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Aquilonastra doranae, O'Loughling & Rowe 2006
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Pauline Corstiana Roos 2017
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Summary | |
Aquilonastra doranae is a micro-species of starfish (Echinodermata: Asteroidea) that was only recently described by OʼLoughling & Rowe (2006). It is part of the family Asterinidae, which are known to thrive in marine aquaria and are often regarded as pests by aquarists. A. doranae was found in large numbers in the University of Queensland aquarium along with other species of Asterinidae. Not much is known yet about some aspects of A. doranae, which presents great potential for research.
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Ecology | |
Like all Echinoderms, A. doranae can only inhabit marine environments (Ruppert et al. 2004). The specimens that were observed in the UQ aquarium were found on the walls of the aquarium or on rock, but not on living substrate. They were most prevalent in the tank containing rubble collected from the reef crest and reef flat on Heron Island. OʼLoughling & Rowe (2006) found their specimen on a shallow seagrass bed near Okinawa (Japan). These two environments are ecologically quite different, however both experience a similar tropical climate.
The range of locations that this species can inhabit indicates that A. doranae is a generalist species, and perhaps this is what allows it to thrive in marine aquaria. Aquarists rely on natural predators such as the harlequin shrimp (figure 4) and Linckia starfish (figure 5) to effectively control numbers of Asterinidae in their aquaria (Reef Central 2009; Parakash & Kumar 2013). It can therefore be expected that these predators also control populations of Asterinidae in the wild.
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Life History and Behaviour | |
* The structures and systems that are mentioned in this section will be further discussed in the ‘Anatomy and Physiology’ section of this page.
Feeding
Like many other asteroids, A. doranae digests its food externally. It does this by moving its mouth over a prey and everting its stomach (Ruppert et al. 2004). The everted stomach secretes digestive enzymes that break down the prey, which is consequently absorbed through the gastrodermis (Ruppert et al. 2004). An example of this can be seen in figure 5, in the previous paragraph, where a Linckia starfish has everted its stomach around an asterina.
A. doranae lacks the claw-like feeding structures, called pedicellariae, that are typical of most asteroid species (OʼLoughling & Rowe 2006). This indicates that A. doranae is not required to manipulate its food before consumption, which points to a diet of organic particulates found on the substrate. This idea is supported by several observations; the small size of this species; the species’ ability to inhabit different ecosystems; and its ability to thrive in marine aquaria.
Locomotion
One of the coelomic cavities in starfish' forms the water vascular system, which includes tube feet that are used for locomotion, among other tasks (Ruppert et al. 2004). The water vascular system is a hydraulic, pentaramous arrangement of internal canals (Ruppert et al. 2004). The tube feet are extensions of the water vascular system on the oral side, and water is pushed into, or withdrawn, from them to move the tube feet in a stepping motion (Ruppert et al. 2004). The video below shows A. doranae using its hydraulic tube feet to move along the side of a petri dish.
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Locomotion of A. doranae along the side of a petri dish (video by Pauline Roos). |
Respiration
Even though a haemal system is present in starfish, it has no role in gas transport (Ruppert et al. 2004). Blood is not actively circulated through the body. Instead, most gas exchange occurs through small structures that are protrusions of the coelomic cavities through the dermal layer and into the water column (Australian Reef Society 1984; Ruppert et al. 2004). Often these structures are referred to as gills. Each coelomic cavity in the starfish’s body has these gills, for example; in the water vascular system the tube feet function in gills; and in the large perivisceral coelom the papulae function as gills (Ruppert et al. 2004).
Excretion
Excretion in starfish occurs at tissue level. Nitrogenous waste is mostly in the form of ammonia, which is excreted through thin coelomic membranes, such as the tube feet or papulae (Ruppert et al. 2004).
Reproduction and development
Asteroids are mostly gonochoric (i.e. separate sexes), and are seasonal breeders (Ruppert et al. 2004). Most are broadcast spawners, where eggs and sperm are released and fertilization takes place in the water column (Ruppert et al. 2004; Byrne 2006). Eggs are mostly lecitotrophic, meaning eggs and larva spend a relatively long time in the water column before settling and metamorphosing into adults (Byrne 2006). The larva of asteroids are of the brachiolaria type, which show some resemblance to the adults in the sense that they show similar shapes (figure 6) (Ruppert et al. 2004; Byrne 2006). Within the Aquilonastra genus a wider variety of reproduction and development strategies has been observed, such as benthic eggs and larva (Byrne 2006). It is therefore uncertain if A. doranae follows the trends described above, or if it has adopted different reproductive strategies.
Most asteroids can also reproduce asexually by means of fission and regeneration. When a ray becomes detached from the body, it is regenerated. Simultaneously the detached ray is capable of regenerating the rest of the starfish’s body, resulting in two complete and identical starfish. It is therefore not uncommon to find A. doranae with more or less than 5 rays. (Ruppert et al. 2004; OʼLoughling & Rowe 2006)
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| Figure 6 |
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Anatomy and Physiology | |
Some specimens that were collected from the UQ aquarium were used to produce microscope slides and to perform dissections. The following anatomical description of A. doranae is based on annotated pictures of these microscope slides and the dissections.
Digestive System
Parts of the digestive system are annotated in figures 7, 9 and 10. The digestive system consists of a through gut, with a mouth and an anus. The mouth is connected to the stomach which consists of two regions; the cardiac stomach and the pyloric stomach. From the pyloric stomach the pyloric ceca extend into each ray, to allow nutrients to be supplied to the distal parts of the body. The digestive system terminates aborally through the anus.
Water Vascular System
Parts of the water vascular system are annotated in figures 7, 8, 10, 11, and 12. The digestive system consists of a net of internal canals. This includes the ring canal in the central disk, from which radial canals extend into each of the rays. Connected to the radial canals are sequences of ampulla and tube feet. The Ambulacural ridge is a calcified barrier that wraps around the radial canals for protection. (Ruppert et al. 2004)
The stone canal leads from the ring canal to the aboral side where the madreporite is located. The madreporite is a specialized porous ossicle that is in contact with surrounding seawater, and lets water into the hydraulic system (Ruppert et al. 2004). Like other Aquilonastra species, A. doranae often has more than one madreporite (OʼLoughling & Rowe 2006).
Nervous System
Parts of the nervous system are annotated in figures 7 and 13. The central nervous system of echinoderms consist of a nerve ring around the mouth and radial nerves that extend into each ray just below the radial canals of the water vascular system. In addition to the central nervous system, echinoderms have two nerve nets; one in the epidermis and one in the coelomic lining. The sensory components of the echinoderm nervous system allow the animal to sense its environment, while the motor components allow for coordinated movement of the ampulla, tube feet, and the body wall muscles.(Ruppert et al. 2004)
A. doranae has various sensing organs. At the tip of each ray there is an eye spot (i.e. ocelli), which contains photo-receptors, and several sensory tube feet, which contain mechano-and chemo-receptors. (Ruppert et al. 2004)
Reproductive System
Figure 7 shows the position in the echinoderm body where the reproductive system (i.e. gonads) would be expected to be found. During the dissections and in the microscope sections, none of the specimens had gonads present. This might indicate that the A. doranae specimens in the UQ aquarium were not breeding at the time of collection. This could be because it was not the breeding season, or because conditions in the aquarium were not favourable for sexual reproduction. It would be interesting to know more about the reproductive strategy of micro starfish like A. doranae, as not much known about this yet (Hart et al. 2004; Byrne 2006).
Body Wall and External Features
Parts of the body wall and other external features are annotated in figures 7 and 14. The dermal layer of echinoderms contains hard calcified structures called ossicles. These ossicles provide body rigidity and sites for muscle attachment (Ruppert et al. 2004). The body of A. doranae is covered in small spines, with long distal spines at the tips of the rays (OʼLoughling & Rowe 2006). The papulae, which are used in respiration, protrude through the thick body wall and are exposed externally.
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| Figure 7 |
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| Figure 15 |
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Biogeographic Distribution | |
When A. doranae was first described by OʼLoughling & Rowe (2006) it was found in the shallow waters around Okinawa, Japan (figure 15). Eventhough this species has never been described to inhabit the Great Barrier Reef, its presence in the UQ aquarium must mean that A. doranae is present here. These two locations experience similar environmental conditions, but it is not clear where the species originated.
If A. doranae originated in Okinawa it would have been very difficult for the species to disperse as Okinawa is quite geographically isolated (figure 15). Adults are small benthic crawlers and do not have a large dispersal range. Even the positively buoyant eggs could not have brought the species to the Great Barrier Reef as surface currents in this part of the world flow northwards. A possible explanation for the spreading of A. doranae to the Great Barrier Reef could be the dispersal of the species through ship balast water. Large container ships often take in balast water and release it at the end of their journey. In doing so they transport panktonic communities, including eggs and larva, between locations.
Alternatively, A. doranae could have originated on the Great Barrier Reef, but has simply been overlooked because of the size and diversity of the Great Barrier Reef, and the small size and camouflage colouring of A. doranae.
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| Figure 15 |
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Evolution and Systematics | |
Currently A. doranae is phylogenetically classified as follows:
Echinodermata (phylum) – Asteroidea (class) – Valvatida (order) – Asterinidae (family) – Aquilonastra (genus) – Aquilonastra doranae (species)
Asteroids are very poorly preserved in the fossil record, making it hard to reconstruct the class’ evolutionary history (Mah & Blake 2012). It is thought that current asteroid diversity radiated from one surviving lineage after the Permian-Triassic mass extinction event (Mah & Blake 2012). On genus, family, and order level the evolutionary history and phylogenetics of A. doranae is still unclear and frequently changed (Hart et al. 2004; Ruppert et al. 2004; Byrne 2006). Therefore there is high phylogenetic research potential for micro asteroids such as A. doranae.
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Conservation and Threats | |
Like all echinoderms, starfish are highly sensitive to changes in ocean chemistry and temperature (Dupont et al. 2010). This is because, as discussed in the ‘Anatomy and Physiology’ section, the coelom of a starfish is in direct contact with the surrounding water, through the papulae, tube feet, and madreporite. Any micro pollutants and contaminants will readily diffuse into the starfish’ body and potentially have lethal effects.
Additionally, as a result of increased levels of atmospheric CO2,ocean acidification is a major concern for starfish as an increasing pH will result in increased dissolution of their calcite ossicles (Dupont et al. 2010).
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References | |
Australian Reef Society (1984), A coral reef handbook, 2nd ed., Australian Reef Society, Brisbane.
Byrne M. (2006), ‘Life History Diversity and Evolution in the Asterinidae’, Integrative and Comparative Biology, 46(3), pp. 243-254
Dupont S., Ortega-Martínez O. & Thorndyke M. (2010), ‘Impact of near-future ocean acidification on echinoderms’, Ecotoxicology, 19(3), pp. 449-462.
Hart M.W., Johnson S.L., Addison J.A. & Byrne M.(2004), ‘Strong character in congruence and character choice in phylogeny of seastars of the Asterinidae’, Invertebrate Biology, 123(4), pp. 343-356.
Mah C.L. & Blake D.B. (2012), ‘Global Diversity and Phylogeny of theAsteroidea (Echinodermata)’, PLoS ONE, 7(4),pp. e35644.
O’Loughling P.M. & Rowe F.W.E (2006), 'A systematic revision of the asterinid genus Aquilonastra OʼLoughlin, 2004 (Echinodermata: Asteroidea)’, Memoirs of Museum Victoria, 63(2), pp. 257–287.
O’Loughling P.M. & Bribiesca-Contreras G. (2015), ‘New asterinid seastars from northwest Australia, with a revised key to Aquilonastra species (Echinodermata: Asteroidea)’, Memoirs of Museum Victoria, 73, pp. 27–40.
Parakash S. & Kumar T.T.A. (2013), ‘Feeding behavior of Harlequin Shrimp Hymenocera picta Dana, 1852 (Hymenoceridae) on Sea Star Linckia laevigata (Ophidiasteridae)’, Journal of Threatened Taxa, 5(13), pp. 4819-4821.
Reef Central 2009, Linckia Starfish, <http://reefcentral.com/forums/showthread.php?t=1745175>, accessed 22/05/2017.
Ruppert E.E., Fox R.S. & Barnes R.D. (2004), Invertebrate Zoology: a functional evolutionary approach, 7th ed., Thomson-Brooks/Cole, Belmont, California.
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