The snail moment you’ve been waiting for …

Welcome to Blog 6, and yes, it’s June. Time marches ever onwards, and we’re half way through the year! Today is the day you’ve been waiting for. The day when I’m going to tell you all about Stephen’s master’s thesis. Thank you for your patience, but the long wait is finally over. Welcome to the discussion of “A classical taxonomic revision of Seraphsidae Jung 1974 (Gastropoda) using a pluralist approach to species assessment”.

Stephen’s thesis demonstrates that a “pluralistic approach to classic taxonomic practice can lead to increased cladistics resolution in a clade, including the recognition of new taxa at all nomenclatural levels”. Okay, so firstly, what is a pluralistic approach? Taxonomists and systematists have a tendency to name species based on particular species conceptions, such as the biological or phylogenetic species concepts. However, often the actual description of the species is not based on that conception. For example, many species are often described based on morphology, but the concept used to define them is based on reproductive compatibility. Pluralism allows a species to be described using multiple concepts, without the need to apply a particular definition that would be limiting. This leads to increased flexibility, and allows for use of all available evidence, rather than limiting the description to only a single kind (e.g. morphology), which often does not match the conception that was used to theoretically distinguish the taxa.

Fig. 1. The fighting conch Strombus pugilus

Stephen’s thesis had three aims: firstly, Stephen wanted to divide the Stromboidea into distinctive evolutionary crown clades using morphological evidence. The intention here was to reinstate the historical understanding of the internal cladistics relationships. A crown clade is defined based on living taxa. The first clade was easy, which is called the Neostromboidea. Animals in this clade contain taxa more closely related to Strombus pugilus (Fig. 1), Terebellum terebellum and Tibia fusus than members belonging to the Struthiolariidae and Aporrhaidae (Fig. 2). The other remaining members of the Stromboidea, the Struthiolariidae and the Aporrhaidae. Defining the other clade in its entirety is, currently, problematic as it would create meaningless ranks.

Fig. 2. The pelican's foot snail Aporrhais serresianus

Stephen’s second aim was to complete an alpha-taxonomic revision of the group defined by Jung (1974) known as the Seraphsidae using a pluralist approach to species conception. I should point out here that he looked at fossils. He first had to compile a list of all the defined taxa. Now this sounds easy, but in reality he had to look for all the synonyms too, so it made it quite a lot of work. He then had to reassess the validity of the synonyms using all available evidence, including morphological variation, historical distribution and temporal positioning. If the named synonymic species could not be validated in the literature, it was not included. Only those with a description that enabled clear resolution of the original author’s intent when describing the species were included. Once he had done this, he had to establish a set of characters for placement in a character state table. He then drew a phylogenetic tree that formed the basis of the internal resolution within the clade (Fig. 3).

Fig. 3. tnt generated maximum likelihood cladogram showing internal cladistic relationships within Seraphsidae Jung 1974 (Taken from Stephen's thesis)

Finally, Stephen’s third aim was to resolve the infrageneric relationships within the Seraphsidae using morphological cladistics. Simply put, what Stephen found here was a need to differentiate and organise this subclade into distinctive inclusive clades that provided an improved understanding of the evolutionary patterns within the group. The first clade represents the Neoseraphsinae nomen novem, with an involute terminal growth pattern. The second clade represents the Neoterebellinae nomen novem, with an evolute terminal growth pattern. Why is this growth pattern important in an evolutionary sense? Because it is related to an essential life history characteristic relating to burial, where the evolute pattern enables detection of burial. That all the Neoseraphsinae nomen novem are extinct indicates that this slight change in shell pattern in the Neoterebellinae nomen novem may have provided an evolutionary advantage, where burial detection may have allowed for additional protection from predation.

The most important message from Stephen’s thesis is that using a pluralistic approach can improve our understanding of internal cladistics relationships in evolutionary trees, and how that can then be reflected in the nomenclature.

You can read more about my research in my publications listed on my blog. You can also find me on ResearchGate, the James Cook University website and Twitter.

A different snail tale …

Welcome to May, and Blog 5. Time continues to fly, and we’re nearly half way through the year and still going strong! I’m going to continue with the current theme of marine gastropods, going back a little way to 2016 to introduce you to a brand new species that Stephen passionately described. Yes, I’m aware that the tension is mounting over his master’s thesis. Remember, patience is a virtue!

The family Cardiidae is an ancient clade that has its origins in the late Triassic. However, the internal resolution within the subclade Tachycardiinae is still unresolved. Within this subclade, two genera are recognised, namely Vasticardium and Acrostergima, and this paper, published in The Festivus, identified a new species falling within the genus Vasticardium.

The new species we describe is called Vasticardium swanae (Fig. 1) in honour of Beverly Swan from Townsville, Queensland, Australia. She made the initial discovery of the type, and has been collecting shells, and has provided support to Stephen, for many years.  

Fig. 1. Holotype of Vasticardium swanae (Maxwell et al. 2016)

V. swanae is located in tropical Queensland, with the type specimen being located on Bramble Reef (Fig. 2). This is currently the only known location of this species, potentially indicating a limited distribution.

Fig. 2. Location of Bramble Reef (Google Maps; 13/05/2019)

The shell (Fig. 1) is quite distinctive, and more colourful for a large Vasticardium (Fig. 3). The dorsum has dark to light brown blotches and flecks, sometimes appearing as ventral purple stain. The shell is moderately strong, equi-valved and ovate to quadrate. The rib morphology is also quite distinctive comparatively. The anterior ribs have strong spines that dorsoventrally increase in size. The ribbing is quite coarse and lacks colour in the aperture. Other characteristics include a fine ligament, fine and well developed anterior and posterior teeth, unequal cardinal teeth, a straight, serrated and interlocking margin that becomes elongated at the anterior dorsal margin and diminishes on the posterior margin, a concave and smooth lunacle, and white umbones (Fig. 1).

Fig. 3. The new species Vasticardium swanae (G) in comparison to other known species of Vasticardium (Maxwell et al. 2016)

The interpretation of new species is largely dependent on the observations of the taxonomist, and their own opinions. In this case, we used morphology to designate the species, while others may use different species concepts to interpret relationships. There aren’t too many important lessons to take away from this paper, although I suggest that we should always be aware that species delimitation and conception are sometimes mixed up, and it should be made clear from the outset how a species has been defined.

You can read more about my research in my publications, listed on my blog. You can also find me on ResearchGate, the James Cook University website and Twitter.

Back to Laevistrombus …

It’s April, and we’re on to Blog 4. Progress is ever-onward, and I can’t believe how quickly the time is flying! Continuing with the current theme of marine gastropods, I’m going to continue with the story of Laevistrombus, discussing a paper that Stephen published in 2017. Yes, I know, I haven’t said anything about his masters thesis yet. All in good time!

Laevistrombus canarium (Maxwell et al. 2019)

Sex-ratio bias is common in gastropods, particularly in the Strombidae, with generally 0.75 males per female. In addition, this sex imbalance is variable on both spatial and temporal scales. Currently, the mechanisms driving sex-ratio bias are little explored. Therefore, in this paper, published in Memoirs of the Queensland Museum - Nature, we explored some of these mechanisms in Laevistrombus canarium (Fig. 1) from two populations.

Four Mile Beach and Alexandra Reefs (Google Maps; 14/04/2019)

We sampled individuals from two reefs in Far North Queensland, Australia, with different benthic compositions, namely Four Mile Beach and Alexandra Reef (Fig. 2). Snails were sampled over nine trips at low tide between June 2013 and May 2016. We collected individuals from bot mating and non-mating clusters when possible. For each individual, we recorded the substrate it occurred on and its sex.

We also found a strong bias towards females, which was consistent across both locations and across benthic substrates (Table 1). Our findings suggest that inherent genetic factors (possibly related to either ZQ-ZZ or XX-XY sex determination mechanisms) regulate natural patterns of sex imbalance in L. canarium. These findings also suggest that some life-history characteristics, yet to be clearly established, are associated with the origin and maintenance of this sex-ratio bias in L. canarium.

* Taken from Maxwell et al. 2017

Again, I hope that you realise that marine snails provide a really rich area for scientific scrutiny. And who knows how much more we will learn in the future. Perhaps the most important lesson that I take out of this paper is not a specific conclusion, or anything that is going to shatter the core of scientific thinking. I suggest that we shouldn’t just assume that patterns will be the same across closely related species. I suggest that we should explore with an open mind, and not just generalise that a snail will behave like any other snail. We really should admire these species, and indeed populations and even individuals within species, for what they are: unique.

You can read more about my research in my publications, listed on my blog. You can also find me on ResearchGate, the James Cook University website and Twitter.