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Home , Acoela, Convolutidae, Flatworm, Tetraselmis

MARINE ECOLOGY PROGRESS SERIES Vol. 58: 41-51, 1989 1 Published December 15 Mar. Ecol. Prog. Ser.

Oceanic mixotrophic *

' Department,Woods Hole OceanographicInstitution, Woods Hole, Massachusetts02543, USA Department of ,University of Bergen,N-5065 Blomsterdalen,Norway Department of , University of Maine, Orono,Maine 04469,USA

ABSTRACT Most reports of photosynthetic flatworms are from benthic or littoral habitats, but small (< 1 mm) acoel flatworms with algal endosymbionts are a widespread, though sporadic component of the open-ocean in warm waters Among oceanic flatworms are specimens harbor~ng prasinophyte or less commonly, dinophyte endosymbionts Photosynthesis was measured by I4C uptake in flatworms from shelf/slope waters in the western north Atlantlc and from the Sargasso Sea Rates were as high as 27 ng C fixed ind -' h-' Assimilation ratios ranged from 0 9 to 1 3 ng C fixed (ng Chlorophyll a)-' h-' Although these acoels were photosynthetic, they were also predatory on other plankton Remains of and radiolanan central capsules were observed in the guts or fecal material of some specimens These acoel-algal associations apparently depend on both autotrophic and heterotrophic nutrition and are thus mixotrophic Among the planktonic protozoa, mixotrophy is a common nutnbonal strategy, it also appears to be common strategy among certain taxa of open-ocean metazoa

INTRODUCTION pelagica, C, schultzei, and Adenopea illardatus (Lohner & Micoletzky 191 1, Dorjes 1970). Acoel flatworms (class , order ) are Planktonic acoels are not unique among oceanic zoo- common inhabitants of the marine littoral environment. plankton in being mixotrophlc. , radiolanans, While reports of pelagic acoels are rare (Brauner 1920, foraminiferans, and acantharians with algal chloro- Hyman 1939, Dorjes 1970, Bush 1984), we have found, plasts or endosymbionts are common in surface waters through special collecting and preservation tech- and are thought to be important in open-ocean plankton niques, that small (< 1 mm) acoels are a common, but communities as sites of enhanced primary production sporadic, component of the plankton in the upper water and as grazers and predators (Anderson 1983, Swan- column in warm, oceanic waters. These as-yet unde- berg 1983, Spero & Parker 1985, Michaels 1988b, scribed species can be abundant but, considering the Stoecker in press). Because of their size (usually > than rarity with which they are reported, they apparently -64 pm), both mixotrophic planktonic acoels and sar- are overlooked in most plankton samples. codines should contribute disportionately for their bio- During the past 13 yr, we have collected incidental mass and primary production to the flux of sinlung data on the occurrence and distribution of acoels in particles out of the mixed layer (Michaels & Silver 1988). surface waters of the open-ocean. All of the species we We document the widespread occurrence of small, have collected harbor algal endosymbionts. Algal sym- mixotrophic acoels in warm, oceanic waters, present biosis is well known among some temperate water, data on their algal endosymbionts, chlorophyll contents littoral acoels, such as Convoluta roscoffensis and C. and gut contents, and give experimental data on the convoluta (Keeble 1910, Ax & Apelt 1965, Taylor 1971, photosynthetic rates of these planktonic acoel-algal Mettan 1979, Bush 1984, Smith & Douglas 1987) and associations. are especially common among tropical, littoral species of Convoluta (Antonius 1968). Endosymbionts also appear in the majority of the described warm-water, METHODS AND MATERIALS planktonic species, specifically Haplodiscus spp., C. - Acoels were observed and collected by SCUBA Contribution no. 7099 from the Woods Hole Oceanographic divers in conjunction with other work on various Institution cruises in the Sargasso Sea, the eastern Atlantic (vicin-

O Inter-Research/Pr~nted in F. R Germany 4 2 Mar. Ecol. Prog. Ser. 58: 41-51. 1989

ity of the Madeira and Canary Islands) and in the were also collected with a 64 pm mesh, 0.5 m diam. net Indian Ocean (vicinity of the Seychelles) (Table 1 and in the top 3 m of the water column (Table 1). The net Fig. 1). In slope waters and in the Sargasso Sea, acoels was deployed during calm weather to wlndward while

20.

Fig. 1 Stations in the North Atlantic where mix- : otrophlc acoel flatworms were collected. o: Con- 0. i voluta sp. 'A'; =: unidentified species 'B'; l:: : unidentified species 'C'; 0: unidentified species I> I> 80. 60' 40. 20- om 'D'. See Table 1 for further information

Table 1 Observations of planktonic acoel flatworms with algal endosymbionts in the open ocean

Position Date Temp ("C) Taxon Size (mm) Color

ATLANTIC OCEAN Slope Waters, NW 39" N, 70" W 31 Aug-2 Sep 1986 Convolufa sp. 'A' Bright green" 41" N. 66" W 3 Jul 1987 Convoluta sp. 'A' Bnght green" Sargasso Sea 29" N, 64" W 24-30 May 1988 Unidentified sp. 'B' Goldena 28" N. 64" W 22 Nov 1975 Unidentified sp. 'C' Dark brown" Eastern Atlantic 36" N, 26" W 9 Jul 1978 Unidentified sp. 'C' Dark brownb 36" N, 15" W 30 Oct 1978 Unidentified sp. 'C' Dark brownb 35" N, 15'W 30 Oct 1978 Unidentified sp. 'C' Dark brownb 33" N, l l' W 31 Oct 1978 Unidentified sp. 'C' Dark brownb 32" N, 11" W 31 Oct 1978 Un~dentifiedsp. 'C' Dark brown" 30" N, 15" W 1 Nov 1978 Unidentified sp. 'C' Dark brownb 29" N, 15" W 1 Nov 1978 Unidentified sp. 'C' Dark brownt' 29" N. 17" W 2 Nov 1978 Unidentified sp. 'C Dark brownh 28" N, 11" W 2 Nov 1978 Unidentified sp. 'C' Dark brown" 27" N,17" W 3 Nov 1978 Unidentified sp. 'C' Dark brownb 19" N, 18" W 6 Nov 1978 Unidentified sp. 'C' Dark brown" 31" N. 16" W 25 May 1986 Unidentified sp. 'D' Bright greenb Unidentified sp. 'C' Dark brownb 26 May 1986 Unidentified sp. 'D' Bright greenb U~dentifiedsp. 'C' Dark brownb 30 May 1986 Unidentified sp. 'D' Bright greenb Unidentified sp. 'C' Dark brownb 4 June 1986 Unidentified sp. 'D' Bnght greenb Unidentified sp. 'C' Dark brownb INDIAN OCEAN 2" S, 55' E 21 Apr 1976 2 8 Unidentified sp. 'C Dark brownh

Collected with a 64 pm mesh, 0.5 m diam. plankton net between 0 and 4 m " Collected by hand with SCUBAin upper 100 m Stoecker et a1 . Oceanic mixotrophic flatworms 43

the ship was allowed to drift. Specimens were also The chlorophyll content of specimens collected from collected off the Canary Islands with a 150 pm mesh, slope waters in 1987 and from the Sargasso Sea in 1988 0.7 m diam. net in short vertical hauls. was determined fluorometrically (Parsons et al. 1984). Freshly collected specimens were checked at sea for One to 5 individuals were rinsed in filtered seawater, the presence of algal endosymbionts; wet mounts were and dropped onto each replicate GF/F filter. The filters made of small, semi-transparent specimens but smears with the acoels were frozen and later ground in 90 76 were made of the larger, more opaque specimens. acetone to extract the chlorophyll. These preparations were then examined with transmit- Photosynthesis was measured by uptake of I4C in ted light and epifluorescence microscopy (a Zeiss mi- specimens collected from slope waters in 1986 and croscope equipped with a 50 W Mercury lamp and a BP 1987 and from the Sargasso Sea in 1988 (Table 1). 450490 nm excitation filter, an LP 520 nm barrier fil- Individuals were transferred through 2 washes of GF/F ter, and an FT 510 nm chromatic beam splitter). filtered seawater prior to incubation in GF/F filtered Endosymbionts appeared as pigmented bodies with seawater with an added ca 0.5 pCi ml-' I4C bicarbonate transmitted light and under epifluorescent illumination (specific activity, 53 mCi m~~lol-';New England Nuc- they had a red fluorescence, indicating the presence of lear). Triplicate 100 yl samples of the spiked seawater chlorophyll a (Chl a). were taken for determination of total activity in the Specimens were preserved for later examination media. (Incubation conditions varied among expen- with light and transmission electron microscopy (TEM). ments and are given in Table and Fig. legends.) The best results were obtained when the acoels were In 1986 and 1987, the samples were prepared for anesthetized in MgC12 isotonic with ambient seawater liquid scintillation counting by transferring individuals for 5 min prior to fixation (Smith & Tyler 1984). Good into scintillation vials and then adding 2 m1 of Scinti- preservations for light and EM were obtained when gest (Fisher Scientific) to kill and digest their specimens were fixed in 3 % buffered glutaraldehyde tissues. After 12 h at room temperature, 2 m1 of 5 O/O (0.1 M sodium cacodylate in 90 O/n seawater, pH 7.2) acetic acid in methanol were added to each vial and the and stored at 4 "C. Some anesthetized acoels were vials were evaporated to dryness (this step removed the fixed in unbuffered glutaraldehyde drawn under a inorganic 14C).Then, 1 m1 of distilled water was added glass coverslip which was used to restrain the speci- to each vial, followed by 10 m1 of scintillation fluid men. Fixation in Bouin's, a recommended method for (ScintiVerse 11, Fisher Scientific). acoels (Smith & Tyler 1984), was tried with the slope Sample preparation in 1988 was modified so that water specimens (Table 1); they disintegrated in this data on chlorophyll content and 14C uptake could be fixative. obtained from the same specimens. After incubation, 5 For light microscopy, acoels were embedded in worms were placed on a GF/C glassfiber filter and Epon, serially sectioned at 2 pm section thickness, and frozen. Later, the filters were ground in 5 m1 of 90 '10 stained with alcian blue, hematoxylin and eosin (Smith acetone and chlorophyll was measured fluorometri- & Tyler 1984). For TEM, some specimens were post- cally. The acetone extract and sedimented debris were fixed in 1 % osmium tetroxide, stained with 0.5 O/O added to a scintillation vial and allowed to evaporate to uranyl acetate, dehydrated in an ethanol series, and dryness at 60°C. Two m1 of 5 % acetic acid in methanol then embedded in Spurr's low-viscosity resin or in were added, and the vials were evaporated to dryness Epon. Some specimens were post-fixed in osmium and again. Sample processing then proceeded as described embedded in LX-112 without prestaining. After sec- above. tioning, the specimens were stained with uranyl ace- Samples were counted on a Packard Tli-Carb 4000 tate and lead citrate and viewed at 60 kV on a Zeiss Series liquid scintillation counter (United Technologies 10CA EM or on a Philips 201 EM. Packard) using the external standard ratio to correct for In 1986 to 1988 some specimens were kept alive and quench. The uptake of 14C due to photosynthesis was transported to the laboratory where they were main- estimated by subtracting the average uptake in the tained for several weeks. Specimens collected in slope dark controls from uptake in the light. Rates of photo- waters in 1986 (Table 1) were held at room temperature synthesis were calculated from light-mediated 14C (-23°C) in GF/F filtered seawater in polycarbonate uptake as described in Parsons et al. (1984). containers. They were exposed to cool-white fluores- cent light at 125 pE m-2 S-' PAR for 14 h d-l. Specimens collected from slope waters in 1987 (Table 1) were kept RESULTS AND DISCUSSION at 15 "C under approximately the same light regime. Small amounts of zooplankton (mostly Acartia tonsa) We have observed at least 3 acoel species in warm were provided as prey. The worms were transferred to (usually > 20 "C) oceanic waters (Table 1 and Fig. 1). fresh, GF/F filtered seawater every 3 to 5 d. Bright green acoels with prasinophyte endosymbionts 44 Mar Ecol. Prog. Ser. 58: 41-51, 1989

(Convoluta sp. 'A') were observed in the western TEM has been used to identify the algal endosym- Atlantic. Chlorophyll contents and photosynthetic bionts in specimens of Convoluta sp. 'A' collected in rates were measured. Specimens of a similar 1986 and 1987 (Table 1). Based on their ultrastructure (unidentified species 'D') were collected in the east- (Figs. 2 and 3) the algae are members of the green line; ern Atlantic. A small, golden acoel (unidentified sp. they have a large central pyrenoid, starch is stored in 'B') with a dinophyte endosymbiont was collected in plates around the pyrenoid and between the thylakoids, the Sargasso Sea. Chlorophyll content and photo- and the thylakoids are somewhat randomly arranged synthetic rates were measured in the golden acoel. A with inter-connections between the grana. We were third type (unidentified species 'C'), a dark brown able to further identify the algae as Tetraselnis sp. acoel associated with gelatinous zooplankton, was subgenus Prasinocladia sp. (family ) collected in both the Atlantic and Indian Oceans. A based on the unique ultrastructure of the pyrenoid prasinophyte endosymbiont was documented to occur (Fig. 2a, b); in this subgenus protrusions of the nucleus in this acoel, but no data were collected on the enter the pyrenoid (Hori et al. 1983). Prasinophyte chlorophyll contents or photosynthetic rates in this endosymbionts occur in C. roscoffensis (Keeble & Gam- acoel-algal association. ble 1907, Oschman & Gray 1965, Gooday 1970, Holligan & Gooday 1975, Douglas 1983, 1985) and C. psammophila (Sarfatti & Bedini 1965),2 l~ttoralspecies. Bright green acoels The endosymbiotic algal cells in our specimens lacked thecae and flagella; loss of these organelles also occurs Among the free-swimming acoels i.n surface waters, in endosymbionts of C. roscoffensis (Douglas 1983). The the most common species is a small (ca 0.9 mm in endosymbionts tended to be concentrated just under the length when mature) undescribed species of Convoluta body wall (Fig. 2A) ostensibly lying in cells of the (Convoluta sp. 'A' in Table 1).This species is usually a peripheral of central . In C. roscoffensis, the bright, grass green color, although in most populations symbionts are often closely associated with muscle a few less-intensely pigmented individuals occur. Sex- fibers of the (Oschman 1966). The same appears ually mature individuals were collected southeast of true of our Convoluta sp. 'A' (Fig. 3). Georges Bank in July, 1987 (Table 1). hght-histologi- Photosynthesis was measured in Convoluta sp. 'A' cal sections of these specimens showed that they collected at both locations in the western North Atlan- belong in the species group containing C. convoluta, C. tic. The specimens from over Alvin Canyon (39"N sordida, etc. (Group 4 in Fig. 80 of Antonius 1968); i.e., 70" W; 24 'C) fixed ca 28 ng C ind.-' h-' at an irra- they had a ciliated male antrum. They did not, how- diance of 125 !LE m-' S-' (Table 2). Rates of photo- ever, match a known species descriphon. We have synthesis in individuals collected in 1987 were less collected small, bright green acoels which we believe than 50 % of the rates in individuals collected in 1986 to be the same, or a closely related species, from the and incubated at the same irradiance (Table 2). Even at eastern Atlantic (unidentified sp. 'D'in Table 1).Small, irradiances which were light-saturating, rates of photo- unidentified, bright green, free-living flatworms have synthesis in worms from 1987 were only a ca 30 % of also been reported from the Indian (Norris 1967) and the rate determined at 125 1sE m-' S-' in 1986. It is Pacific Oceans (Michaels 198813). possible that the observed differences on the 2 occa- In slope waters and in the eastern Atlantic when we sions were due to differences in temperature or incuba- have encountered them, the bnght green acoels were tion conditi.ons (refer to Table 2) or due to the phys- patchy in distribution. Swarms sometimes reach iological state of the worms and/or their endosym- densities of an estimated 10 to 100 m3. In the Canary bionts. The specimens collected southeast of Georges Islands in 1986, short vertical hauls collected several Bank were sexually mature whereas those collected in hundred bright green worms and divers reported that 1986 were not. the worms occurred at densities of 10's to 100's m-3 in a Chlorophyll measurements are only available for the narrow zone. It is possible that Convoluta sp. 'A' or mature worms collected in 1987; they contained an similar specles are widespread in warm oceanic waters, average of 7.5 ng chl a 1nd.-' when collected; 18 d later but further sampling and taxonomic study are neces- their chl a content was about the same and the assimi- sary before this can be confirmed. lation ratio was ca 1.2 ng C (ng chl a)-' h-' at a saturat-

Fig. 2. Convoiuta sp. 'A' collected over Alvin Canyon (39'N 70-(1?; sce Table I). A: Algal endosymbionts (thin arrow) are concentrated under the body wall (thick arrow) of thc flatu,orm. Scale bar = 5 m. B: The endosymbiont can be tdentified as a sp. hdsed on its chIornplast structure Starch grains (thin arrow] II~between the thylakoids and a starch shell surrounds the core of the pyrenoid (thick arrow). The protrusion of the algal nucleus (N)into the pyrenoid is chdracterist~cof the subgenus Prasinocladla (Hori et al. 1983).Scr~le bar = 1 ILrn

4 6 Mar Ecol. Prog. Ser. 58: 41-51, 1989

Fig. 3. Convoluta sp. 'A' collected south of Georges Bank (41'-N,66OW; see Table 1). The algal lies close to a muscle fiber (the point of contact is indicated by the arrow). This contact is shown at a higher magnification in the inset. Note the numerous starch grains in the alga (also a Tetraselmis sp.). The ciliated of the is shown at the bottom of the micrograph Scale bar = 1 urn

Table 2. Convoluta sp. 'A' Chlorophyll a contents and photosynthetic rates. Means  SD

Site ng chl a md.-' Irradiance (WEnl ' s-'1 ng C fixed ind.' h-' I

' no data

Â¥ 12 d post-collection; 1 h incubation at 23 '(1 ' 1 d post-collection; 6 h incubation at 18 "C 18 d post-collection; 6 h incubation at 20 "C; assimilation ratio: - 1.2 ng C (chl a)' h-' ing irradiance (Table 2). This is similar to what might added to their culture containers disappeared. In the be expected in late exponential/early stationary phase dark or dim light, these acoels aggregated on top of free-living phytoplankton (Glover 1980). maimed and appeared to be browsing on cuticle, along with other unidentifiable them. This behavior ceased immediately if lights were material, was present in the central (where turned on. We do not know how the worms capture digestion occurs) of the mature Convolutasp. 'A' intact copepods. In the field, the worms swim freely, obtained in 1987 (Fig. 4). When we maintained Con- turning frequently in what could be an effective search voluta'A' in the laboratory, we observed that copepods pattern. Many of the littoral acoels with symbionts prey Stoecker et al.. Oceanic

on small (Sarfatti & Bedini 1965, Taylor 1971) although C, roscoffensis stops feeding once it acquires algal endosymbionts (Keeble 1910, Holligan & Gooday 1975).

Golden acoel

During 1988, we collected a second type of sinall acoel with symbionts (unidentified sp. 'B' in Table 1). This type was a semi-transparent golden color. All the specimens we collected were immature and thus none could be identified. We observed golden-brown gym- nodinoid cells ca 18 to 22 pm in length in the worms. The mean number of symbionts ind. -' was 103 (SD = 36, n = 21). TEM confirmed our field identification of the endosymbiont as a (Fig. 5) based on the mesokaryotic nucleus characteristic of this family and on chloroplast structure (Dodge 1979). We have tentatively identified the dinophyte as a Syrnbiodinium sp. based on its symbiotic habit, gymnodinoid shape, and peripheral lobed chloroplast. Dinoflagellate Fig. 4. Convoluta sp. 'A' collected south of Georges Bank endosymbionts have been reported in littoral, tropical (41"N 66'MI; sec Table 1). Light micrograph of a cross-section stained with alcian blue and hernatoxylin and eosin. Algal acoels; Amphiscolops usually has an Amphidiniurn endosymbionts (thin arrow) are located under the epidermis; endosymbiont but Haplodiscus can have both cuticle (thick arrow) and other remnants are visible Amphidinium and endosymbionts (Tay- in the central syncytium. Scale bar = 100 pm lor 1971, Trench & Winsor 1987). However, the Sym- biodiniunl sp. in our acoel does not have a stalked see Swanberg 1979) This species has been observed pyrenoid like that found in the Symbiodinium found in mostly on the surfaces of colonial radiolarians and Haplodiscus by Trench & Winsor (1987). other gelatinous zooplankton. Divers have rarely On-deck I4C ~ncubationswere done on 2 dates with observed free-swimming individuals. Acoel 'C' has the golden acoels from the Sargasso Sea. The acoels been observed in association with colonial radiolarians collected on May 25, 1988 had an average chl a content in the Sargasso Sea, the eastern Atlantic, and the of 2.5 ng ind.-' (SD 0.7, n = 21) and a maximum rate of Indian Ocean (Table 1). None of our preserved speci- photosynthesis (P,,,) of 2.5 ng C ind.-' h-', SD = 0.6 mens were sexually mature or the genitalia were not 11.0 ng C (ng chl a)-' h-'] (Fig. 6A). Worms collected on intact; therefore it has not been possible to identify this May 27 had a lower chl a content, 1.6 ng ind.-' (SD = 1.0, type to species but the presence of a single bursa1 n = 21), a lower photosynthetic rate ind-' (P,,, 1.4 ng mouth-piece in 1 sectioned specimen is consistent with ind-' h-', SD = 0.0) and a lower chlorophyll specific rate being a species of Convoluta. of photosynthesis 1- 0.9 ng C (ng chl a)-' h-'] (Fig. 6B). Freshly collected specimens of acoel 'C' deposited We suspect that these differences between the 2 experi- yellowish fecal material that contained radiolarian ments reflect differences in the physiological state of the central capsules as well as other unidentifiable acoel/algal associations collected on the 2 d. However, material (Swanberg 1979). It seems likely that this as has been observed in symbiotic foraminiferans (Spero acoel preys on colonial radiolarians or consumes & Parker 1985), there was little or no photoinhibition in injured radiolarian tissues. It is also possible that it either experiment (Fig. 6); this suggests that the acoel/ scavenges prey captured by the radiolarians or by algal associations are adapted to high-light such as symbionts associated with them. occurs at the sea surface. Tissue smears of fresh material indicated that acoel 'C' contained algal endosymbionts but TEM was necessary to identify the symbiont as a prasinophyte (Fig. 7). Dark brown acoel Identifying characteristics were the large central pyre- noid surrounded by a starch shell, the thylakoid arrange- The third type of acoel flatworm with algal symbionts ment, and protusion of the nucleus into the pyrenoid that we obse~vedwas a dark brown species ca 0.5 to (Dodge 1979). No data are available on the chlorophyll 1.0 mm in length (unidentified sp. 'C' in Table 1; also content or rates of photosynthesis in acoel 'C' 48 Mar Ecol. Prog, Ser 58. 41-51. 1989

Fig. 5. Dinophyte endosymbiont (arrow)in acoel sp. 'B' (see Table 1). The mesokaryon nucleus (N) characteristic of dinophytes is evident. Scale bar - 1 urn. Note the peripheral, lobed chloroplast

CONCLUSION in most studies of oceanic plankton, probably because they become unrecognizable with the fixation pro- Photosynthetic, mixotrophic acoel turbellarians are a cedures routinely used for plankton samples (they con- widespread component of the open-ocean plankton in tract into a tight sphere if they are not anesthezed prior warm waters. These metazoans have been overlooked to fixation).

Percent lo Percent lo

Fig. 6. Photosynthesis vs irradiance curves for acoel sp. 'B' (see T'ible 1).A: on May 25 (I' = 2507 !E rn-^s-I): B' May 27 (I0= 1976 FE ni st).Specimens were incubated during mid-day for 2 h in an on-deck flow-through incubator: seawater temperature was 2.5 :C. In the first incubation (A),the mean c-hl d c-ontont was 2.5 (SD = 0.71 ng ind:' rind tho assimilation ratio was at. 1.0 ng C (ng chl a) ' h ' at saturating irrariiance (average for 4 highest lqht I~vels).In the second inruhation (B).the chluropliyll content was 1.6 (SD = 1.0) ng ind" and the assimilation ratio at saturatinq irradiance iaveraqe for 3 highest light levels) was 0.9 ng C (nqchl ril' h' Stoecker et al.. Oceanic m~xotroph~cflatworms 49

Large (> 64 pm) mixotrophic organisms appear to be be extremely efficient utilizers of both carbon and ni- common in warm oceans. Among the large protozoan trogen derived from prey; thus, they may be more mixotrophs are the colonial radiolarians (Swanberg important in carbon and inorganic nutrient cycling than 1983),many solitary radiolarians (reviewed in Anderson their biomass alone would suggest (Taylor 1982, Ander- 1983),and many planktonic foraminiferans (reviewed in son 1983, Michaels 1988a).The assimilation ratios, 0.9 to Taylor 1982, Spero & Parker 1985, Gastrich & Bartha 1.2 ng C fixed (ng chl aj-' h-', that we observed in the 1988), and acantharians (Michaels 1988a, b). Our data acoels were about 10-fold lower than those that have show that mixotrophic metazoans may also be important been observed in planktonic radiolarians with algal in the open ocean. Some of the planktonic hyd- symbionts (Swanberg 1983, Rivkin & Lessard 1986). romedusae and scyphomedusae also have algal Nevertheless, acoels, like colonial radiolarians (Swan- endosymbionts (reviewed in Taylor 1984, Muscatine et berg 1983),should be important as localized, microscale al. 1986, Trench 1987, Kremer et al. unpubl.).Although sites of hlgh primary productivity. These symbiotic each of these types of mixotroph probably makes only a associations are also important because of their size; small contribution to the total biomass and chlorophyll they are large compared with most phytoplankton. They (Swanberg 1983, Michaels 1988a), as an assemblage, should contribute disproportionately for their biomass to large mixotrophs may be quite important in oligotrophic the flux of energy and materials out of the mixed layer oceans (Caron & Swanberg 1989). (Michaels & Silver 1988). The ecology of metazoan as Photosynthetic mixotrophs often have high photo- well as protozoan associations with endosymbiotic synthetic rates per unit chlorophyll and are thought to algae needs to be studied in the open ocean.

Fig. 7. Prasinophyte endosymbionts (ar- rows) in acoel sp. 'C' collected in the vicinity of the Canary Islands (30°N 15" W; see Table 1). Note the large cen- tral pyrenoid surrounded by a starch shell in the algal cell on the right. A protusion of the nucleus can be seen in the pyrenoid. Scale bar - l :trn 50 Mar Ecol. Prog. Ser 58: 41-51, 1989

Acknowledgements. This work was partially supported by Keeble, F., Gamble, F. W. (1907). The origin and nature of the NSF grants OCE-860765 and OCE-880684 to D.K.S. We thank green cells of Convoluta roscoffr~nsis.Q. J1 mlc!osc. Sci. 51 Dr D Angel for making his data ava~lableto us, MS J. Lndsey 167-219 and MS S. Houghton for electron microscopy, Ms Y Coursey Loehner, L., blicoletzky, H. (1911). Ueber zwei neue for maintenance and observation of acoei 'A' in the laboratory. pelagische Acoelen des GoIfes von Triest (Convoluta Drs G. R. Harbison and R. J. Olson for opportunities to collect pelagica und Monochoerus illardatus). Z. wiss. Zool. 98: specimens on their cruises, and Drs A. F. Michaels, M. E. Putt 381-429 and an anomymous reviewer for their comments and sugges- Mettan, C. (1979). A northern outpost of Convoluta roscoffen- tions. sis in South Wales. J, mar. biol. Ass U.K. 59 251-252 Michaels, A. F. (1988a). Vertical distribution and abundance of Acantharia and their symbionts. 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This article was submitted to the editor Manuscript first received: May 10, 1989 Revised version accepted: August 4, 1989