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Pentaceraster regulus (Müller &Troschel, 1842)




Luke Dekkers 2016

Summary

Pentaceraster regulus, first identified by Müller &Troschel in 1842, is a medium sized sea star and the specimens used for this characterisation were observed and collected around Amity Point on North Stradbroke Island around the seagrass habitats (Lat -27.40449, Lon 153.43655).This webpage aims to investigate and subsequently educate about this species, from morphology, life history to evolutionary theory. It is not uncommon for there to be a lack of specific literature at this level of classification and unfortunately P. regulus is no exception. A broader stance on the topics at hand aside from first hand observations shall therefore be utilised to inform. An in-depth analysis of tube foot structures is presented as the focal point of anatomical discussion due to their critical functions and unique status to echinoderms.

Physical Description

Pentaceraster regulus exhibits a robust body form characterised by a thick body with a flattened ventral and convex dorsal side with size ranging from 150mm to 220mm diameter. Pentaradial symmetry is exemplified by 5 arms, extending radially from the central disk, bend upwards towards the tips to allow elevation of sensory tentacles and eye-spots above the substrate. Dorsal plates ( A in Figure 1) are arranged in rows extending outwards from the central disk and also parallel with the lateral margins of the arms. Whereas within the central disk, 5 large plates are pentagonally arranged around the anus (B in Figure 1). Colouration of observed specimens mainly light brown with orange dorsal plates however larger organisms exhibited a wider range of plate colouration with many variations of browns to reds displayed.

For a more-indepth description for scientific purposes please see Hee Lee & Shin (2009)
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Figure 1

Ecology

Habitat

Although P. regulus has a wide geographic spread as detailed in the biogeographic section, the habitat in which the specimens used for this report were collected from was a very shallow seagrass bed, roughly 10m offshore at a depth of around 1-1.5m. This benthic environment was soft, sandy substrate with little sedimentation disturbance due to very low wave energy and high abundance of seagrass.

The Encyclopedia of Life attempts to define a set of environmental parameters within which P. regulus can be located however due to lack of evidence and cited literature to substantiate claims along with our collection occurring 5 degrees south of the defined maximal latitude, the set is deemed inadequate for reporting. 

Commensalism

As a large and relatively slow moving animal, P. regulus serves as a commensal host for other, smaller animals. Observed upon the collected specimens were small ophiuroids identified as Ophiothela danae (Figure 2) and two forms of scale worms, Asterophila culcitae (Figure 3) and Gastrolepidia clavigera (Figure 4A). 
Given these two forms of scale worms displayed reddish colouration and consistently sought to remain under the ventral oral surface around the ambulacral grooves and tube feet, it is hypothesised that their primary nutrient resource is scavenging particulate food as it is passed along or over by the tube feet.
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Figure 2
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Figure 3
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Figure 4

Life History and Behaviour

​Asteroids typically only reproduce once a year, with the mass spawning events resulting in large masses of planktonic larvae. The first larval form is the bipinnaria (figure 5A), a bilaterally symmetrical, suspension feeder. It feeds and moves with ciliated bands, phytoplankton and other particulate food is swept by the generated currents towards the mouth. This forms develops into the brachiolaria (figure 5B), with three adhesive arms and a sucker utilised for settlement. (Rice et al 2006) Metamorphosis involves the shift from ancestral bilateral symmetry to radial symmetry of the juvenile form and subsequent consumption of the brachiolaria tissue by the developing juvenile form. This biphasic lifecycle, consisting of a plantonic dispersal period followed by benthic settlement is iconic to many marine invertebrates and asteroids are no exception.
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Figure 5

Anatomy and Physiology

Internal Structures and Systems

​Body Wall and Skeletal Structure
The body wall can be broadly divided into 4 layers; cuticle, single-layered epidermis, thick mesodermal layer of connective tissue, and the specialised coelomic ciliated myoepithelium. The mesodermal layer binds small calcareous plates, called ossicles, together with collagenous connective tissue and ligaments. A notable characteristic of this particular form of this connective tissue is that if stimulated by the nervous system it can increase rigidity, reducing muscular stress. (Ruppert et al 2004, Lawrence 2013)

​Gas Exchange
Sea stars exhibit specialist gills, called papulae (Figure 5) which are outgrowths of the coelomic fluid through canals in the aboral surface of the animal. Bidirectional flow of the coelomic fluid through these papula allows direct gas exchange to occur and essentially function as gills. The internal coelomic myoepithelium acts to circulate the coelomic fluid, delivering oxygen and removing waste products from the internal organs. the papulae are then capable of excreting the nitrogenous waste through diffusion. Tube feet are observed to have a similar function as well as their role in locomotion. (Ruppert et al 2004, Hickman et al 2014, Lawrence 1987)

Nervous System
The ectoneural system is comprised of a nerve ring around the mouth with large radial nerves projecting into each arm. The hyponeural system lies aborally the ectoneural system, consisting of a ring nerve around the anus​ with radial nerves projecting along the 'roof' of each arm. Finally an epidermal nerve net acts to connect these systems and can respond to tactile stimulation. (Ruppert et al 2004, Hickman et al 2014)


​Reproduction and Regeneration
Most echinoderms are gonochoric, separate sexes, and P. regulus is no exception. Two gonads are present in each arm, and can fill the space supplied if ripe. Ejection of gametes occurs through gonopores along the arm into the surrounding seawater, supporting the idea of mass spawnings in response to some external stimuli. 

If the central disk with madreporite is left undamaged, regenerative processes can lead to a full recovery of the individual. Sea stars can also elect to automatize an arm if significantly disturbed.
(Ruppert et al 2004, Hickman et al 2014)


​Feeding and Digestive Processes
P. regulus was observed feeding through eversion of the stomach through the mouth, controlled by gastric ligaments. Partial external digestion takes place and the nutrients are moved into the upper portion of the stomach, where pyloric ceca in each arm act to further digestive processes. A short intestine leads from the stomach to the anus. (Hickman et al 2014)


Water Vascular System

The water vascular system (WVS) (Figure 6) is arguably the defining feature of echinoderms, a coelomic hydraulic system that performs a critical function in locomotion, respiration and feeding.

Seawater enters the WVS through the madreporite, a calcareous plate on the external aboral surface of the sea star, highly noticeable due to striking difference in colouration compared to surrounding tissue (Figure 8).

The madreporite is filled with many micro-channels,allowing water flow from the external environment to the internal coelomic systems, which is all lined with a ciliated myoepithelium.

This inward flow is controlled by ciliary action within the madreporite, with both inward and outward current generated. This conflicting current system appears to allow the influx of water whilst preventing foreign particulate entry to the system. The presence of many micro-channels as opposed to a large single channel also suggests an evolved function to minimise foreign entrants along with reduced dependency upon a single input (Boolootian 1966)

The water flows from the madreporite into the ring canal via the stone canal, named for its musculature reinforced with calcareous deposits. This ring canal acts as the connection for all fiveradial canals, each one extending along the length of an arm. Also attached to this radial canal areTiedemann’s bodies and polian vesicles, producing coelomocytes (coelomic leukocytes) and acting as pressure-regulation bodies respectively. Lateral canals divert off the radial canals and each one acts as a one-way valve system to an ampulla and tube foot. (Ruppert et al 2004, Hickman et al 2014, Lawrence 1987, Nichols 1967)



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Figure 6
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Figure 7
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Figure 8

Tube Feet and Locomotion

Tube feet consist of 4 tissue layers; the myomesothelium (A),connective tissue (B), nerve plexus (C) and the outer epidermis (D), which enclose the lumen (E) which fills with coelomic fluid. The outer epidermis also typically displays a more concentrated section of glandular cells for adhesion processes to be discussed (F).
They exhibit a variety of distal morphologies depending upon usage, Santos et al (2005) identified 2 main broad morphologies of tube feet; knob-ending and simple disc-ending (Figure 9). They hypothesise that sea stars exhibiting know-ending tube feet engage with softer substrates and undertake burrowing behaviour whereas disc-ending tube feet likely developed for angled surfaces or areas with high energy events, as mechanical suction ability is thought to be enhanced. 

The sensory tentacles at the tips of the arms are thought to be specialised tube feet, with a greater sensitivity to stimuli, likely chemo and somatic. In order to garner an understanding of the structural differences between these two forms of tube feet, histological examination utilising hematoxylin and eosin (H&E) staining for light microscopy (Figure 10 & 11) and DAPI stains for fluoroscopic analysis (Figure 12). However due to the small size of the tube feet, the orientation of samples upon the slides resulted in inadequate representation for a comparative analysis and shall therefore support this generalist discussion upon tube feet. 

Based upon visual analysis of the microscopy results as well as macro-level photography (Video 1) 
P. regulus appears to exhibit simple-disc ending tube feet, offering a greater amount of flexibility for adaption to a variety of substrates and habitats.

Adhesion

The ability for adhesion to surfaces is a critical capability for sea stars as they often inhabit inter-tidal zones with high energy events. The tube feet perform this largely through rapid adhesive mucus production with mechanical suction developing if the animal remains stationary for a period. Specialised glandular cells within the distal epithelium of the tube feet allow for secretion of the adhesive mucus, followed by the secretion of a substance to break it down again, in a relatively rapid sequence.  (Ruppert et al 2004)

Locomotion

Movement in tube feet is generated by extension an attachment of the tube feet to the substrate, force-generation i.e. either pulling or lever-like movements and then detachment and retraction of the tube feet. This process begins with the ampulla contracting and forcing fluid into the tube foot, allowed by the closure of the valve structure within the connected lateral canal. Due to the musculature and nerve plexus, the tube foot is capable of complex control and once it comes into contact with a surface the distal end begins secretion of adhesive mucus and developing mechanical suction. This process is undertaken by each individual tube foot in a highly coordinated manner, giving the appearance of the sea star gliding along the substrate. (Lawrence 1987, Ruppert et al 2004)




Video 1. Short clip illustrating differences between sensory and locomotor tube feet, distal morphology of tube feet and flexibility of the structures
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Figure 9
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Figure 10
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Figure 11
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Figure 12

Biogeographic Distribution

Information gathered from marine species databases shows a wide Indo-Pacific spread of P. regulus. The World Register of Marine Species offers records of collection indicating geographical spread. Locations include; Thailand, Japanese, Chinese and Korean waters, New Caledonia, Bay of Bengal, East Indies, The Philippines and Australian coastal waters. This is broadly defined as Indo-Western Pacific.  

Evolution and Systematics

The 'backwards' shift from bilateral symmetry to a more basal form of radial symmetry is well explained by a historical shift to sessile-suspension feeding, seen in the first echinoderm fossils from the Cambrian period. Currently extant echinoderms can be broadly split into 2 groups, the sessile, oral-end-up Crinoidea and the motile, oral-side-down Eleutherozoa (asteroids, echinoids, holothurians) with the crinoids seen as the basal sister group to the Eleutherozoans. 

Molecular phylogeny studies undertaken by Lafay et al (1995) show a rapid divergence in asteroid lineages at the end of the Triassic, with Valvatida emerging around mid-Jurassic. However the accuracy of these measurements was questioned in light of the rapid divergence leading to differences in phylogeny very difficult to pinpoint. 

The currently accepted systematics of Pentaceraster regulus are as follows:
Kingdom - Metazoa
Phylum  - Echinodermata
Subphylum - Asterozoa
Class - Asteroidea
Superorder - Valvatacea
Order - Valvatida
Family - Oreasteridae
Genus - Pentaceraster
Species - Pentaceraster regulus

Conservation and Threats

Climate change and associated disturbances coupled with anthropogenic-induced effects appear to be the most significant threats to not just P. regulus but many marine invertebrates. The effects of ocean acidification will likely have a negative effect upon the larval stages of asteroids, potentially leading to higher rates of mortality and developmental abnormalities (Dupont et al 2010). However the effects upon adult forms of asteroids as investigated by Gooding et al (2009) supported the concept of increased growth rates in higher temperature and lower pH environments. It could be hypothesised that the factors will likely have differing effects upon different life stages of organisms. 

 Anthropogenic effects such as induced-eutrophication or increased sedimentation will likely have a negative effect upon the habitats and trophic interactions utilised by P. regulus. Although there is no official recognition of a threatened status for P. regulus, steps should still be undertaken to mitigate negative consequences of these impacts for the longevity of all marine fauna and flora. 

References

Dupont, S., Ortega-Martinez, O., & Thorndyke, M. (2010) Impact of near-future ocean acidification on echinoderms, Ecotoxicology. 19(3). pp. 449-462

Gooding, R.A., Harley, C.D., & Tang, E. (2009) Elevated water temperature and carbon dioxide concentration increase the growth of a keystone echinoderm, Proceedings of the National Academy of Sciences, 6(23). pp. 9316-9321

Rice, M.E., Young, C.M., & Sewell, M.A. (2006) Atlas of Marine Invertebrate Larvae. Elsevier. Amsterdam. 9780123736086

Lawrence, J.M. (2013) Starfish: Biology and Ecology of the Asteroidea. Johns Hopkins University Press. Baltimore. 9781421407876

Lawrence, J.M. (1987) A Functional Biology of Echinoderms. Johns Hopkins University Press. Baltimore. 9780801835476

Lafay, B., Smith, A.B., & Christen, R. (1995) A Combined Morphological and Molecular Appraoch to the Phylogeny of Asteroids (Asteroidea: Echinodermata). Systematic Biology. 44(2). pp. 190-208

Hickman, C.P., Roberts, L.S., Keen, S.L., Eisenhour, D.J., Larson, A., & I'Anson, H. (2014). Integrated Principles of Zoology (16ed). McGraw-Hill Education. New York. 9780073524214 

Nichols, D. (1967) Echinoderms (3ed). Hutchinson & Co. London. 

Ruppert, E.E., Fox, R.S., & Barnes, R.D. (2004) Invertebrate Zoology: A Functional Evolutionary Approach (7ed). Brooks/Cole. Belmont. 9780030259821

Hee Lee, J., Shin, S. (2009) A New Record of Sea Star (Asteroidea:Valvatida) from Korea. Korean Journal of Systematic Zoology. 25(3). pp. 291-294

Santos, R., Haesaerts, D., Jangoux, M., & Flammang, P. (2005) Comparative Histological and Immunohistochemical Study of Sea Star Tube Feet (Echinodermate, Asteroidea). Journal of Morphology. 263(3). pp. 259-269

World Register of Marine Species. http://www.marinespecies.org/