Pseudopterogorgia Elizabethae

Gorgonians

Gorgonians are predominantly arborescent in nature, though a few single unbranched stem species do occur; they are subdivided into two main groupings determined by the nature of their central supporting structure. In the Scleraxonia this is called an axis and in the Holaxonia, the central core. This structure is a product of the coenenchyme, and as such is produced by the colony, not the polyps. The Scleraxonia have a central axis composed of horny material called gorgonin, and usually, fused sclerites, while the Holaxonia central core has an horny outer layer (cortex) surrounding a chambered core of gorgonin; sclerites are not present (Bayer, 1973; Hyman, 1940). The coenenchyme can be thick in some genera (e.g. Eunicea) and thin in others (e.g. Pseudopterogorgia). It contains the gastric cavities of the polyps, solenia, axial canals and sclerites.

Polyps extend roughly half their length into the coenenchyme and are connected by solenia running throughout the coenenchyme and along the axial skeleton. In some genera the polyps are completely retractable into an inconspicuous calyx (e.g. Plexaurella) while in others they retract into a very pronounced

Figure 1.6 Longitudinal and cross-section views of a typical gorgonian

Modified from Bayer et al., 1983

Coenenchyme

Gastric cavity Peristome

Pharynx

Coenenchyme

Gastric cavity Peristome

Pharynx

Pinnules

Gorgonian Bayer

Axial sheath

Pinnules

Axial sheath and heavily spiculated calyx (e.g. Muriced). The calyx can often be surround with sharp calcareous spines and lids that can cover the calyx w hen the anthocodium retracts (e.g. Muriced). Spicules often extend onto the anthocodium and can form eight sharp spines, one along each tentacle. The polyps can be distributed all over the branches, restricted to just the edges or just on one side of the branch, depending on the genus. Polyps are rarely dimorphic; most notably in the precious red coral Cora Ilium (Hyman, 1940). Gorgonians are the most plant-like of corals, and their bright colours (yellow, orange, purple, red, and blue ) add a brilliant touch to the underwater landscape. Colonies can be feather-like, bushy or fanlike, branching in only one plane or in several. See chapter seven for descriptions of the genera commonly kept in aquaria.

Order Pennatulacea (Sea Pens)

Sea pens represent the highest order of octocoral in terms of colonial complexity, polymorphism of polyps, functional specialization of polyps and colonial integration (Bayer, 1973). In fact, sea pens act as "super organisms" with various types of polyps performing circulatory and respiratory functions, feeding and digestion, sexual reproduction, and locomotory functions (Bayer, 1973). They tend to form fleshy colonies with a single axial polyp, from which numerous secondary polyps arise. The secondary polyps are dimorphic. The axial polyp usually lacks an anthocodium on the end and is divided into two regions. The peduncle is devoid of anthocodia and is used to anchor the colony into the substrate, usually fine sand or mud. There are no stolons or filaments on the end to anchor the peduncle but there may be a bulb-shaped swelling on the tip. The other end, the rachis, bears the secondary polyps. In the more primitive genera such as the one covered in this volume, Cavernularia, the rachis is a rounded cylinder that bears polyps on all sides. More advanced genera show the characteristic feather quill shape from which the common name Sea Pen was derived. For more detailed information on sea pen anatomy we recommend Hyman (1940) or Bayer (1973).

Reproduction

Reproduction in octocorals is a poorly understood topic. There have been scattered notes and papers in the scientific literature, but it was not until the early 1980's that detailed papers began to appear dealing with sexual reproduction in Alcyonium, Heteroxenia, Lobophytum, Parerythropodium, Sarcopbyton and Xenia. In fact, it was only recently discovered that soft corals on the Great Barrier Reef, Australia exhibit the same type of mass spawning event in November as do stony corals (Alino and Coll, 1989).

As in all other anthozoans, octocorals can reproduced both asexu-ally and sexually. Unfortunately, it appears the scientific community has a greater interest in sexual methods, while hobbyists are more interested in using the magnificent recuperative powers and growth rates of octocorals to propagate colonies asexually, for sale and trade among ;:ellow aquarists. However, the vast potential of sexual reproduction for aquaculture should not be dismissed! A South Pacific species of Heteroxenia at the Waikiki Aquarium in Hawaii regularly releases brooded planulae that settle out all over the stony coral culture tanks and within the exhibits. Internal and external brooders offer the potential of vast numbers of planulae each year for grow-out. Heteroxenia is especially attractive for culturists in that it is hermaphroditic (Benayahu and Loya, 1984a). Many of these brooding corals can be made to expel planulae simply by handling.

Asexual Reproduction

The most commonly encountered form of octocoral reproduction seen in aquaria is asexual. In the wild, asexual reproduction has been recognized as an important mechanism for increasing local numbers of individuals in octocorals (Lasker, 1990). With few exceptions, all octocorals have the capacity to reproduce asexually. This most commonly occurs via colony growth and fragmentation. Other methods include budding, transverse fission and pedal laceration. Colony growth will be discussed later in this chapter.

Fragmentation

Soft corals employ many different means of fragmentation to advantage. In arborescent genera, the most common method is a general pinching-off of a branchlet with five to eight polyps by the parent colony. This then falls to the substrate and reattaches rapidly. In Dendronephthya, this is a very common occurrence (Dahan and Benayahu in prep.). The fragment attaches by means of numerous fine filaments that extend from the base. These "root" the fragment to the substrate and full attachment continues via growth, a process that takes approximately ten days in Dendronephthya hempnchi (Fabricius et al., 1995b). A similar process occurs in some Caribbean gorgonians. Small branchlets are produced and then through fission they are pinched off of the mother colony, to settle and growT nearby (Lasker, 1990). It is likely that this occurs in several other soft coral genera. This tendency to produce lots of fragments naturally can be used by indigenous peoples to easily collect fragments for sale. Simply placing loose rubble rock, PVC plates or ceramic tiles underneath and around colonies on the reef, would yield hundreds of fragments, ready for shipping.

Other genera such as Alcyonium, LenmaUa, Litophyton, Nephthea and Scleronephthya may encourage the growth of algae as a means of separating the growing tip of a branch. Strands of filamentous algae grow around a branchlet and constrict it until it effectively pinches off. The severed branchlet then falls to the bottom or is carried away by currents to land and reattach elsewhere. Some soft corals such as Capnella employ swelling ol branch tips with water just before they become severed from the original colony.

Xenia and Anthelia species, which grow quickly and spread over substrates via vegetative budding of polyps and daughter colonies, also employ two special forms of fragmentation to help spread the colonies over longer distances. They spontaneously fragment individual polyps that are carried away in the currents. About midway down the column of each individual polyp a flattened swelling sometimes forms indicating the site of imminent fission. The separated polyps readily attach to substrates downstream and form new colonies. Other times Xenia spp. employ fragmentation of the colony stalk or encrusting tissue. This technique resembles transverse fission in anemones. As soon as the polyp-bearing capitulum is thus naturally severed it is apparent that the nubby stalk left behind already has some polyps branching out (Matt Rigberg, pers. comm.).

Fragmentation can also occur through the actions of predators and natural disturbances such as storms and hurricanes (called typhoons west of the International dateline). The attack of predatory7 worms, snails, fish, etc. on Sarcophyton (Leather corals), may destroy the individual colony, but produce numerous offspring from the remaining tissue. Natural disturbances would do much the same but on a massive scale. Some Sarcophyton spp. will also form necrotic areas on the head

Sarcophyton spp. form necrotic areas on the capitulum and the partially severed fragments that result may drop off and form new colonies. J. C. Delbeek ss

JVH j'fffi

MGv if*

Stab jrT

A colony of Tubipora growing on a submersible pump at Tropicorium in Romulus Michigan. This young colony may have been produced by sexual reproduction in the aquarium. Alternately it may be the result of asexual reproduction, wherein a fragment of the stolon from another colony drifted in the water and became sucked against the intake screen of the pump, sending out new polyps and encrusting tissue from there. J. Sprung

Water Pump Intake Screen

that can cause a "flap'1 of tissue with polyps to separate and settle to the bottom, and a new colony may thus be formed.

Fragmentation occurs in genera with creeping stolons and runners such as Briareum, Clavularia and Pachyclavularia and Tubipora. In this case the stolon or runners, become gradually thinner and eventually sever. The stolon can also be severed by falling rocks or coral heads, and storm-tossed rubble.

Budding

In massive octocorals such as Sarcophyton, budding off of new individuals is not uncommon. These usually appear near the base of the stalk but can also occur just under the edge of the capitulum (see

Sarcophyton in chapter 7). If budding at the base occurs when the colony is still small, the offspring and the mother colony can grow together to form multiple stalked colonies. If they occur when the mother colony is very large, these small offspring tend to remain stunted in growth due to shading by the mother colony.

Fission

As we mentioned earlier, some soft corals such as Xenia spp. exhibit transverse fission, wherein the top of the colony bearing

Sarcophyton sp. budding a daughter colony from the column. S. W. Michael

Xenia exhibiting reproduction via longitudinal fission. J. C. Delbeek

Xenia Splitting

the capitulum becomes severed from its base. Another form of fission occurs as soft corals develop a thick stem. Splitting of a soft coral lengthwise is most common in arborescent genera such as Alcyonium (Colt coral), Lemnalia, Litopbyton, Nephtbea, etc. and in Heteroxenia and Xenia. In this case, the stalk will begin to split towards the base along a vertical line between its two largest branches, eventually resulting in two equally-sized corals. While in Xenia spp. it may occur within as little as one week, this process usually takes several months to complete fully. Branching Cladiella spp. and Sinularia spp. are also known to split this way.

Pedal laceration

Some soft corals such as Cladiella, Nephtbea and Xenia are known to actually move across the substrate, trailing basal tissue. This tissue can remain attached or become detached, eventually developing into a new coral.

Sexual Reproduction

Soft corals employ different modes of sexual reproduction. Some species have separate sexes, while in other species the colonies contain both male and female germ cells. When egg and sperm unite a planula larva is formed. This larva may be brooded or it may form in the water column. Many species spawn seasonally, while some spawn year round according to cues such as moon phase and tide. The vast majority of octocorals have separate male and females colonies (gonochoric /dioecious), however, a few hermaphroditic species of Heteroxenia (e.g. H. elizabethae, H. fuscescens, H. ghardaqensis) and Xenia (e.g. X. viridis) have been identified (in Benayahu and Loya, 1984a).

There are three main modes of sexual reproduction: 1) broadcast spawning with external fertilization; 2) internal fertilization with planula brooded internally in special endodermal pouches and then released through specialized structures and; 3) internal fertilization with planulae then brooded in external pouches on the surface of the polyp and then released.

Broadcast Spawning

Broadcast spawning involves the release of gametes into the water column where fertilization then occurs. Broadcast spawning has been reported in species belonging to the following genera: Alcy-onium, Cladiella, Dendronephthya, Lobopbytum, Sarcophyton and Sinularia (Alino and Coll, 1989; Benayahu and Loya, 1986; Fabricius et al., 1995b; Yamazato et al., 1981). It appears that this form of sexual reproduction is the most common in alcyoniids.

Gonads are located on the mesenteries of the autozooids. In studies of Lobopbytum, Sarcophyton and Sinularia it was found that egg devel-

This "Colt Coral," Alcyonium sp„ has eggs visible within its translucent branches. R. Mascarin

Pictures Isolated Polyp Alcyonium

Tentacles

Figure 1.7

Longitudinal section through polyps of Lobo-phytum crassum showing the developing eggs and testes in the autozooids From Yamazato eta!., 1981

Female

Testes

Tentacles

Oocytes (small group)

Oocytes (large group)

Testes

Female

Pseudopterogorgia elisabethae, a Caribbean photosynthetic gorgonian, produces white eggs from November through December. These fall off the colony like sand grains. We have never witnessed the release of sperm in this species, nor the formation of planulae. J. Sprung

Photosynthetic Gorgonians

opment generally takes two years as opposed to one year for sperm (Alino and Coll, 1989; Benayahu and Loya, 1986; Yamazato et al., 1981). Therefore it is not unusual to find different size classes of eggs in the same polyp. Eggs vary in size, but they are generally much larger than stony coral eggs and can range from 625 um in Lobophytum spp. to 810 um in Sinularia spp. (Alino and Coll, 1989; Yamazato et al., 1981) Spawning usually takes place after dusk one to two weeks after the full moon, in the spring and/or summer. Fertilization occurs within a few hours, within forty eight hours planulae are formed, and one to two weeks later settlement occurs (Alino and Coll, 1989; Benayahu and Loya, 1986; Yamazato et al., 1981). Reproductively mature polyps occur on inner branches in arborescent species of octocorals such as gorgonians, and near the centers of massive soft corals such as Sarco-pbyton and Lobophytum (Yamazato et al., 1981). See chapter seven for further descriptions of reproduction in each genera.

Internal Brooding

In the Xeniiclae family several genera have been shown to brood their planulae internally. Fertilization occurs within the gastrovas-cular cavity and the planulae begin to develop there. Eventually they move into pouches within the mesogloea called inter-siphonozooid spaces, to complete development (Zaslow and Benayahu, 1996). Both Heteroxenia spp. and Xenia spp. employ this mode of reproduction. Most gorgonians are also internal brooders (Brazeau and Lasker, 1989). Release of mature planulae occurs a few hours after dusk in the spring and summer, usually a few days after the new moon. In some cases planulation occurs year round e.g. Heteroxenia fuscescens. See chapter seven for detailed descriptions of reproduction in each genera.

External Brooding

This method is very similar to internal brooding except the fertilized eggs are shed from the polyp and adhere to the side in mucus pouches where they then develop until they are released. Planulae are released a few hours after dusk. This method was first discovered in the alcyoniid coral Parerythropodhim fulvum fulvum and has since been observed in Briareum, CI avid aria, Efflatounaria and Pachyclavularia, and the Mediterranean gorgonian Paramuricea clavata (Alino and Coll, 1989; Benayahu and Loya, 1983; Coma et al., 1995). See chapter seven for detailed description of reproduction in these genera.

Asexual Planulae (Parthenogenesis)?

Brazeau and Lasker (1989) reported a population of Plexaura sp. off the coast of San Bias Islands, Panama that did not contain any male or hermaphroditic colonies, yet the females produced fertile eggs four to seven days after the full moon every month from May to July. The eggs would begin to develop just prior to or at the time of release. These observations suggest that this particular species reproduces parthenogenetically, producing planulae asex-ually, A subsequent study by Lasker and Kim (1996 1 showed that very few planulae were produced parthenogenetically. The vast majority were the product of broadcast spawning and external fertilization. Male colonies were more common at other sites than in the previous study, and were observed in aquaria to release barely discernable wisps of sperm.

Defense Mechanisms Chemical Defenses

Octocorals produce a wide range of chemicals that play an important role in their ecology, particularly in competition for space, ant i-fou ling against algae, defense against pre elation and in enhancing reproductive success (Sammarco, 1996). Although the structures of these compounds are chemically simple, their function cannot be easily predicted from their structure (Sammarco, 1996). For example, many of these chemicals are specific in their actions i.e. they may affect certain stony corals but not others (see Sammarco et al., 1983). Also, not all species within a genus are equally susceptible. For example, Xenia puertogalerae was shown to allow7 the planula of Acropora spp. to settle amongst its colonies and develop (Atrigenio and Alino, 1996). However, in another study (Coll et al., 1985) showed that a species of Xenia severely affected the growth of Acropora spp. Finally, the func tioning of many of these compounds now appears to be synergistic. That is on their own some appear to have little affect but when combined with other secondary metabolites, they exhibit certain abilities. Therefore it is entirely possible that these need to be tested in the presence of other secondary metabolites and perhaps in the same concentrations as found in the host tissues, in order to exhibit any bioactivity (Sammarco, 1996).

Figure 1.8

A summary of ecological interactions in soft corals that are mediated by secondary metabolic compounds Modified from Coll and Sammarco, 1986

Many of the chemical compounds found in corals (both soft and stony) as well as zoanthids, sea cucumbers, gastropods, and anemones have both medical and commercial potential. For example, Sinularia flexibilis contains anti-cancer diterpenes such as sinularin and dihydro-sinularin. Mycosporine-GLY from stony

Defenses

Toxicity Payability* Morphology

Reproduction and Growth

Sexual Reproduction

External Fertilization s - planktonic planulae

External Brooding of Planulae

Prédation

Reproduction and Growth

Sexual Reproduction

External Fertilization s - planktonic planulae

External Fertilization

Specialists co-evolved predators Ovula ovum

Phyllodesmium longicirra

Soft Coral

External Brooding of Planulae

Specialists co-evolved predators Ovula ovum

Phyllodesmium longicirra

Competition for Space

Hard Coral

Soft Coral

( Hard Coral

Figure 1.9 Structures of diterpenes isolated from Sinularia flexibilis and Lobophytum sp.

Modified from Aceret et ai, 1995

Was this article helpful?

0 0
The COMPLETE guide to Aquariums

The COMPLETE guide to Aquariums

The word aquarium originates from the ancient Latin language, aqua meaning water and the suffix rium meaning place or building. Aquariums are beautiful and look good anywhere! Home aquariums are becoming more and more popular, it is a hobby that many people are flocking too and fish shops are on the rise. Fish are generally easy to keep although do they need quite a bit of attention. Puppies and kittens were the typical pet but now fish are becoming more and more frequent in house holds. In recent years fish shops have noticed a great increase in the rise of people wanting to purchase aquariums and fish, the boom has been great for local shops as the fish industry hasnt been such a great industry before now.

Get My Free Ebook


Post a comment