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Student Project

Aquilonastra byrneae

Sophia Hannah Martin 2018


Asteroidea is a class in Echinodermata commonly known as sea stars, or starfish. Asteroidea are found in all the Earth's oceans, and they can inhabit any depth due to a lack of internal gas pockets (Hickman et al., 2014). Like all other members of Echinodermata, asteroidea is exclusively marine due to the inability to osmoregulate (Hickman et al., 2014). Echinoderms are interesting, because despite being a mobile organism, they have radial symmetry. The discovery of adult, bilaterally symmetrical echinoderms from the Cambrian period confirms that this pentaradial symmetry was a derived characteristic (Hickman et al., 2014).This is thought to be beneficial as they live a benthic lifestyle (Raff & Byrne, 2006).

Aquilonastra byrneaeI is a species of asteroid a part of the family Astrinidae, and was first described by O'Loughlin & Rowe (2004). The family Astrinidae has 147 species worldwide, with 42 occurring off the coast of Australia (Byrne & O'Hara, 2017). Astrinidae is difficult family to identify, because the adult forms often have extremely similar morphology. This often means that there is trouble with traditional taxonomic characteristics, such as pedicillaria and spines (O'Loughlin, P.M., & Bribiesca-Contreras, 2015; Gale, 2013). However, astrinidae has the most diverse variety of life histories known thus for, within Asteroidea. This could be an indicator that the evolution of life history exerted considerable pressure upon speciation (Byrne, 2006). Thus, when identifying members of the family, it is prudent to focus on developmental characteristics (O'Loughlin & Rowe, 2004).

A. byrneae is found on the underside of intertidal benthic rubble, and graze on the biota located there. They can be found in bands, at the 1.5m line above Lowest Astronomical Tide (Byrne & Walker, 2007). The specimen used in this project was collected at Heron Island. Until recently, A. byrneae was often confused with A. cepheus , a species with a similar geographical distribution and morphology. However, the discovery of oral gonopores on A. byrneae (instead of aboral), prompted a taxonomoic revision (Byrne & Walker, 2007). The known distribution of A. byrneae thus far is limited to the Southern Great Barrier Reef (Living Atlas of Australia; O'Loughlin & Rowe, 2004). This could be due to their reproductive strategy, which, unlike many members of Asteroidea, lacks a pelagic distribution stage. They have benthic non-feeding larvae (a derived characteristic) (Byrne & Walker, 2007). Thus, their dispersal ability may be quite limited (Byrne, 2013).

Figure 1

Physical Description

Aquilonastra byrneae is a five-rayed asteroid with colouring that ranges from mottled brown, orange, cream, olive green, to red (O'Loughlin & Rowe, 2004). Individual size varies dependant on sex, the males were smaller with an average diameter of 8mm , females were 15mm (Byrne & Walker, 2007). As the specimen used for this project measured 14mm in diameter, it is likely that this individual is female. They are dorsoventrally flattened, with their oral side facing downwards. Their ossicles on the aboral surface tended to have 3 curved ends (fig.2). No pedicellaria are present. The significant diagnostic feature of this species is their oral gonopores (fig. 3, 4 & 5) (O'Loughlin & Rowe, 2004).

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Figure 5


Aquilonastra byrneae live in distinct bands in the upper intertidal zone (1.5m above Lowest Astronomical Tide). A. byrneae occupies the underside of coral skeletons, grazing on the biota (Byrne & Walker, 2007). Reef rubble is made of dead coral, which was shifted onto the reef crest by persistent wave action (Shannon et al., 2013). Unfortunately, little scientific research has been completed on this habitat, making it problematic to understand their ecology (Byrne & Walker, 2007).

A. byrneae appears to be a solitary species, as it was uncommon to find more then one conspecific under the same boulder. They were found under large boulders, which ranged from 200cm2 to 780cm2. Their abundance was 1.4 individuals per 4m2. Unfortunately, their ecological role and importance is largely unknown (Byrne & Walker, 2007).

Life History and Behaviour

The family astrinidae has the greatest diversity of life histories within asteroidea (Mercier & Hamel, 2013; Byrne, 2006). Aquilonastra byrneae , like the majority of asteroidea, has a biphasic life cycle with a larval and an adult phase. However, unlike many asteroidea (who are broadcast spawners), A. byrneae lays negatively buoyant eggs on the benthos that need to be fertilised by a direct deposition of sperm (Byrne 2006; Byrne 2013). Byrne and Walker, (2007) found that there was sexual dimorphism, where the males were smaller in diameter (8mm) compared females (14mm), which caused them to suspect that A. byrneae is a protandric hermaphrodite. This means that they start out in life as a male, and then turn into a female. Byrne and Walker, (2007) found that they had mature gonads in May/June, indicating that they would be ready to lay eggs in Winter. By November, gonads had shrunk considerably no gametes were evident. The A. byrneae specimen was dissected in May, and was found to have large gonads which was consistent with Byrne & Walker, (2007).

There are four typical mechanisms that enable successful breeding: (1) aggregation, (2) asynchronous release of gametes by females and males, (3) simultaneous mass spawning, or (4) some form of communication before or during the spawning event (Mercier & Hamel, 2013). As A. byrneae is solitary, logically they would employ one or more of these to maximise fertilisation success. However, there has not been any research into this area to determine this.

A. byrneae produces lecithotrophic (yolk bearing) eggs, which also makes them less fecund. This is because lecithotrophic eggs require more energy to make (Byrne, 2006). We know this, because their eggs are 420μm in diameter, which makes them large. A large egg is defined as >300μm, while a small egg is defined as 100-150μm (Byrne, 2013). These eggs give rise to benthic brachiolaria larvae which are highly modified to attach to the substrate (Byrne & Waker, 2007; Byrne, 2013). Interestingly, the chemical composition of the egg reflects life history. Lecithotrophic benthic eggs are high in yolk proteins, while lecithotrophic pelagic eggs are high in lipids. The biochemistry gives negative buoyancy and positive buoyancy respectively (Byrne, 2013).

The larval part of the life cycle is often the least understood, and A. byrneae is no exception (Metaxis, 2013). Typically, after an asteroid benthic egg mass hatches, there are two options. Either the larvae develop on the substrate unattended, or they are attended to by an adult of the species (Byrne, 2013). As brooding is a less common life strategy throughout asteroidea, and has yet to be discovered within the genus aquilonastra, it is less like that A. byrneae are brooders (Byrne, 2006). It is also worth noting that A. byrneae cannot reproduce asexually through fission as many other asteroidea can (Byrne & Walker, 2007).

Anatomy and Physiology

Digestive System:

The stomach is located above the mouth and has two chambers, the cardiac and the pyloric chamber. The cardiac chamber can be everted through the mouth throughout feeding, and is located above the mouth (fig. 6). The pyloric chamber is directly above the cardiac chamber, and is connected pyloric ceca (which are digestive glands) in each ray (Hickman et al., 2014).

Respiration and excretion:

Asteroids have no dedicated circulatory or excretory systems (O'Hara & Byrne, 2017). Gas exchange and nitrogenous waste excretion occurs primarily through diffusion through the thing walls of the tube feet and papulae (Hickmen et al., 2014). Papulae protrude through the ossicles on the aboral surface, and contain extensions of the coelom which increases the surface area for respiratory exchange (O'Hara & Byrne, 2017) ( fig. 4, 6 & 7).

Nervous system:

The nervous system is pentaramously symmetrical and has three components. The circumoral nerve ring is located in the central disk, connecting to five radial nerves which follow the ambulacral grooves (Hickemen et al., 2014). This central nervous system is highly derived, but has homologous linking to that of the chordates (Byrne & O'Hara, 2017).

Sensory reception:

Asteroidea rays are a key part of how they sense the world around them. One the oral side, there are open ambulacral grooves which have numerous disc ended tube feet which are equipped with sensory cells. These cells can be either limited to the disc ending, or they can be scattered further up the tube foot (Hennebert et al., 2013). The tips of the asteroid arms are a particularly important aspect of the sensory system. They have elongated, knob ended tube feet which are constantly moving. These play a role in chemo and mechano-sensing (Hennebert et al., 2013). The tip of the arm bends upwards and at the very end of the ambulacral grooves, , there is an eyespot. (fig. 2 & 8). This is a compound eye lacking a lense. Up until recently, there had been no research done on Asteroidea eyesight. Garm & Nilsson, (2014) completed experimental research on the visual behaviour and navigation on the species, Linkia laevigata . Garm & Nilsson, (2014) found that they had slow, colour blind eyes which were capable of image formation. Asteroidea are the only echinoderms (except for one species of Holothurian) which have image forming eyes. The primary colour that L. laevigata could see were blues. Their maximum light sensitivity was 450nm, which is the same wavelength as the open ocean. Therefore, the open ocean would appear bright to them. In comparison, coral blocks were would appear dark, as they were only the lower end of their visual spectra (Garm & Nilsson, 2014). When relocated, L. laevigata could navigate itself back to the reef .This behaviour only worked when close to the reef. Therefore these eyes would not be useful when seeking new habitat. Due to the slow and colour blind nature of the eyes, it was determined that they were unsuitable for predator avoidance or predation (Garm & Nilsson, 2014). Unfortunately, there has been no research completed on the eyesight and visual behaviour ofA. byrneae . As their environment is different to that of the reef asteroid, L. laevigata, it is possible that their eyes would have different adaptations to light spectra. This is potentially a future area of research.

Water vascular system and tube feet:

The water vascular system is a coelomic compartment which is unique to the echinodermata (Hickman et al., 2014). It has multiple components which include the madreporite, the stone canal, the ring canal (central disk), the radial canals (each ray), ampullae and the tube feet (fig.9)(O'Hara & Byrne, 2017). These form a hydraulic system which is used for locomotion, foraging, respiration and excretion (Hickman et al., 2013). The madreporite is located on the aboral surface, and opens the water vascular system to the outside via small pores. The madreporite connects to the ring canal via the stone canal. The radial canals diverge from the ring canal into the ambulacral grooves of each ray (fig. 10) (Hennebert et al., 2013). Also attached to the ring canal are polian vesicles and Tiedemann's bodies. These are much smaller then the radial canals, and don't leave the central disk of the star. The number of Tiedemann's bodies and polian vesicles can vary from species to species. Polian vesicles serve as fulid storage, while Tiedemann's bodies produce coelomocytes. The radial canals connect to numerous lateral canals, which are connected to the tube feet (Hickman et al., 2013). As tube feet are hydraulic structures, they require fluid to be maintained at sufficient hydrostatic pressure to function (Hennebert et al., 2013). The fluid in the water vascular system is similar in composition to seawater, but has higher quantities of potassium and is hyperosmotic. Thus, the required hydrostatic pressure is generated by the hyperosmolarity of the fluid. This then causes water to enter through the wall of the water vascular system (Prusch & Whorisky, 1976).

Tube feet morphology also indicates the environment that the asteroid lives in (Vickery & McClintock, 2000). A. byrneae has a suckered tube foot adapted to rocky environment (Santo et al., 2005). The only order with tube feet that aren't suckered are Paxilloda. They instead have pointed tube feet, which are adapted to an infaunal habitat (Hennebert et al., 2013).

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Figure 10

Biogeographic Distribution

Aquilonastra byrneae has thus far only been found on the Southern Great Barrier Reef.

Evolution and Systematics

All extant asterinids are from the same lineage of neoasternids, which first appeared in the early Triassic. Their radial bauplan is derived, because they come from a bilaterally symmetrical ancestory (Byrne, 2013). The most basal group within the neoastrids is the order, paxillosida (Gale, 2013). They are the only members of asteroidea with non-suckered, pointed tube feet. They also lack brachiolaria larvae. Paxillosida live as infaunal predators in soft sediments (Gale, 2013). Traditional phylogeny has been primarily based off the morphology of spines and pedicellaria. This has been problematic, due to homoplasy (which is the independent evolution similar morphology). Even the advent of molecular data did not clear up the confusion (Gale, 2013). However, there were a few congruencies between the molecular and morphological data. The orders paxillosida and forcipulatida are monophyletic groups, and astrinidae is sister taxa to solarista (Gale, 2013).

Sensory system phylogenetics

The appearance of image forming eyes within asteroidea is unusual. Garm and Nilsson (2014), only completed the study on one species, Linkea laevigata. As this is a singular species, one must be careful before drawing conclusions for the rest of asteroidea, which is why further research should be completed in this area. However, they did find some intriguing results. L. laevigata had a combination of the two major types of photoreceptors. These are not normally found in the same organism (Garm & Nilsson, 2014). The two main types of photoreceptors are rhabdomeric and ciliary (Plachetcki et al., 2005). The traditional theory was that these arose from two separate evolutionary lines (Eakin, 1979). Ciliary photoreceptors are traditionally associated with invertebrates, and rhabdomeric photoreceptors are traditionally associated with vertebrates. However, more recent molecular research suggested that the two cell types are probably homologous (meaning that they can be traced back to the same common ancestor) (Plachetcki et al., 2005). As the echinoderms are an early branch within the deuterostome clade, the appearance of their eyes might help scientists understand the divergence of the two cell types (Garm & Nilsson, 2014). Both photoreceptors increase the surface area which can be exposed to light, but they achieve this in different ways. Rhabdomeric photoreceptors use microvilli which are membrane covered tubes. Ciliary photoreceptors use cilia, which are small hair-like structures on the surface of the cell (Plachetcki et al., 2005). L. laevigata demonstrated visually guided behaviour to return to its preferred habitat (Garm & Nilsson, 2014).

Astrinidae phylogenetics

Astrinidae is a unique family with asteroidea. They are characterised by morphologically similar adults, that have a great diversity of life history strategies (Byrne 2006). Lecithotrophy arose six times independently, and viviparity, twice (Byrne, 2013). This could represent an overall shift from plankotrophic larvae (ancestral) to lecithotrophic larvae (derived). This shift may have been enabled by the fact that lecithotrophs can only produce descendants with non-feeding larvae (Byrne 2006). There also needed to be a change in the composition of the eggs. Small plankotrophic eggs tend to have high quantities of triglicerides, with low levels of wax esters, while larger lecithotrophic eggs have some triglicerides, but are dominated by wax ester (much higher quantities then plankotrophic). It's thought that the wax esters play a role in the enabling eggs to float (Villinksi et al., 2002). The eggs of A. byrneae have a different composition again due to their negative buoyancy. Thus, eggs size and composition reflects larval ecology (Raff & Byrne, 2006; Byrne, 2013). There are various strengths and weaknesses to being lecithotrophic over plankotrophic. Lecithotrophy is a safer developmental mode, with less loss and less reward. As lecithotrophic eggs are more energetically costly, typically less will produced (Byrne, 2006). However, the larvae have a guaranteed nutrition supply and shorter larvae duration, making them independent of the local plankton supply. These protect against starvation and predation (Byrne, 2013). The downside to lecithotrophy though, is that larvae tend to disperse shorter distances which reduces population connectivety. Plankotrophic species tend to follow a "boom and bust" pattern, which is less stable, as approximately 68% of echinoderms have developed the derived lecithotrophy over plankotrophy. It seems like the evolutionary trend is towards the lower risk, lower gain strategy (Byrne, 2013).

Conservation and Threats

As so little is known about the ecology, developmental mode and geographical distribution of A. byrneae , it is difficult to know the conservation state that the species is in. Further research is needed in all areas of knowledge regarding A. byrneae .


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