Coral reproduction is diverse; many species have chosen specific strategies in order to maximize chances of survival for their offspring. During the last decades, biologist have unravelled much about the sex life of stony and soft corals. Recently, the mysterious Dendronephthya species have been extensively studied.

During the last years, scientists have uncovered many different ways in which corals reproduce; they can be gonochoric or hermaphroditic. Some corals can change sex, which is called sequential hermaphroditism (see the Coral Science archive, section Reproduction). Some species do not even need males, as they can self-fertilize their eggs. This last mode of reproduction is called parthenogenesis, and may be used by corals from the genera Fungia, Porites and Pocillopora (plants can do this as well). A lot of research has been conducted on tropical, shallow water stony corals. Last year however, Korean scientists discovered more details about the sex life of Dendronephthya’s (figure 1). 

Figure 1: Where the coral reef meets the forest: Dendronephthya sp. growing on an old tree branch (copyright Hans Leijnse).

 Deep water gems

Soft corals (subclass Octocorallia, order Alcyonacea) are beautiful corals which often receive little attention by marine aquarists. They display interesting morphology and underwater movement, although their colors are often not that striking. This is not true however for the corals from the genera Dendronephthya and Scleronephthya (family Nephteidae), which display colors ranging from yellow to red. These corals occur in shallow and deeper waters, and are difficult to maintain alive in aquarium systems for prolonged time.

Just like stony corals, soft corals reproduce in various ways (Table 1). For an overview of how stony corals (order Scleractinia) reproduce, we recommend the article by Dana Riddle in Advanced Aquarist, Volume VII, Issue IX. Next to broadcast spawning (the release of eggs and sperm) and brooding, soft corals also brood their larvae externally. When the larvae have developed properly, they are released and will start looking for a suitable substrate to settle on. Just like their stony coral relatives, soft corals reproduce annually during the summer and fall, and they also use the moon for cues. The factors which stimulate corals to reproduce are complex, and they include moon cycles, water currents, water temperature, light in general and food availability⁴. Unlike stony corals however, soft corals also reproduce during the full moon as well as around new moon. Temperate soft corals are known reproduce all year round².

Table 1: Reproductive patterns documented in the literature on Alcyonacea (soft corals). BS broadcast spawner; EB external brooder, G gonochoric, H hermaphroditic, G/H gonochoric or hermaphroditic, IB internal brooder, P parthenogenetic, U unknown (modified from Hwang & Song, Marine Biology, 2007).

Taxon:                    species # Sexuality                       Mode of reproduction      

Most soft corals are gonochoric (Table 1), although some of them are hermaphrodites. When soft corals mature sexually, their gonads (sex organs) develop along the mesenteries (connective tissue) inside the polyps. When they are completely mature, the gonads will detach from them. The reproduction of the Nephteidae family has not been studied that well, and only some species have been properly described. These include Lithophyton arboreum, which is a gonochoric internal brooder (of which the females are fertilized and brood their larvae), Dendronephthya hemprichi, a gonochoric broadcaster (releasing eggs and sperm) and Capnella gaboensis, which broods its larvae externally. 

Dendronephthya gigantea reproduction

Last year, Korean scientists studied Dendronephthya gigantea, and determined all aspects of its reproduction; sexuality, sex ratio, mode of reproduction, fecundity, gametogenesis and larval development. This species is abundant in Korea, and inhabits waters ranging from 14 to 26°C (57 to 79°F). Most of the colonies occur on vertical and horizontal rocky substrate from 2 to 35 m in depth, and show peak abundance between depths of 10 and 20 m. For their research, the scientists collected 5 cm (2 inches) fragments of these corals regularly and placed them in small tanks along the coast of Munseom Island (figure 2, red arrow). All of the Dendronephthya gigantea colonies examined were gonochoric, as determined by microscopic and histological examination.

Figure 2: Munseon Island (red arrow),the sample area of this study,which is close to the large Island of Cheju (source: Google earth)

Among a total of 98 colonies analyzed, 46 were found to be female, 22 male, and 30 had no gonads. During summer, they found about 4 mature eggs per coral polyp (figure 3).  Their eggs and sperm arose from the gastrodermis, and gradually moved into the gastrovascular cavity (polyp cavity) as they grew. Sperm cells were only detected from June to October, whereas oocytes were found at all times of the year. This is interesting, as producing egg cells is more energy-demanding compared to producing sperm. The reason might be that the males have simply adapted their cycles to those of the females, as it is of little use to produce sperm when no fertile female colonies are present.

Figure 3: Dendronephthya gigantea egg development, called o1 to o5, where o5 is the mostly developed egg. A: Cluster of Stage I oocytes (eggs) embedded in mesentery (connective tissue). B,C,D: Developing oocytes with a nucleus. E: Stage V oocyte  with numerous yolk bodies, which will feed the future larva. F: Planula brooded in the gastrodermal canal. The scale bar represents 50 micrometer (m mesentery, n nucleus, nu nucleolus (nucleus body), o1 Stage I oocyte, o2 Stage II oocyte, o3 Stage III oocyte, o4 Stage IV oocyte, o5 Stage V oocyte, gc gastrodermal canal, pd pedicle, pl planula Hwang & Song, Marine Biology, 2007)

The scientists found that the size of eggs and sperm was related to seawater temperature (figure 4). Egg formation began between October and November, when seawater temperatures were decreasing.  The rapid increase of seawater temperature between May and July strongly stimulated egg development. The eggs became mature in the summer, when water temperature reached a peak. When water temperature dropped again, egg sizes (mean diameters and volumes) decreased rapidly. The formation and maturation of spermaries (clusters of sperm cells) occurred at the same moment, from June to October, when seawater temperatures were between 19 and 24°C. They did not find any spermaries after October, when water temperatures fell too low.

Figure 4: Dendronephthya gigantea reproduction cycle, which showed seasonal fluctuations in monthly mean diameter and egg volume. This was correlated to  seawater temperature. Higher temperatures stimulated egg development and the release of planula larvae (indicated by red arrows). The production of sperm cells following a similar, seasonal pattern (modified from Hwang & Song, Marine Biology, 2007).

Dendronephthya larvae

D. gigantea was found to be a gonochoric brooding species, which released developed larvae into the water column. Histological analysis showed that gonads were produced from the mesenteries, which then migrated and matured in the polyp. The larvae also developed here. The scientists found that D. gigantea colonies released larvae between July and September during full and new moon, when their sex organs were fully developed. This was also the time when water temperatures were highest. It seems that these corals start preparing for reproduction in the fall, and are fully ready when summer temperatures increase. This reproductive aspect might have evolved because food availability for offspring is highest during this period. Interestingly, fragments which were kept in aquaria for prolonged time released larvae continuously, which was not related to moon cycles.

The corals released larvae of various shapes and sizes; both contracted and elongated ones, which were orange- and yellow-colored. The length and diameter of the orange and yellow planulae ranged from 600 to 1,500 µm and from 200 to 300 µm, respectively. The larvae showed contraction and extension, which caused them to become round or cylindrical (figure 5). They were also found to be ciliated; these are small hairs which allow them to swim, and many coral larvae show this characteristic. The larvae were also negatively buoyant after release; when the corals released them in the field lab, they started swimming along the bottom of culture plates instead of in the water column or close to the surface.

Figure 5: Dendronephthya gigantea. A: Orange–yellow mature eggs in the gastrodermal canals of the coenenchyme of a female colony. B: Free-swimming planula. C: Polyp with tentacle lobes undergoing metamorphosis. D: Primary polyp with pinnate tentacles and well-developed mouth (gc, gastrodermal canal; mo, mature oocyte (egg); pt, pinnate tentacle; tl, tentacle lobe, modified from Hwang & Song, Marine Biology, 2007).

It is known that brooding coral species release developed larvae with negative buoyancy, which might indicate a narrow range of dispersal. This was also true for D. gigantea; its larvae started to settle after only 2 days, which is quite soon compared to other coral species (some corals are known to have swimming pelagic larvae for several weeks). This may be because D. gigantea larvae are lecithotrophic, which means they feed on egg yolk while swimming. They need to settle and start feeding as soon as possible, as their yolk depletes quickly.

After settlement, D. gigantea larvae metamorphosed into primary polyps (figure 5D). One day after settlement, the first tentacles appeared. They were covered in pairs of pinnules; small feather-like structures which filter-feeding corals use to sift out plankton. This observation suggested the primary polyps started feeding. After one week, the polyps were around 2.3 mm (0.09 inches) in size, and showed many white sclerites on their bodies and tentacles. After this stage, they slowly became lighter in color, and after one month the first asexual budding of polyp clones was observed. 

In contrast to brooding coral species, broadcasting ones such as D. hemprichi have larvae which are able to settle after 2 months, which means they have a long so-called competency period (figure 6).  Broadcasting species usually have eggs with positive buoyancy, which float near the water surface, and larvae with pelagic development. During their development, these larvae can feed and many species ingest Symbiodinium algae ( zooxanthellae, which provide the larvae with glucose by means of photosynthesis) before they settle. This allows them to have a long-competency period and the potential for wide dispersal.

Figure 6: D. hemprichi development, a broadcast spawner. Left: A sperm cell  fertilizes an egg cell (scale bar 1 µm). Middle: A 32-cell stage embryo. Right: A developed planula larvae covered in cilia, which is ready to swim and settle on the reef (modified from Dahan & Benayahu, Invertebrate Biology, 1998).

Coral reproduction; as flexible as the weather

Corals really are highly diverse when it comes to their reproduction; some are brooders, whereas others broadcast sperm and eggs. They can be gonochoric, hermaphrodites, sequential hermaphrodites and even parthenogenetic. Soft corals are no exception to this rule. Although gonochorism is known to be the dominant reproductive pattern in most soft corals, a few species from the Alcyoniidae and Xeniidae families have been shown to be hermaphroditic ^2,3,4.  Furthermore, D. gigantea is a gonochoric brooder, whereas D. hemprichi is a gonochoric broadcaster, already showing diversity of reproduction in a single genus.  The scientists theorize that Dendronephthya gigantea reproduction is mainly controlled by seawater temperature/algal blooms (production of eggs and sperm) and moon cycles (release of larvae). 

References:

1.     Cordes EE, Nybakken JW, VanDykhuizen G (2001) Reproduction and growth of Anthomastus ritteri (Octocorallia: Alcyonacea) from Monterey Bay, California, USA. Mar Biol 138:491–501

2.     Sebens KP (1983) The larval and juvenile ecology of the temperate octocoral Alcyonium siderium Verrill. I. Substratum selection by benthic larvae. J Exp Mar Biol Ecol 71:73–89

3.    Benayahu Y, Weil D, Kleinman M (1990) Radiation of broadcasting and brooding patterns in coral reef   alcyonaceans. Adv Invertebr Reprod 5:323–328 Benayahu Y, Weil D, Malik Z (1992) Entry of algal symbionts into oocytes of the coral Litophyton arboreum. Tissue Cell 24(4):473–482

4.      Hwang S-J, Song J-I (2007), Reproductive biology and larval development of the temperate soft coral Dendronephthya gigantea (Alcyonacea: Nephtheidae), Marine Biology, 152:273–284

5.  Dahan M, Benayahu Y (1998), Embryogenesis, planulae longevity, and competence in the octocoral Dendronephthya hemprichi,  Invertebrate Biology 117(4): 271-280.