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Comanthus wahlbergii Species Profile
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Benjamin Durrington 2017
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Summary | |
Comanthus wahlbergii (Müller 1843) is a widespread crinoid (the most basal group of echinoderms) that can be locally abundant in some parts of Australia (Vail 1987a). The specimen used for this profile was collected from Heron Island from an area of coral rubble. This species belongs to the Order Comatulida, better known as the featherstars. Most members of this group are not fixed to the substrate, and are capable of movement, which may help them avoid predators and find food (Meyer and Macurda 1977). Comanthus wahlbergii exhibits many of the unique adaptations and behaviours of this group that have allowed them to exploit shallow marine habitats around the world (Meyer and Macurda 1977).
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Physical Description | |
Comanthus wahlbergii varies in color and
morphology, but shares the basic feather star body plan, which is described in
Messing (2017) and Summers et al.
(2014). Feather stars have a bony endoskeleton with units called ossicles
connected by ligaments and muscles. Syzygies and synarthries are two common
articulations for feather stars: a syzygy connects ossicles with ligaments,
while a synarthry has ligaments and muscles. Feather stars have an adoral (Figure 1) and aboral (Figure 2) side.
The
main regions of the body are the centrally-located theca, the rays, and the
cirri. The theca includes a calyx, which holds internal organs, and a tegmen,
which covers them. Rays branch off from the theca, and may divide multiple
times. The ossicles of a ray located proximal to a division comprise a single
brachitaxis. The number of branching points of an ray is equal to the number of
brachitaxes. The ossicles past the last division point comprise the
non-branching arm. The ossicles of a ray are called brachials, and they can be
referred to using Roman numerals to denote brachitaxes, and Arabic numbers to
denote brachials (starting proximally; summarized in Summers et al. 2014 and Messing 2017; see Figure 3). At the
aboral side (bottom) of the theca, there is an ossicle called the centrodorsal,
which is the only remnant of the stalk found in mature feather stars. The cirri are connected to the centrodorsal
and help feather stars grip onto the substrate.
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| Figure 1 |
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| Figure 2 |
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| Figure 3 |
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Ecology |
Habitat | |
The
specimen used for the profile was collected in an area of coral rubble to the
inside of the reef crest at Heron Reef, Queensland. Individuals have also been
observed sheltering under coral rubble and reef cavities at Lizard Island, Queensland (Vail
1987a). In more temperate areas with less coral, this species may exploit rocky
habitats (Rowe et al. 1986). During its time in captivity, the specimen has remained
associated with pieces of coral rock.
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Predation | |
Feather
stars are preyed upon by a variety of fish, including triggerfish, snappers,
and damselfish (Meyer and Macurda 1977). They are known to exhibit several
behaviors in response to predators, including diel movements (see Life History
and Behaviour Section), crawling, and swimming (Meyer and Macurda 1977; Vail
1987a). They are able to regrow most of their organs after loss or damage,
including arms and viscerae (Mozzi et al.
2006), and they have several morphological traits that minimize damage during
predation events (Oji and Okamoto 1994). In particular, feather stars,
including Comanthus wahlbergii,
typically branch the most at the base of the rays (Oji and Okamoto 1994). This
is an anti-predation technique, since a bite to an exposed non-branching arm results
in the loss of less tissue than a bite below a branching point (Figure 3; Oji
and Okamoto 1994). Sea lilies, by contrast, typically have more
distally-located branching points to maximize food collection (Oji and Okamoto
1994). All crinoids have non-muscular articulations (called syzygies in feather
stars) that facilitate breakage of arms during autonomy or predation events,
another strategy used to reduce tissue loss (Oji and Okamoto 1994). All the
arms of my specimen of C. wahlbergii
showed evidence of regrowth, suggesting that it may have suffered from multiple
predation events. (Figure 4). Using various metrics related to damaged and
undamaged arms in a population of Cenometra
bella (another feather star), Baumiller and Gahn (2013) estimated that this
species loses arms about every 10 days. This would be an interesting parameter
to investigate in the future for Comanthus
wahlbergii.
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| Figure 4 |
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Life History and Behaviour |
Life History | |
Early development in Comanthus wahlbergii has not been studied, but larvae of the comatulid order of crinoids are lecithotrophic (dependent on yolk reserves) and generally known to exhibit a simple morphology (without feedling structures) (McEdwardand Miner 2001). There are two pre-metamorphosis larval stages and two post-metamorphosis larval stages. The former includes a uniformly ciliated stage and an ovoid doliolaria stage, which has up to five transverse rows of cilia and an apical tuft of cilia (Haig and Rouse 2008; McEdward and Miner 2008). These cilia allow locomotion in the water column, and may not be present in crinoid species that brood their larvae (Haig and Rouse 2008). Doliolaria also have a ventral adhesive pit that is involved in attachment to the substrate during the sessile post-metamorphosis phase, and a ventral vestibule that is later enclosed (Haig and Rouse 2008; McEdward and Miner 2008).
During their metamorphosis into cystidean larvae, the doliolaria continue to develop the adhesive pit, as well as a stalk of columnar ossicles, and the basic elements of the calyx: the basal, radial,and oral ossicles (Haig and Rouse 2008). Cystidean larvae attach to the substrate but do not feed. The final larval phase, the pentacrinoid phase, involves the opening of oral plates, commencement of feeding using podia, and production of brachial ossicles that branch out from the radial ossicles (Haig and Rouse 2008). Comatulids lose the stalk ossicles during the juvenile phase (Haig and Rouse 2008).
Some comatulids (brooders) protect their young within or on their pinnules, while others (broadcast spawners) release eggs into the water column to be fertilized (Vail 1987b). Comanthus parvicirrus, a close relative of C. wahlbergii, is known to be a broadcast spawner (Eléaume,Baumiller and Améziane 2003; Vail 1987b). In addition, Capillaster multiradiatus and Anneissia japonica (members of closely related genera) are known to have pelagic larvae (McEdward and Miner 2001). Thus, it may be the case that Comanthus wahlbergii produces pelagic, simple, lecithotrophic larvae through broadcast spawning. This hypothesis is supported by the fact that the species has a large Indo-Pacific distribution,and is found even in South Africa, where the larvae may have been transported by the Agulhas Current (Rowe et al.1986).
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Feeding | |
Feather stars, like other crinoids, are filter feeders that depend
on food that falls on their arms; however, they do have greater capacity to
maximize access to food by changing feeding sites (Brusca et al. 2016; Meyer and Macurda 1977). Particles generally ranging
from 0.05 to 0.4 mm are collected with the tube feet and first transferred to
cilia-lined ambulacral grooves, and then along the arms to the
centrally-located mouth (Brusca et al.
2016; Fatheree 2013). Crinoids are known to eat various types of zooplankton,
phytoplankton, as well as detritus (Fatherree 2013). They can also absorb
dissolved organic material and may rely on this when their viscerae have been
damaged through predation (Mozzi et al.
2006).
The Comanthus wahlbergii
specimen used for this profile was fed a high-concentration solution of phytoplankton,
including Isochrysis sp. (a
haptophyte), Thalassiosira weissflogii (a diatom), Nannochloropsis sp. (a
eustigmatophyte) and Pavlova sp. (a haptophyte). The solution was squirted
over the pinnules on all the rays. Upon exposure to the solution, individual
pinnules began undulating, perhaps to enhance feeding, in a behavior not
previously observed (Figure 5). Ingestion of the phytoplankton was confirmed afterwards because
the crinoid’s fecal matter was bright green.
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| Figure 5 |
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Diel Movements | |
In a study of crinoid behavior on Lizard Island, Comanthus wahlbergii and other crinoid species were observed to
vary their degree of exposure from shelter sites over the course of a day (Vail
1987a). Most members of the C. wahlbergii
population were visible during part of the day (52%), while others were
nocturnal (42%) or visible at all times (6%). When visible, C. wahlbergii individuals were never
completely exposed, but they did expose more of their arms when it was dark. In
this study, Vail measured light intensity and confirmed that crinoids may
respond to darker (i.e. cloudy) days by becoming more active. This is
concordant with the behavior of C.
wahlbergii in the lab. During observation of the individual with a dissecting
scope, it repeatedly avoided the highly illuminated parts of its container by
crawling away. The periodic and negatively phototactic behavior of C.
wahlbergii may help it avoid predators, feed more efficiently, or coordinate
reproduction (Vail 1987a).
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Anatomy and Physiology |
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Internal Anatomy | |
Key features of a crinoid’s internal soft anatomy include the
U-shaped gut which connects to the mouth and anus on the surface of the tegmen,
and a water vascular system. The water vascular system is composed of a ring
canal that connects to stone canals and coelomic sinuses in the calyx, as well
as radial canals in the arms. Tube feet branch off from the radial canals and
surround the ambulacral grooves on the surface of the arms and pinnules. (Brusca
et al. 2016). Crinoids also have
hemal and nerve rings which branch into rays through axial canals (Brusca et al. 2016; Messing 2017).
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Physiology | |
Crinoids, like other echinoderms, cannot osmoregulate and are only
found in marine environments (Brusca et
al. 2016). They exchange gases passively across their thin tube feet, but
generally require less oxygen than other invertebrates (Baumiller and LaBarbera
1989). And, like many other echinoderms, crinoids possess toxins such as
polyketide sulfates which may discourage predation (Rideout et al. 1979). Comanthus parvicirrus is one of the species in which production of
toxins has been confirmed, so they are likely present in Comanthus wahlbergii
as well (Rideout et al. 1979).
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Biogeographic Distribution | |
Comanthus wahlbergii is widespread in the Indo-Pacific, and can be found in Australia, New Zealand, South Africa, Japan, New Caledonia, Samoa, and many other locales (Rowe et al. 1986).
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Evolution and Systematics | |
Crinoids (Class Crinoidea) have been present on the earth for almost half a billion years: the first fossil membersof the group are from the Early Ordovician (Guensburg and Sprinkle 2009). Over this period of time, many lineages have emerged and become extinct, but the 650 species alive today are all members of the subclass Articulata (Rouse et al. 2013; Summers et al. 2014). This group diversified after the Permian-Triassic extinction, and the first fossils known are from the Early Triassic (Rouse et al. 2013;Schubert et al. 1992). Notably, this group inherited complex muscular brachial articulations facilitating greater capacity for movement (Meyer and Macurda 1977). The most mobile of this group are those from the order Comatulida, the comatulids, which represent most of today’s crinoid diversity (Summers et al.2014). Most members of this group are feather stars which lose the stalk after early development, although there are some which have secondarily retained it (Rouse et al. 2013). Feather stars are the only extant crinoids which live in shallow water, and this may because their mobility allows them to avoid predators, maximize their access to food,or avoid stressful abiotic conditions (Meyer and Macurda 1977; Rouse et al. 2013).
The Comatulidae family of feather stars is at least as old as the Miocene, and now has 95 species with a center of diversity in the Indo-Pacific, including 26 species in Australian waters (Rowe et al. 1986; Summers et al. 2014). They are characterized by combs present at the tips of their proximal pinnules and by mouths which are usually off-center (Summers et al. 2014). They are the most common and diverse group of feather stars found on tropical coral reefs (Summers et al. 2014). Comanthus (AH Clark, 1908) belong to the Comatulidae and includes 11 currently accepted species (Messing 2015). Comanthus can be distinguished from other genera in the Comatulidae family by a transverse tooth found on the proximal portion of their pinnule combs (Summers etal. 2014). In Australia, Comanthus wahlbergii can be distinguished from other shallow-water members of the genus by the centrodorsal which usually completely covers the radial ossicles and its “robust” cirri (Rowe et al.1986).
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Conservation and Threats | |
No threats are known to specifically target Comanthus wahlbergii. Given its large distribution and population size at some locations (Vail 1987a), it is not likely currently in need of conservation effort.
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References |
Acknowledgements | |
I would like to thank Charles Messing (Nova Southeastern University, Fort Lauderdale, FL) for confirming the likely ID on this specimen.
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Bibliography | |
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