages.

Acknowledgements. I wish to thank Ms. G. A. Brenchley, Mr. P. Gilman, Drs. J. B. C. Jackson, C. F. Nyblade, R. T . Paine, S. Stanley, G . Vermeij, and D. Young for discussion and critical readings of various stages of the manuscript. Drs. V. Gallucci, A. J. Kohn, J. Levinton, H. Sanders, and J. Yorke provided stimulating discussion. I was supported by NSF Grant No. GA-42611 during a portion of this study.

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Received: 8 July, 1975; revised: 27 October, 1975.