Contributions to Zoology, 81 (1) 43-54 (2012)
The phylogenetic position of the solitary zoanthid genus Sphenopus (Cnidaria: Hexacorallia)
James D. Reimer1, 2, 7, Meifang Lin3, Takuma Fujii4, David J.W. Lane5, Bert W. Hoeksema6
1
Molecular Invertebrate Systematics and Ecology Laboratory, Rising Star Program, Transdisciplinary Research
Organization for Subtropical Island Studies (TRO-SIS), University of the Ryukyus, Senbaru 1, Nishihara, Okinawa
903-0213, Japan
2
Marine Biodiversity Research Program, Institute of Biogeosciences, Japan Agency for Marine-Earth Science and
Technology (JAMSTEC), 2-15 Natsushima, Yokosuka, Kanagawa 237-0061, Japan
3
Biodiversity Research Center, Academia Sinica, Nangkang, Taipei 115, Taiwan
4
Molecular Invertebrate Systematics and Ecology Laboratory, Graduate School of Engineering and Sciences,
University of the Ryukyus, Senbaru 1, Nishihara, Okinawa 903-0213, Japan
5
Department of Biology, Faculty of Science, Universiti Brunei Darussalam, Jalan Tungku Link BE1410, Brunei
Darussalam
6
Department of Marine Zoology, Netherlands Centre for Biodiversity Naturalis, PO Box 9517, 2300 RA Leiden,
The Netherlands
7
E-mail: jreimer@sci.u-ryukyu.ac.jp
Key words: Anthozoa, azooxanthellate, biodiversity, DNA marker, free-living, marine invertebrate, monostomatous,
Sphenopidae
Abstract
The zoanthid genus Sphenopus (Cnidaria: Anthozoa: Zoantharia), like many other brachycnemic zoanthids, is found in shallow subtropical and tropical waters, but is uniquely unitary
(solitary, monostomatous), azooxanthellate, and free-living.
With sparse knowledge of its phylogenetic position, this study
examines the phylogenetic position of Sphenopus within the
family Sphenopidae utilizing specimens from southern Taiwan
and Brunei collected in 1999-2011, and furthermore analyzes
the evolution of its unique character set via ancestral state reconstruction analyses. Phylogenetic analyses surprisingly show
Sphenopus to be phylogenetically positioned within the genus
Palythoa, which is colonial (polystomatous), zooxanthellate,
and attached to solid substrate. Ancestral state reconstruction
strongly indicates that the unique characters of Sphenopus have
evolved recently within Palythoa and only in the Sphenopus
lineage. These results indicate that zoanthid body plans can
evolve with rapidity, as in some other marine invertebrates, and
that the traditional deinitions of zoanthid genera may need reexamination.
Contents
Introduction .....................................................................................
Material and methods ....................................................................
Specimen collection .................................................................
Specimen identiication ..........................................................
DNA extraction, PCR ampliication, sequencing .............
Phylogenetic analyses .............................................................
Ancestral character state reconstruction ...........................
43
45
45
45
47
47
49
Results ...............................................................................................
Phylogenetic analyses – mt16S rDNA .................................
COI ..............................................................................................
ITS-rDNA ...................................................................................
Ancestral character state reconstruction ...........................
Discussion ........................................................................................
Acknowledgements ........................................................................
References ........................................................................................
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49
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53
53
Introduction
The zoanthids are an order (=Zoantharia, or Zoanthidea) of benthic cnidarians related to scleractinian
corals and sea anemones within the subclass Hexacorallia, class Anthozoa. Similar to order Scleractinia,
zoanthids are generally colonial (modular or polystomatous), but unlike these stony corals they are not
calciiers; instead most zoanthids incorporate sand
and other detritus into their body wall to contribute to
their structure. Zoanthids in the genus Palythoa can
be up to 60% encrustation by weight (Haywick and
Mueller, 1997). This encrustation impedes internal
examination of zoanthids, making observation of the
sphincter muscles, mesenteries, and other characters
dificult (Reimer et al., 2010). Furthermore, many zoanthid species show much intraspeciic morphological variation, compounding the dificulty of identiication (Muirhead and Ryland, 1985; Burnett et al.,
Reimer et al. – The solitary zoanthid Sphenopus
44
Fig. 1. a-c. In situ images of Sphenopus marsupialis specimens from Brunei. Note sandy sediment habitat background in a and b, and
open oral disk in c. For specimen and collection information refer to Table 1. Scale = approx. 1 cm.
Specimen
number
Collection
locality
Depth (m)
Collection
date
Collector(s)
mt 16S rDNA
COI
ITS-rDNA
Reference
Table 1. Specimens of Sphenopus marsupialis utilized in this study, collection details, and GenBank Accession Numbers for DNA
sequences. Abbreviations: n/a = not available, or not acquired; MISE = Molecular Systematics and Ecology laboratory (U. Ryukyus),
BRCAS = Biodiversity Research Center Academia Sinica (Taiwan), DJWL = DJW Lane, BWH = BW Hoeksema.
T1 (MISE)
S1
S2
S3
S4
S5
S6
S7
S8
S9
S10
S11
S12
I1-I3
Shitzwan, Taiwan
SW of Pelong Rocks, Brunei
SW of Pelong Rocks, Brunei
SW of Pelong Rocks, Brunei
SW of Pelong Rocks, Brunei
SW of Pelong Rocks, Brunei
SW of Pelong Rocks, Brunei
SW of Pelong Rocks, Brunei
SW of Pelong Rocks, Brunei
SW of Pelong Rocks, Brunei
SW of Pelong Rocks, Brunei
SW of Pelong Rocks, Brunei
SW of Pelong Rocks, Brunei
Bintan, Riau Archipelago,
Indonesia
Suao, Taiwan
n/a
13
13
13
13
13
13
13
13
13
13
13
13
n/a
1999
23.iv.2011
23.iv.2011
23.iv.2011
23.iv.2011
23.iv.2011
23.iv.2011
23.iv.2011
23.iv.2011
23.iv.2011
23.iv.2011
23.iv.2011
23.iv.2011
vi.1995
BRCAS
DWJL, BWH
DWJL, BWH
DWJL, BWH
DWJL, BWH
DWJL, BWH
DWJL, BWH
DWJL, BWH
DWJL, BWH
DWJL, BWH
DWJL, BWH
DWJL, BWH
DWJL, BWH
DWJL
n/a
JQ323164
JQ323165
JQ323163
JQ323166
JQ323160
n/a
JQ323169
JQ323168
n/a
JQ323162
JQ323167
JQ323161
n/a
n/a
JQ323180
JQ323177
JQ323174
JQ323170
JQ323173
n/a
JQ323171
JQ323172
n/a
JQ323178
JQ323176
JQ323175
n/a
n/a
n/a
n/a
JQ323159
JQ323158
n/a
n/a
JQ323157
JQ323156
n/a
n/a
n/a
n/a
n/a
This study
This study
This study
This study
This study
This study
This study
This study
This study
This study
This study
This study
This study
This study
Not given
n/a
n/a
BRCAS
n/a
n/a
AB441420 Fukami et al.
2008
45
Contributions to Zoology, 81 (1) – 2012
1995; Reimer et al., 2004). Thus, a clear understanding
of the species richness of this order has not yet been
achieved. Recent examinations of shallow-water zoanthids (Suborder Brachycnemina) utilizing allozymes
(Burnett et al., 1997) and DNA phylogenetic analyses
(Reimer et al., 2006a, b, 2007b, 2008) have led to
speculation that many currently described species in
the literature are actually inadvertent redescriptions
(Burnett et al., 1997), and it is possible that species
numbers in the coral reef genera Zoanthus, Isaurus,
and Palythoa are lower than currently believed. Additionally, other recent phylogenetic examinations have
questioned the current taxonomic placement of lesserknown coral reef zoanthid genera such as Acrozoanthus (Reimer et al., 2011b) and Neozoanthus (Reimer
et al., 2011a).
The phylogenetic placement of one brachycnemic
zoanthid genus, Sphenopus, has not yet been comprehensively examined. Sphenopus was originally deined and described by Steenstrup (1856), and is placed
together with Palythoa in the family Sphenopidae. Unlike most other zoanthids, Sphenopus is always unitary (as deined by Ryland and Lancaster (2003); =
monostomatous, solitary, not colonial or modular, unless budding), and usually not attached to any substrate
(i.e., free-living). Instead, the large polyps (up to 3 cm
in diameter, 4.5 cm in length) are generally rounded
and bulbous or anchored at the aboral end and are
found partially embedded in sandy, coral reef environments. Specimens have been reported in popular handbooks and ield guides from various localities, such as
the Seychelles (Den Hartog, 1997), Malaysia and Indonesia (Erhardt and Knop, 2005), Papua New Guinea
(Colin and Arneson, 1995) and eastern Australia
(Zann, 1980). They possess some limited mobility
(Soong et al., 1999) and their mode of nutrition is suspension feeding. Aside from some investigations on
reproductive ecology (Soong et al., 1999) and use in
one phylogenetic study as an outgroup (Fukami et al.,
2008), very little is known about Sphenopus phylogeny
and diversity.
In this study, we examine the phylogenetic position
of Sphenopus with specimens of S. marsupialis (Gmelin 1791), the type species of this genus, from both Taiwan and Brunei, and generate phylogenetic trees using
sequences of the mitochondrial DNA markers cytochrome oxidase subunit 1 (COI), 16S rDNA (mt 16S
rDNA), and the nuclear internal transcribed spacer region (ITS-rDNA). We also attempt to map both the
evolution/devolution of symbioses with Symbiodinium
and the unitary and free-living body plan within the
suborder Brachycnemina by ancestral state reconstruction. Our results lead us to reconsider the deinition of Palythoa and Sphenopus, and demonstrate the
relative rapidity in which radically different body
plans and strategies can evolve in zoanthids.
Material and methods
Specimen collection
Sphenopus specimens from Brunei (n=12) were collected on 23rd April 2011 at a sandy/muddy bank (depth
approximately 13 m) 1.5 km southwest of Pulau Pelong-Pelongan (Pelong Rocks) and 3.5 km from the
Brunei coastline (5°04’10.08”N, 115°02’35.1”E). Collected specimens were photographed in situ and subsequently in a dish of seawater, with the polyp disc
allowed to expand (Fig. 1). Preservation was carried
out using 70% analytical grade ethanol. A specimen
from Taiwan (n=1) was collected in 1999 at Shitzwan
ish landing site, southwestern Taiwan (22°37’28.53”N,
120°15’39.08”E) from a bottom trawl sample, depth
unknown, and preserved in 70% ethanol. Three additional specimens from Indonesia collected in 1995
have been included in the specimen list (Table 1) to
increase information on the distribution of this species, but were not examined in this study.
Specimen identiication
Currently the genus contains three described species
(Reimer, 2011), the type species S. marsupialis (Gmelin, 1791), S. arenaceus Hertwig, 1882 and S. pedunculatus Hertwig, 1888. The latter two have not been reported on for over 80 years. The type species S. marsupialis, is worldwide in distribution (Soong et al.,
1999), including reports in the Paciic from the Great
Barrier Reef (Burnett et al., 1997) and Taiwan (Soong
et al., 1999). If S. marsupialis in fact consists of several sibling species, these would likely be very closely
related (Soong et al., 1999), and to date no evidence of
genetic differentiation among S. marsupialis specimens has been found (Burnett et al., 1997). This species has a rounded bottom portion, and is earthen-gray
in colour (Hertwig, 1882).
Sphenopus arenaceus Hertwig, 1882 (not mentioned
since Pax, 1924), is similar to S. marsupialis in being
unitary and free-living, but it has a rusty red colour,
while S. pedunculatus Hertwig, 1888 (not mentioned
since Delage and Hérouard, 1901) is heavily furrowed
46
Reimer et al. – The solitary zoanthid Sphenopus
Fig. 2. Maximum likelihood (ML) tree of an alignment of mitochondrial 16S ribosomal DNA sequences for zoanthid specimens. Values
at branches represent ML and neighbor-joining (NJ) bootstrap values, respectively. Sequences newly obtained in this study in bold.
Thick branches indicate Bayesian posterior probabilities >0.95. Sequences from previous studies in regular font with GenBank Accession Number. For specimen information see Table 1.
Contributions to Zoology, 81 (1) – 2012
and attached with a long ‘foot’ or ‘stalk’ to small pieces
of stone, as in specimens illustrated by Erhardt and
Knop (2005). All three species are solitary, azooxanthellate, free-living and live in sandy environments.
Specimens in this study best it the description of S.
marsupialis (solitary, not attached to substrate, sand
encrustation, inhabiting sandy/muddy bottoms, azooxanthellate, earthy-gray in colour) and were thus identiied as S. marsupialis.
Specimens collected from Brunei in April 2011
have been deposited in the collection of the Netherlands Centre for Biodiversity Naturalis at Leiden (catalogue number RMNH.Coel.40119). Additional material, collected on 30 November 2011 from the same
location, is deposited in the Universiti Brunei Darussalam Department of Biology reference collection
(catalogue number UBDM.6.00001).
DNA extraction, PCR ampliication, sequencing
Genomic DNA was extracted from portions of specimens either using spin-column Dneasy Animal Extraction protocol (Qiagen, Santa Clarita, CA) according to the manufacturer’s instructions, or by following
a guanidine extraction protocol as described in Sinniger et al. (2010). PCR ampliication using template
genomic DNA was conducted using HotStarTaq DNA
polymerase (Qiagen) according to the manufacturer’s
instructions. Mitochondrial 16S ribosomal DNA (mt
16S rDNA), cytochrome oxidase subunit 1 (COI) and
nuclear internal transcribed spacer region (ITS-rDNA)
were ampliied using primers and ampliication conditions following Sinniger et al. (2005, 2010), Reimer et
al. (2007a), and Reimer et al. (2007b), respectively.
Ampliied products were visualized by 1.0% agarose gel electrophoresis, and positive PCR products
were treated with Exonuclease I and Shrimp Alkaline
Phosphatate (Takara) prior to sequencing reactions. Sequencing was performed by MacroGen Japan (Tokyo).
Phylogenetic analyses
New sequences obtained in this study were deposited
in GenBank (accession numbers JQ323156-JG323180).
Sequences of all three DNA markers were aligned
with publically available sequences of family Sphenopidae (Palythoa), with Zoanthus (Zoanthidae) sequences utilized as outgroups for mt16S rDNA and
COI, as the monophylies of these two families and
their sister-group relationship has previously been
demonstrated (Sinniger et al., 2005). For the ITS-rD-
47
NA alignment, only Sphenopidae sequences were included, as this marker has been shown to have high
variability in Zoanthus (Reimer et al., 2007c).
All alignments were constructed manually based
on previously published and publically available
Brachycnemina (primarily Palythoa and Zoanthus)
sequence alignments, inspected by eye, and any ambiguous sites in the alignments were removed from the
dataset prior to phylogenetic analyses. Three alignment datasets were generated: 1) 757 sites of 39 sequences (mt 16S rDNA), 2) 462 sites of 35 sequences
(COI) and 3) 955 sites of 72 sequences (ITS-rDNA).
Alignment data sets are available from the corresponding author and at the homepage http://web.me.com/
miseryukyu/.
For the phylogenetic analyses of the data sets, the
same methods were independently applied. Alignments were subjected to analyses with the maximum
likelihood (ML) with PhyML (Guindon and Gascuel,
2003) and neighbour-joining (NJ) methods. PhyML
was performed using an input tree generated by BIONJ with the general time-reversible model (Rodriguez et al., 1990) of nucleotide substitution incorporating a discrete gamma distribution (eight categories)
(GTR+). The discrete gamma distribution and base
frequencies of the model were estimated from the
dataset. PhyML bootstrap trees (1000 replicates) were
constructed using the same parameters as the individual ML tree. The distances were calculated using a
Kimura’s 2-parameter model (Kimura, 1980). Support
for NJ branches was tested by bootstrap analysis
(Felsenstein, 1985) of 1000 replicates. CLC Free
Workbench 3.0 (Aarhus, Denmark) was used for NJ
phylogenetic analyses (1000 replicates).
Bayesian trees were made by Mr. Bayes 3.1.2 (Ronquist and Huelsenbeck, 2003) under GTR + I + Γ. One
cold and three heated Markov chains Monte Carlo
(MCMC) with default-chain temperatures were run for
2 million generations, sampling log-likelihoods (InLs),
and trees at 100-generation intervals (20,000 InLs and
trees were saved during MCMC). The likelihood plots
for COI, mt 16S rDNA and ITS-rDNA datasets suggested that MCMC reached the stationary phase after
the irst 300,000 generations for COI and mt 16S rDNA
(standard deviation of split frequencies = 0.006620 and
0.004511, respectively), and after 500,000 million generations for the ITS-rDNA analysis (standard deviation
of split frequencies= 0.052928). Thus, the remaining
17,000 trees of COI and mt 16S rDNA, and the remaining 25,000 trees of ITS-rDNA were used to obtain
clade probabilities and branch-length estimates.
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Reimer et al. – The solitary zoanthid Sphenopus
Fig. 3. Maximum likelihood (ML) tree of an alignment of mitochondrial cytochrome oxidase subunit 1 (COI) sequences for zoanthid
specimens. Values at branches represent ML and neighbor-joining (NJ) bootstrap values, respectively. Sequences newly obtained in this
study in bold. Thick branches indicate Bayesian posterior probabilities >0.95. Sequences from previous studies in regular font with
GenBank Accession Number. For specimen information see Table 1.
49
Contributions to Zoology, 81 (1) – 2012
Ancestral character state reconstruction
To reconstruct ancestral evolution in Brachycnemina,
ancestral character state reconstructions were performed with both ML and maximum parsimony (MP)
methods by tracing the character states of colony form
and zooxanthellae symbiosis over a ‘reduced taxa’ mt
16S rDNA ML tree utilizing Mesquite v.2.7.4 (Maddison and Maddison, 2010). The reduced taxa ML tree
contained only one sequence for each species or species group, and consisted of 757 sites in 13 taxa, with
the same basic alignment as in the mt 16S rDNA alignment in the previous section, with ML analyses also
performed as in the previous section. Species’ colony
form characters were assigned as: 0 (= colonial, attached to some substrate) for all species except S. marsupialis, 1 (= unitary, not attached to substrate/freeliving) for S. marsupialis; and 0 (= zooxanthellate) for
all species except S. marsupialis, 1 (= azooxanthellate)
for S. marsupialis.
Results
Phylogenetic analyses - mt 16S rDNA
The maximum likelihood (ML) tree resulting from the
analysis of the mt 16S rDNA alignment showed two
clear groups, one consisting of Zoanthus (family Zoanthidae) outgroups, and another clade with Sphenopus and Palythoa (Sphenopidae) sequences (Fig. 2).
All acquired S. marsupialis sequences were identical.
Support for the Sphenopidae clade was very high
(neighbor joining [NJ] = 100%, ML = 100%, Bayesian
posterior probability [Bayes] = 1.00). Within Sphenopidae, two subclades were seen. The irst consisted
of Sphenopus marsupialis (Gmelin, 1793) and Palythoa heliodiscus Ryland and Lancaster, 2003 sequences, but was only weakly supported (NJ = 56%, ML =
64%, Bayes < 0.50), while the second subclade included P. mutuki Haddon and Shackleton, 1891, P. tuberculosa Klunzinger, 1877, P. sp. ‘sakurajimensis’ sensu
Reimer et al. (2007a) and related sequences (NJ =
77%, ML = 69%, Bayes = 0.83).
+ Palythoa) formed another, very highly supported
clade (NJ = 99%, ML = 100%, Bayes = 1.00). Again,
all acquired S. marsupialis sequences were identical,
except for the sequence from specimen S5, which differed by one base pair. Within the Sphenopidae, resolution was poorer than observed in the mt 16S rDNA
tree, with three species (S. marsupialis, P. mutuki, P.
tuberculosa) appearing particularly poorly resolved,
i.e., no clear subclades and no strong support values
for each species group. All S. marsupialis sequences
formed a weakly supported clade (NJ = 63%, ML =
63%, Bayes = 0.87) together with sequences from P.
mutuki and P. tuberculosa, and most of the S. marsupialis sequences (S1, S2, S3, S7, S8, S10, S11, S12)
were identical to many of the P. mutuki and P. tuberculosa sequences, with only S5 being slightly unique to
the other S. marsupialis sequences.
ITS-rDNA
The ML tree for ITS-rDNA was once again similar in
overall topology to both mt 16S rDNA and COI, but
apart from the lack of a Zoanthidae outgroup, there
were some other small but noticeable differences (Fig.
4). Foremost, the tree showed good resolution, with all
species groups forming clear clades with relatively
high (e.g. ML>75%) bootstrap support. The Palythoa
heliodiscus group was seen to be most distant from
other species, followed by the very-well supported S.
marsupialis group (NJ = 100%, ML = 99%, Bayes =
1.00), which was sister to a well supported (NJ = 100%,
ML = 100%, but Bayes = 0.50) P. sp. ‘sakurajimensis’
+ P. mutuki + P. tuberculosa + P. sp. ‘yoron’ sensu
Reimer et al. (2007b) + P. caribeaoreoum clade. In the
Bayesian analyses, the P. heliodiscus subclade (Bayes
= 1.00) and the Sphenopus subclade (Bayes = 1.00)
were sister (Bayes=1.00) and within a P. mutuki + P.
tuberculosa + P. sp. ‘yoron’ + P. caribeaoreoum clade.
The S. marsupialis clade of ive sequences had some
variation between individual sequences (59/905 base
pairs = 6.5%), particularly in the spacers ITS1 and
ITS2, but similar or higher levels of ITS-rDNA sequence variation have previously been observed within other Sphenopidae species (Palythoa, see Reimer et
al., 2007b).
COI
Ancestral character state reconstruction
The ML tree for COI had a very similar overall topology to the mt 16S rDNA tree, albeit with some small
differences (Fig. 3). Again, Zoanthus spp. sequences
formed one clear clade, and Sphenopidae (Sphenopus
Both ML and MP analyses very strongly indicated that
colonial and zooxanthellate character states were ancestral in Sphenopidae (ML proportional likelihood =
50
Reimer et al. – The solitary zoanthid Sphenopus
Fig. 4. Maximum likelihood (ML) tree of an alignment of nuclear internal transcribed spacer region (18S, ITS-1, 5.8S, ITS-2, 28S) ribosomal DNA sequences for zoanthid specimens. Values at branches represent ML and neighbor-joining (NJ) bootstrap values, respectively. Thick branches indicate Bayesian posterior probabilities >0.95. Sequences newly obtained in this study in bold. For specimen
information see Table 1.
51
Contributions to Zoology, 81 (1) – 2012
ML
Z. kuroshio
MP
Z. sansibaricus
Z. gigantus
P. heliodiscus
S. marsupialis
P. sp. 1167
P. sp. sakurajimensis
P.sp. 1142
P. tuberculosa
(Ogasawara)
P. tuberculosa
(Israel)
P. tuberculosa
(Amami)
P. mutuki
0 colonial, attached, zooxanthellate
1 unitary, not attached, azooxanthellate
(Miyake)
P. mutuki
Fig. 5. Ancestral state reconstruction of gross
colony morphology and state of Symbiodinium (zooxanthellae) symbioses in brachycnemic zoanthids utilizing maximum likelihood
(ML; left) and maximum parsimony (MP;
right) methods traced on an identical ML tree
of mitochondrial 16S ribosomal DNA. Note
that gross colony morphology and symbiosis
state results gave identical results.
(Iriomote)
0.994-1.000 for all internal nodes), and that S. marsupialis alone has uniquely evolved into a unitary, freeliving and azooxanthellate state (Fig. 5).
Discussion
From the phylogenetic results of this study, Sphenopus
is unequivocally within the Palythoa generic level
clade (Figs 2-4), and even shares identical COI sequences with Palythoa tuberculosa. From these unexpected results, several conclusions can be drawn.
First, these results demonstrate that short (~460 bp)
COI sequences alone are not enough to distinguish all
zoanthid species from one another. Thus, any ‘DNA
barcoding’-type of identiication of zoanthids should
utilize additional mt 16S rDNA sequences, as suggested in Sinniger et al. (2008). Furthermore, these results
demonstrate the slow evolution of mt DNA in Anthozoa, as previously suggested (Shearer et al., 2002;
Huang et al., 2008). On the other hand, these results
demonstrate that any difference(s) in mt DNA sequences between zoanthid specimens is likely indicative of a species-level difference.
Secondly, the combined phylogenetic and ancestral
state reconstruction results demonstrate that changes
in gross morphology (e.g. body shape, colonial/unitary, etc.) and ecology (attached/free-living, zooxanthellate/azooxanthellate) can evolve with rapidity
within brachycnemic zoanthids (Fig. 5). From the present analyses, it appears that Sphenopus has made a
switch from the ancestral state (colonial, attached, zooxanthellate) in the Palythoa clade to a unitary, freeliving, azooxanthellate mode of life. While we did not
calculate a molecular clock time for the divergence
between Sphenopus and its closest Palythoa relative
(P. tuberculosa), the topology of the three DNA marker trees and previously estimated rates of anthozoan
DNA evolution (Medina et al., 2006) indicate that the
switch undoubtedly occurred within recent evolutionary history. In this context it is notable that a recent
molecular study of mushroom corals (Scleractinia:
Fungiidae) shows that evolutionary switches in morphology occur within clades, in this case from a freeliving mode of life towards attached and encrusting
growth forms, and that such changes are more common than expected (Gittenberger et al., 2011; Benzoni
et al., subm.). Phylogenetic reconstructions of the
Fungiidae indicate that the overall morphology (habitus) of corals can change rapidly while similarity in
microstructures of the coral skeleton are more consistent within evolutionary lineages (Hoeksema, 1991;
Gittenberger et al., 2011). In zoanthids, Sphenopus is
the only extant group that has taken the evolutionary
path to an unattached mode of life, and apparently
very recently in evolutionary terms.
The switch from a modular to solitary body plan is
another character state transformation unique to Sphenopus among zoanthids. Although Sphenopus polyps
are relatively large compared to those in Palythoa, by
being solitary the whole body size as compared to encrusting forms appears more constrained. A small
52
body size allows Sphenopus individuals to live partly
buried in sand or on top of it, apparently enabling
some degree of mobility and a subsequent capacity to
shed sediments, as seen in free-living mushroom corals (Hoeksema, 1988; Bongaerts et al., 2012). The
mushroom coral family Fungiidae shows several evolutionary lineages with trends from solitary (monostomatous) to modular (polystomatous) coral shapes
(Hoeksema, 1991; Gittenberger et al., 2011; Benzoni et
al., subm.). The smallest free-living solitary mushroom
coral species (several Cycloseris spp.) are most abundant on sandy substrates and have been found co-occurring with Sphenopus individuals in the Spermonde
Archipelago, South Sulawesi (Hoeksema, pers. obs.).
These Cycloseris corals can maintain a small body
size and perform asexual reproduction by fragmentation through autotomy (Hoeksema and Waheed, 2011).
In contrast, some other Cycloseris species appear to be
polystomatous and encrusting (Gittenberger et al.,
2011; Benzoni et al., subm.). The largest mushroom
coral species, either free-living or attached, are all
polystomatous and occur on solid substrates (Hoeksema, 1991; Gittenberger et al., 2011), although some of
them may also use fragmentation for reproduction and
dispersal (Hoeksema and Gittenberger, 2010). Even if
the evolutionary development from modular to solitary
growth forms appears less common among anthozoans than the reverse, among zoanthids it is most likely
connected to the colonization of sandy habitats.
Thus, the unexpected phylogenetic position of
Sphenopus despite its unique body plan leads to the
question of what a zoanthid genus encompasses. For
obvious reasons, it is desirable to keep Sphenopus as
a valid genus separate from the Palythoa clade, yet
this does not relect phylogeny and evolution. The traditional image of Palythoa being colonial and zooxanthellate may not be correct as additional, undescribed, azooxanthellate Palythoa species from coral
reef caves have been found in Okinawa (Reimer,
2010), and it appears this genus encompasses a much
wider diversity of lifestyles and ecologies than previously thought. A re-examination of Palythoa and its
generic deinition is obviously needed to reconcile
taxonomy and nomenclature with the data presented
here. Despite very different gross morphologies and
ecologies, Sphenopus and Palythoa do have many
common features, including: 1) being brachycnemic
and having sand encrustation in the mesoglea, 2) having zoanthella (not zoanthina) larvae, and 3) lacking
b-mastigophore nematocysts (Ryland and Lancaster,
2003). Thus, a future merging of these genera after
Reimer et al. – The solitary zoanthid Sphenopus
additional conirmation is not as far-fetched as it may
initially seem.
An analogy exists among mussels (Mytilidae) boring in live corals. While shells of species classiied
with Leiosolenus, which live as endosymbionts in a
wide range of host corals, are more or less cylindrical
and torpedo-shaped, those belonging to Fungiacava,
exclusively boring in mushroom corals (Fungiidae),
are typically lat and heart-shaped. Although based on
molecular evidence Fungiacava is part of the Leiosolenus clade, its unique shell shape and host speciicity justify its status as a separate genus (Owada and
Hoeksema, 2011).
The results of this study resemble other recent phylogenetic results in which it was seen that the rediscovered zoanthid genus Neozoanthus (Neozoanthidae) is apparently very closely related to Isaurus (Zoanthidae), calling into question the existence of Neozoanthidae as a valid family (Reimer et al., 2011a). As
well, the zoanthid genus Acrozoanthus was demonstrated to be within Zoanthus (Zoanthidae), despite
having a unique ecology (Reimer et al., 2011b). In
contrast to the suborder Macrocnemina, in which different genera apparently have long evolutionary histories with various other organisms that they utilize as
substrates (Sinniger et al., 2010), it appears that brachycnemic zoanthids, although generally restricted in
distribution to shallow subtropical and tropical waters
(Swain, 2010), can evolve new life history strategies
and change their gross morphology relatively rapidly,
allowing species to inhabit the many various microhabitats of coral reef ecosystems. It may be that the
high levels of intraspeciic morphological variation
observed in some brachycnemic species (Burnett et
al., 1994, 1995; Reimer et al., 2004) are adaptive in
allowing species to diversify rapidly when encountering changes in environments.
This and other recent studies (Gittenberger et al.,
2011; Owada and Hoeksema, 2011; Reimer et al.,
2011a; Benzoni et al., subm.) demonstrate that in invertebrates with relatively simple and/or modular body
plans morphological or ecological characters thought
to be diagnostic may not always be so. Comprehensive
analyses utilizing both molecular and morphological
methods will allow researchers to re-assess relationships not only between zoanthids, but also in many
other understudied marine invertebrate groups. At the
same time, it is hoped that as an end result of such
studies, the classiication and identiication of zoanthids can become more accessible, allowing a clearer
understanding of this order of hexacorals.
Contributions to Zoology, 81 (1) – 2012
Acknowledgements
JDR was supported in part by the Rising Star Program and the
International Research Hub Project for Climate Change and
Coral Reef/Island Dynamics (both University of the Ryukyus).
Collection of Brunei specimens was funded by a Science &
Technology Grant from the Brunei Government (grant number
UBD/GSR/S&T/14). Three anonymous reviewers’ comments
greatly improved the manuscript.
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Publications.
Received: 3 August 2011
Revised and accepted: 14 December 2011
Published online: 31 January 2012
Editor: R.W.M. van Soest