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Asterophilia culcitae


David Willoughby 2016

Summary

Kingdom: Animalia

Phylum: Annelida

Class: Polychetae

Order: Phyllodocida

Family: Polynoidae

Genus: Asterophilia

Species: culcitae


Asterophilia culcitae is a small species of Polynoidae (scale worm) polychaete that was recently described by Britayev and Fauchald in 2005. The name Asterophilia literally translates into star love (astero = star; philia = love) and was given in description of the intimate commensal association exhibited between the species of this genus Asteroidea, commonly known as sea stars (Britayev & Fauchald, 2005; Hanley, 1989). Asterophilia culcitae was named after the most common Asteroidea host found in the study area of the paper that identified A. culcitae; Culcita novaeguineae, the cushion starfish. A. culcitae is also documented to live commensally with crinoids (sea ferns), another family of Echinodermata (Antokhina & Britayev, 2012; Britayev & Zamishliak, 1996).

Physical Description

Identification

The identifying features of the Asterophilia genus where defined by Hanley 1989, with one species, A. carlae.
The features described;

  • 15 pairs of large, transparent and delicate elytra
  • A bilobed prostomium
  • Two palps
  • Three antenna
  • Attachment of the median antenna between the prostomium lobes
  • The first segment not dorsally visible, with two pairs of tentacular cirri
  • The second segment forms a nuhcal fold, supports the first set of elyra and ventral buccal cirri
  • Biramous parapodia
  • Small anterodorsal notopodia
  • Large neuropodia
  • Long to short curved and serrated chaetae with notched tips
  • Dorsal cirri on segments that have no elyra
  • Antenna and cirri with subdistal inflation and filiform tips
  • Short and tapered ventral cirri
  • And commensal with Asteroidae (for which they are named)

Since then the number of species in this interesting and elusive genus has doubled, from one to two species. In 2005 Britayev and Fauchald identified the species A. culcitae. The differences that separate these two species needs to be observed under a microscope;

  • Distinctive frontal pocketing of the elytra (instead of folding)
  • Presence of micropapillae on the elytra
  • No enlarged basal row of serrations on the neurochaetae
  • The middle and lower neurochaetae have serrated edge

Refer to Figures 1 – 4

There are also many online resources to help in the identification of polynoids in general.


Taxonomic description

Asterophilia culcitae, Britayev and Fauchald
(Figure 1, 2, 3, 4)

Type material. -27.40449, 153.43655, Australia, west side of Amity point, from ambulacral groove of Pentaceraster regulus, 1.5m, 30th April 2016

Description. The specimen found had a very fragile, dorsally and ventrally flattened scarlet body with 38 segments and an elliptic outline. 15 relatively large, very delicate and antero-posteriorly overlapping pairs of Elytra; attaching on chaetigers 2, 4, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 26, 29, 32. Posterior segments have dorsal cirri and lack elytra. Elytra transparent, basically oval in shape with an irregular outline as well as micropapillae and between 2 – 4 prominent whitish or yellowish opaque spots along the dorsal posterior edge. The larger elytra with frontal pocketing along edge near elytrophore (Figure 1).

The ventral and dorsal surfaces of the body are smooth. Dorsally, each body segment has two transverse ciliary bands for connection to the cirrophores or elytrophores (attachment sights for the cirri and elytra). 

Dorsal anteriorly a wide, bilobed prostomium with a median notch with a ceratophore where the median antenna attaches between the distinctive prostomial lobes. On the prostomial lobes two pairs of brown ovate eyes, roughly the same size with the anterior pair being found more laterally on the widest part of the prostomial lobes.
The median antenna is approximately twice as large as the lateral antennae and 4 times longer then the prostomium. The lateral antennae attach beneath the prostomial lobes and the median antenna into slender ceratophores. All antennae have filiform tips and are subdistally inflated (Figure 2).
Palps lack the subdistal inflation and attach ventral-laterally to the lateral antennae on the palpophore.
Ventrally, the first segment is visible with two pairs of long tentacular cirri that resemble the antennae and are approximately equal in length to the median antenna. These tentacular cirri attach dorsal-laterally above the mouth which is surrounded by three lips; two lateral lips and an upper lip with a distinctive median ridge.
The second segment has biramous parapodia with a ventral pair of buccal cirri with slight subdistal inflation and filiform tips. Dorsally, the second segment also forms the nuchal fold that covers the posterior of the prostomium (Figure 2 and 3). 

The cirrostyles and elytrastyles attach dorsally to the parapodia. The dorsal cirri attach to cirrostyles and emerge through the frontal pocketing of the elytra on the next posterior segment. Dorsal cirri resemble antenna and tentacular cirri with subdistal inflations and filiform tips (Figure 1). The pygidium has 2 large inflated dorsal cirri. All dorsal cirri are similar in size and length, apart from anterior and posterior cirri that taper down in size to match the anterior antenna. All subdistal inflations of the antenna, tentacular and dorsal cirri fade from the scarlet colour of body into a milky white.
Ventral cirri taper gradually, attach posterior-ventrally to the parapodia and are found on all chaetigers except for chaetigers 1 and 2.

The parapodia are biramous with nuero and notopodia (Figure 4). The notopodia attach on the anterior dorsal side of the parapodia. They are reduced in comparison to the neuropodia, with the aciculae penetrating the surface near the end of the notopodia conical acicular lobe. A vertically orientated fan of 3 to 15 notochaetae dorsal to the acicular digitiform lobe. The notochaetae are round in cross section, and towards the tips are slightly curved, with short marginal serrations and slightly notched tips.
The neuropodia are significantly larger than the notopodia, with a rounded postchaetal lobe and a long triangular prechaetal lobe, ending in a digitiform acicular lobe. The acicular may penetrate the acicular lobe. The neurochaetae are more numerous than the notochaetae forming a vertically orientated fan of between 24 and 34 chaetae.
The upper neurochaetae are long, slender, with 22 – 24 rows of serration, slightly curved away from serration with hooded notched tips.
The middle neurochaetae are also long and slender, the slightly curved tip has fewer serrated rows, approximately half the length of the serrations found on the upper neurochaetae and ends in a hooked uni- or bidentated tip.
The lower neurochaetae are relatively shorter with a stout, shorter cured unidentate tip with between 8 – 9 rows of serrations.

Remarks.
i) Specimen was found sharing host Pentaceraster regulus with a Gastrolepidia clavigera specimen.
ii) During observation of the live specimen, the palps where able to be extended to more than double the length of median antenna, in what appeared to an investigatory or exploratory behaviour. Suggesting the palps serve as some form sensory organ – potentially chemosensory.
iii) While handling the specimen, moving it from host to petri dish it broke apart approximately two thirds of the way down the body. The posterior end continued to move for roughly 45minutes after detaching. The anterior end swam back to and settled on original host before returning to ventral side of the P. regulus. In following weeks, the anterior section regrew its posterior section. The posterior end remained still and within a week had dissolved / rotted away.
1
Figure 1
2
Figure 2
3
Figure 3
4
Figure 4

Ecology

Polynoids are a large family of polychaetes with approximately 200 genera and nearly 900 species described so far. The family Polynoidae also accounts for the largest number of symbiotic polychaetes; parasitic, commensal or mutualistic (Petersen & Britayev, 1997). 

Despite symbiotic polychaetes being a relatively common phenomenon, collecting specific species can be problematic due to individual rarity (Britayev & Antokhina, 2012). This makes research on these symbionts difficult to undertake and much of the knowledge is generalized from better studied species (Britayev & Martin, 2006; Martin & Britayev, 1998). As a result, many of the relationships between symbiotic polynoids and their hosts are labeled as commensalism as the true nature of the relationship remains unclear. These symbioses do not appear to be obligate relationships for the polynoids and there have been many cases of finding free living individuals or even with other host species (Petersen & Britayev, 1997). Such is the case appears for the specimen used in this report; A. culcitae is documented to be commensal on three taxa of star fish Culcita novaeguineae, Protoreaster nodosus, and Linckia laevigata (Antokhina & Britayev, 2012; Britayev & Fauchald, 2005), however the A. culcitae specimen gathered for this report was found in the ambulacral groove of P. regulus. Suggesting that A. culcitae chooses hosts based on something other then the species but more widely base potentially chemo sensory or physiological. This would be an avenue of future research, to identify what attracts Asterophilia to hosts.

The A. culcitae specimen found was also co-commencing on the host, with a much larger Gastrolepidia claoigera specimen (Figure 5). A. culcitae did not exhibit any signs of interspecific aggression or territorial behaviour such as fighting. Finding these two specimens on the same host gave a valuable first hand opportunity to see just how similar Asterophilia is to G. clavigera. This similarity is essentially the reason Asterophilia was only described in 1989 while G. clavigera was described in 1861(Britayev & Fauchald, 2005; Hanley, 1989).
In the lab, when first identifying the scale worms from P. regulus, the Asterophilia was first thought to be juvenile of G. clavigera. 

The small size of the Asterophilia and similarity in appearance may assist it to co-commence on host with larger G. clavigera. There are well documented cases of adult polynoids co-commencing with numerous juveniles of the same species (Martin & Britayev, 1998). This could be an avenue of future investigation to potentially explain some of the evolutionary development of Asterophilia traits.
5
Figure 5

Life History and Behaviour

Reproduction

Asterophilia culcitae is yet to have its life history investigated thoroughly as are many other polynoids, however by looking at other known aspects of A.culcitae’s ecology, it is possible to infer other aspects of its life history. As polychaetes possess a multitude of sexual reproduction and developmental modes and the presence of one form of reproductive does not necessarily mean that closely related species will use the same or even similar modes of reproduction (Martin & Britayev, 1998; Wilson, 1991). It is this plasticity and variation in the polychaete life histories that has allowed them to be so successful in all marine environments (Wilson, 1991).

Some polychaetes have the ability to reproduce both sexually and asexually but most polynoids are heterosexual and require the presence both sexes to to successful reproduce (Jollivet et al., 2000; Martin & Britayev, 1998). A common form of reproduction in the marine environments, is that of broadcast spawning. Some polynoids employ this broadcast spawning and synchronise the release of thousands of small gametes during one or two large annual spawning events. These spawning events are controlled species wide by a number of influences. In some species the major factors appearing to be the presence of the opposite sex and day length, together, influencing an internal control mechanism that triggers the spawning event (Garwood, 1980). Once spawned, these eggs are fertilised in the water column and developed into planktonic feeding larvae. The developing larvae may stay in the plankton for weeks to months, in which time, due to global tide and currents, they may have travelled huge distances (Garwood, 1980; Martin & Britayev, 1998; Pernet, 2000). This trait can be suggested through very large species distributions (Pernet, 2000). This form of reproduction does not force a symbiont species to develop any specialised adaptions (morphologically or behaviourally) to successfully reproduce from its host.

This is the hypothesised method of reproduction for A. culcitae due to its large distribution (see Biogeographic Distribution) and it is unlikely that this species processes the ability to asexually reproduce, supported by lack of examples of polynoids ability to asexually reproduce in literature and by observation iii in the taxonomic description.

Behavioral Observations

Introduction:

As only one specimen of A. culcitae was collected, no valid experiment could be carried out that would avoid or account for pseudo-replications and confounding variables. As a consequence, the results found are little more than conjecture and one set of observations from one subject A. culcitae. These are important points to keep in mind while reading the following experimental write up.
Symbionts require the ability to in some way detect their host or at least an individual from a host species(‘). This implies the presence of organs or systems that allow the symbiont to pick up on some cue(s) that the host(s) produce either actively or inertly. There are experiments that have shown that commensal polychaetes are able to detect their host(s) to varying degrees; from a distance, through water from the host and via touch (Martin & Britayev, 1998). Petersen and Britayev wrote in 1997 that a common trait among some of the commensal polynoids (including A. culcitae) appeared to be the presence of larger or more pairs of anterior eyes. 

From the remarks in the taxonomic description the observation was made that the live specimen was able to extended its palps to more than double the length of it’s median antenna, in what appeared to an investigatory or exploratory behaviour. Suggesting that the palps may serve as some form of sensory organ; potentially a chemosensory organ. 

The aim of this study is to investigate the role of chemosensory organs in the detection and location of host(s) in Asterophilia culcitae, using a simple y maze experiment. I hypothesised that the A. culcitae will be able to detect the chemo-trail of host of the host and follow that scent up the host arm of the maze regardless of visual stimuli status.

Methods:

A large, rectangular, flat based, white tray (50cm /30cm) was filled with sea water to a mark on the side, 1cm above the tray base. Two clear containers had six 5mm square holes cut into the bottom of one side and the same was repeated with 2 opaque containers of the same size. The A. culcitae specimen was removed from the host and placed into a petri dish. The host was placed in a container of for 15 minutes to concentrate any compound it released into the water. Both clear containers were prepared to be place into tray; host was randomly allocated to one container. Containers were placed into the tray at one end, side by side, with the 5mm holes facing out towards the opposite end of the tray, randomly. The containers were then filled with water; container holding the host was filled with the water the host had been conditioning for the 15 minutes, while empty container was filled with an equal amount of un-conditioned sea water. A. culcitae was released at the opposite end of the tray. Timed and watched for 10 minutes, with response(s) recorded. After 10 minutes has elapsed, host was captured, the equipment cleaned with fresh water and the experiment reset for a second replication using clear containers. Then repeated the entire process using the opaque containers.

 

Observational Results:

In all replicates, the flow of water from the containers, created a clockwise swirl of water in the tray. In replications using clear containers, A. culcitae followed flow of water from container holding host up the host arm (Figure 6). In the replications using the opaque containers, A. culcitae failed to move from release spot for entire allocation of time.

*note*
As a result of time restraints and limited availability and access of needed resources, the replicates using the opaque containers were carried out exactly 1 week after the replicates using the clear containers. Originally, three replicates per container type were attempted, however after the second replicate using the clear containers, the A. culcitae specimen had become lethargic and was carried around the tray with the water movement; synonymous with exhaustion. Hence the number of replicates was reduced.

Discussion: 

The results do no support my hypothesis that the A. culcitae would be able to detect and react to the chemo-trail of host regardless of visual stimuli. When the clear containers were used, the specimen moved toward the host and when the opaque containers were used the specimen failed to detect, react or follow chemo-trail of host. These results would be a beautifully simple and elegant display of a commensal scale worm’s reaction to a combination of host based stimuli (visual and smell), if these result were not gained from one specimen over two separate weeks. The minute sample size limits the statistical power of these results, neither allowing me to reject or accept the null hypothesis.
I would present these results solely as observations of a singular A. culcitae reaction’s to separation from its host. I would completely ignore second week’s data, not because that aligns the findings with the hypothesis of the experiment but rather because of the unnatural environmental pressures and the deterioration of the singular specimen. At the time of the first experiment the symbiont and its host had already been kept in captivity for three weeks. In the first week of captivity, the A. culcitae was handled extensively and resulted in the loss of the posterior 1/3 of its body and many of its elytra. The symbiont then under went weekly inspections of prolonged separation from its host, while it was exposed to high levels of stress. The regrowth of its posterior end was monitored and after two weeks (by the time of the first experiment) it had regrown its entire posterior section and all the missing elytra. That is a massive amount of energy expenditure to endure for any organism, let alone in captivity. By the time of the opaque container replications, the A. culcitae had been in captivity for just less then a month.
Additionally, A. culcitae is a commensal worm that spends its entire adult life on the ventral side of a sea star, with it’s food and shelter are provided (insert every reference on reference list), which is now being required to repeatedly partake in a stressful dash back towards it’s host and safety.

The first replications of the experiment did provide an interesting observation. The flow of water in the tray created a clockwise current that in both replications the specimen followed to the container holding the host in it (Figure 6). The fact that the A. culcitae actually followed the flow of water and did not take the shortest route suggests the presence of some chemo-sensory organ or system(Gaudron, Watson, & Bentley, 2007; Martin & Britayev, 1998)

For future studies, in order to reduce the error and increase the statistical power of the experiment, a larger sample size is required, with better spaced replications (if specimens were to be tested more then once). If specimens were collected from the wild, they should ideally be tested while they are of healthier condition, rather then allowing their condition to deteriorate as was the case in this experiment. 
6
Figure 6

Anatomy and Physiology

A brief literature review shows that there have been no studies of the anatomy and physiology of Asterophilia relating to the commensal habit although some studies have been conducted in other Polynoidae (Britaev, Krylova, Aksyuk, & Cosel, 2003; Britayev, Antokhina, & Marin, 2015; Britayev & Zamishlyak, 1994; Dimock & Dimock, 1969; Eckelbarger, Watling, & Fournier, 2005; Freeman, Richardson, & Seeda, 1998; Gaikwad, 1993; Pettibone, 1991; Ruff, 1991; Uchida, 1975).
Research in this field would be valuable and should probably focus on respiration (Is the respiration enhanced or compromised by the commensal habit?), reproduction and larval biology (Do the larvae detect host species at time of settlement? and If so, how?)

Biogeographic Distribution

The known distribution of A. culcitae is currently patchy at best. Figure 7 is a collaboration of all the A. culcitae specimens documented in papers (that I was able to find). The species distribution seems to stretch from Fiji in the South Pacific Ocean, stretching into the Coral sea and Arafura Seas of Australia’s east and northern coast lines, continuing into the South China sea and up into the East China sea.Despite being documented to be commensal with three main taxa of star fish, it is suggested by observations within the literature and from this paper that A. culcitae is able to commence of a a wider range of star fish (Britaev & Antokhina, 2012; Britayev et al., 2015; Britayev & Martin, 2006; Lyskin & Britaev, 2005; Martin & Britayev, 1998; Petersen & Britayev, 1997). 
7
Figure 7

Evolution and Systematics

All polynoids have some form of hard elytra, spins or bristles as defence from predation, however it has been observed that many symbiotic polynoids have soft or even delicate elytra (Britayev & Zamishliak, 1996; Martin & Britayev, 1998; Petersen & Britayev, 1997). It is this trend that suggests that delicate and transparent elytra are a symbiotic feature of polynoids (Martin & Britayev, 1998). The symbiotic polynoid behaviour of being located on the ventral surface or in the case of of the Asterophilia species, in the ambulacral groove of the host is another evolutionary response to a commensal lifestyle (Britayev & Zamishliak, 1996; Martin & Britayev, 1998). As the host acts as their defence again predators there is less need to divert energy into the construction of these hard protective features, allowing them to turn that energy into other adaptions to increase the success as a symbiont.

One such avenue is the use of colouration and mimicry. This is a well documented adaptation of many commensal polynoids. Symbiont polynoids have colouration or patterns that match or are similar to the basal colouration of the host(s) they are found living symbiotically with(Martin & Britayev, 1998). A. culcitae found living on blue star fish exhibit bluer or even more transparent shades, while specimens living on red or brown hosts exhibit similar shades(Britaev & Antokhina, 2012; Martin & Britayev, 1998). The colouration, the elliptical outline dotted with cirri that resemble tube feet of A. culcitae resembles an ambulacral opening along the ventral surface of a star fish remarkable well, and is a perfect example of mimicry (Antokhina & Britayev, 2012; Britayev & Antokhina, 2012; Britayev & Fauchald, 2005; Britayev & Martin, 2006; Hanley, 1989). Symbiont polynoids have also evolved adaptations to help keep them attached to their hosts, in the form of hooked, hooded, notched and serrated tips of the neurochaetae (Britayev & Fauchald, 2005; Britayev & Zamishliak, 1996; Hanley, 1989). 

Conservation and Threats

We live in a period characterised by an increased rate of extinctions and a decrease in global biodiversity. This is due, in part to the rapid growth, development and pollution generated by the human race (Colwell, Dunn, & Harris, 2012; Fahrig & Merriam, 1985; Hoffmann & Sgró, 2011). Co-extinction of commensal species can result from the primary extinctions of host species. Recording co-extinctions can be difficult because of confounding factors such incomplete knowledge of the symbionts life history, distribution or host list(Colwell et al., 2012).

As the knowledge of A. culcitae and its life history is incomplete it is difficult to identify the current treats to the species, however as it is a commensal species of polynoid and as a result, faces the potential of its host(s) extinction(s). This treat is reduced considerable as A. culcitae does not seem to be an obligate symbiont, but is able to commence on a wider number of Asteroidae and at least one Crinoid species then originally thought (Britayev & Fauchald, 2005; Britayev & Martin, 2006; S. A. Lyskin & T. A. Britaev, 2005; Martin & Britayev, 1998). 

References

Acknowledgements

I would like to thank the following people:

My lectures, Bernie and Sandy Degnan,  tutors and lab staff for setting up the laboratory and being there to help for every 5 hour practical.

A massive thank you to my colleagues; Luke Dekkers and Cari Rivers, for collecting the Star fish that supplied the specimen for this study.

A big thank you to Dr. Robin Wilson for assisting my in the identification of the specimen for this paper and guidance while writing this paper.

A special thank you to Eunice Wong for sharing her knowledge on all things polychaete.

This would also be a good time to thank the Museum of Victoria and ask you to give a little bit back to the museums. Visit; museumvictoria.com.au/donate

References

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