Dendronephthya Hemprichi

Coral Reefs (1997) 16:5Ð12 Reports Clonal propagation by the azooxanthellate octocoral Dendronephthya hemprichi M. Dahan1, Y. Benayahu2 1 Department of Environmental Sciences and Energy Research, Weizmann Institute of Science, Rehovot 76100, Israel 2 Department of Zoology, George S. Wise Faculty of Life Sciences, Tel Aviv University, Ramat Aviv, Tel Aviv 69978, Israel Accepted: 2 February 1996 Abstract. The azooxanthellate octocoral Dendronephthya of many bottom-dwelling aquatic invertebrates (e.g., Jack- hemprichi (Octocorallia, Alcyonacea) is the most abun- son 1977, 1985; Hughes 1989). Such individuals are termed dant benthic organism inhabiting the under-water surfa- ramets and a genet consists of all ramets that are derived ces of oil jetties at Eilat (Red Sea); however, it is very rare from the same zygote (e.g., Harper 1977; Coates and on Eilat’s natural reefs. This soft coral exhibits a newly Jackson 1985; Hughes et al. 1992). Asexual reproduction is discovered mode of clonal propagation that results in the clonal proliferation of individuals by separation of autotomy of small-sized fragments (2Ð5 mm in length). ramets, either at the time of ramet formation or by partial They possess specialized root-like processes that enable mortality and/or mechanical fragmentation of previously a rapid attachment onto the substrata. An autotomy event united groups of ramets (Jackson 1985). In most clonal is completed within only 2 days; large colonies can bear invertebrates the potential number of ramets per genet is hundreds of pre-detached fragments. Temporal fluctu- unlimited because modular construction generally elimin- ations in the percentage of fragment-bearing colonies indi- ates surface to volume constraints on overall clonal size cate that autotomy is stimulated by exogenous factors, (e.g., Jackson 1977). The consequences of producing dis- probably flow-related events. Recruitment of fragments junct modules in terms of spreading the risk of mortality onto PVC plates placed horizontally adjacent to and exploitation of patchily distributed resources have D. hemprichi colonies, was immediate and remarkably already been discussed in several studies (Cook 1979; high. Attached fragments were observed 2 days after McFadden 1986; Hughes and Cancino 1985; Hughes et al. placement of plates; after 52 days, densities of more than 1992). Clonal organisms are characterized by great mor- one recruit per cm2 were recorded. The negative buoyancy phological flexibility, by the potential to accumulate large of fragments causes them to land on horizontal surfaces biomass, and often by the potential for rapid growth. rather than on vertical ones. However, their survivorship These characteristics enable the dispersal of clones to on vertical surfaces is much higher. This pattern corres- opportunistically exploit patches of resources as well as ponds with dominance of D. hemprichi on vertical substra- generating large aggregations (Hughes and Cancino ta at the oil jetties and on natural vertical reefs of the 1985). Cloning by somatic division, i.e., budding, fission or northern Red Sea. Such a mode of clonal propagation fragmentation, occurs among a wide variety of an- provides an e¦cient mean for genets of D. hemprichi to thozoans, including scleractinian corals (e.g., Highsmith exploit food resources within a zooxanthellate-dominated 1982; Harrison and Wallace 1990), zoanthids (Karlson reef community. 1988), sea anemones (e.g., Chia 1976), and octocorals (Lasker 1988, 1990). Soft corals (Octocorallia, Alcyonacea) tend to form large monospecific aggregations on coral reefs, composed of numerous colonies, most probably derived by asexual processes (Benayahu and Loya 1977, Introduction 1981). Octocorals exhibit a particularly large range of mechanisms of clonal propagation (Lasker 1988). Among The ability to form more than one individual of identical the described mechanisms, budding of daughter colonies genetic composition is a common feature in the life history is found in Sarcophyton gemmatum (Verseveldt and Be- nayahu 1978), fragmentation in Junceella fragilis (Walker and Bull 1983), colony fission in Capnella gaboensis (Far- rant 1987), Xenia macrospiculata (Benayahu and Loya 1985), X. lepida and Nephthea cf. cupressiformis, and the generation of new colonies by stolons in E§atounaria sp. Correspondence to: Y. Benayahu (Lasker 1988; Karlson et al. in press). 6 The species examined in this study is the azooxanthel- Results late octocoral Dendronephthya hemprichi of the family Nephtheidae, which includes several hundreds of reef-in- Features of the fragments habiting species across the Indo-Pacific basin, (see Bayer 1981). Dendronephthya hemprichi is abundant on many Clonal propagation of D. hemprichi is performed by reefs of the northern Red Sea (Benayahu 1985), where it autotomy of fragments, 2Ð5 mm in length, each composed forms aggregations composed of numerous colonies with of several polyps. Autotomy is first observed as a gradual a wide array of colors. This species is the most abundant narrowing of the coenenchyme connecting the parent col- benthic organism inhabiting artificial underwater con- ony and the incipient fragment. Within 20 hours of com- structions at Eilat, but is very rare on Eilat’s natural reefs. mencement of the autotomy process, only a thin cord Unlike the vast majority of the coral-reef octocorals, all of tissue connects the predetached fragment to the Dendronephthya species are azooxanthellate, i.e., they are colony (Fig. 1a). At this stage, root-like processes, not associated with endosymbiotic algae at any stage in each approximately 1 mm long, develop at the basal part their life cycle. However, D. hemprichi successfully inhabits of the fragment and subsequently the fragment detaches reef sites dominated by numerous zooxanthellate species from the colony (Fig. 1b). Laboratory observations in- of stony corals, soft corals, zooanthids and sea anemones. dicated that completion of an autotomy process takes In this work we describe a unique mode of clonal only 2 days. After detachment of the fragment, its root-like propagation exhibited by D. hemprichi, resulting in processes continue to elongate and additional ones may autotomy of small fragments. The study also examines appear. The number of processes per fragment ranges short- and long-term dynamics of recruited fragments. between 2Ð15, each with a diameter of 200Ð800 lm. His- The consequences and benefits of this mode of propa- tological studies indicated that the processes have a cen- gation for rapid recruitment and space utilization are tral cavity that is continuous with the gastrovascular demonstrated. canals of the fragment. The wall is 7Ð12 lm in width, composed of an ectoderm rich with nematocysts and separated by a thin mesoglea from the internal ciliated Materials and methods endoderm. The frequency distribution of detached fragments, in The study was conducted at the oil jetty area, located 4 km south of terms of number of polyps per fragment, indicates that Eilat (Gulf of Elate, Red Sea) during 1989Ð1990. The underwater most of them are small (Fig. 2). In 84.4% of the fragments steel jetties were built in the early 1970s and since then have been (n"506) the number of polyps ranged from only 4 to 12. densely colonized by a diverse benthic community (Goren 1992). The fragments were negatively buoyant; their settling ve- Dendronephthya hemprichi is the most abundant soft coral at this site locity, measured in a glass cylinder, was 3.6$0.6 (SD) (20% cover), growing on the vertical supporting pillars of the jetties cm/s (n"88 fragments). Field observations indicated and on the surrounding coiled barbed wire. Numerous colonies were maintained in running sea water at the that, while sinking, fragments may drift in water currents Marine Biological Laboratory at Eilat for detailed examination of to lateral distances of over 60 m away from the parent the autotomy process. The fine structure of the fragments was colony, within several minutes. During this period, if the studied by histological sections, following the methodology of Be- fragments encountered appropriate solid-substratum, nayahu and Loya (1983). Colonies of D. hemprichi ('20 cm) were they attach on it. monitored monthly in the field during SeptemberÐDecember 1990 The fragments rapidly adhere to a variety of artificial for the presence of predetached fragments. Surveys were conducted and natural substrata (Fig. 1c). Firm attachment is along 6 m line transects on each of four vertical pillars at a depth of 2Ð8 m. Short- and long-term dynamics of recruited D. hemprichi achieved within 5Ð10 h. During attachment, the fragments fragments were studied on white PVC plates, measuring 20]20 cm rise into an upright position and become vertically by 3 mm thick, placed adjacent to large D. hemprichi colonies. The oriented on the substratum, anchored by their root-like plates were individually tagged and wired to the jetties at depths of processes. This position appears to be maintained by 6Ð15 m. Throughout the study period, recruits appearing on the hydrostatic pressure within the cavities of the processes. plates were counted and those '3 cm in length were measured. Within 2 weeks of attachment, the root-like processes Short-term recruitment was examined on 30 plates placed horizontally in September 1990 and monitored 2, 4, 9, 22 and 52 days after disappear and the basal part of the small colony remains placement. Long-term recruitment was studied on 124 plates, which attached to the substratum. were monitored

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