Rosa Rubicondior: Creationism Refuted - Evolution By LOSS of Genetic Information

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Pycnogonum litorale, adult female feeding on a sea anemone.

C: Georg Brenneis

What the sea spider genome reveals about their bizarre anatomy

Creationists frequently argue that macroevolution without divine involvement is impossible because it supposedly requires the creation of new genetic information to code for novel structures. They assert that such new genetic information cannot arise through natural processes, claiming this would violate the Second Law of Thermodynamics. However, try getting a creationist to explain what the Second Law of Thermodynamics actually is, how it relates to genetic information, and why it supposedly forbids gene duplication, and it quickly becomes apparent that they haven’t the faintest idea what they’re talking about.

Of course, this entire argument hinges on a distorted definition of macroevolution, namely the claim that it must involve the appearance of entirely new structures not present in ancestral forms. Like so many creationist arguments, it is built on misinformation and the misrepresentation of fundamental biological concepts. Macroevolution refers to evolutionary changes above the species level, while evolution more broadly is defined as a change in allele frequencies in a population over time.

Another familiar plank in the creationist propaganda platform is the patently absurd claim that evolution cannot occur through a loss of genetic information, on the grounds that lost genetic material is always deleterious—if not fatal—and therefore cannot be passed on to subsequent generations. This claim, too, wilfully ignores well-established mechanisms in evolutionary biology.

So, a recent paper from an international team including researchers from the University of Vienna and the University of Wisconsin–Madison (USA) should present a problem for that narrative. The study shows that the bizarre body plan of marine arthropods known as sea spiders (Pycnogonida) is the result of a lost gene.

If creationists were intellectually honest, they might take this as a cue to question why creationist ‘scientists’ (to use the term loosely) are misleading them. More likely, however, they’ll claim that it’s the mainstream biologists who are doing the lying—despite the fact that the latter group provide empirical evidence to support their conclusions.

The research is detailed in an open-access paper in BMC Biology.

What information is there on the evolution and biology of Pycnogonida? Sea spiders (class Pycnogonida) are a fascinating and somewhat enigmatic group of marine arthropods whose evolutionary relationships and biology have long intrigued zoologists and evolutionary biologists. Here's a concise yet comprehensive overview of their biology and evolutionary history:

Biology of Pycnogonida

General Anatomy

Body Plan: Sea spiders have a highly unusual morphology. Their bodies are small and narrow, but they possess very long legs in relation to body size—sometimes over 90% of their total body length.
Segmentation: Their body is divided into a cephalon (head) and trunk, with a reduced or absent abdomen. They usually have four pairs of walking legs, although some species have five or six pairs.
Proboscis: A prominent, tube-like proboscis is used to suck fluids from prey such as cnidarians and bryozoans.
Digestive and Reproductive Systems in the Legs: Due to their reduced body size, parts of the digestive, excretory, and reproductive systems extend into the legs.

Nervous System and Sensory Organs

They have a simple nervous system with a brain and ventral nerve cord.
Many species have eyes, typically four, arranged on a central tubercle. However, some deep-sea species are eyeless.

Reproduction and Development

Sexual dimorphism is common; males carry the fertilised eggs in external egg sacs located on special legs called ovigers.
Larval forms are often protonymphon larvae, which are free-swimming and bear only a few appendages initially.
Development is indirect and sometimes complex, with multiple moults.

Habitat and Ecology

Pycnogonids are found in all marine environments, from shallow tidal pools to the deep sea (some species live at depths >7,000 m).
Their diet includes soft-bodied invertebrates: sea anemones, hydroids, bryozoans, sponges.
Despite their spider-like appearance, they are not true spiders, nor are they closely related to them in any functional sense.

Evolutionary Relationships and Phylogeny

Evolutionary Placement

Pycnogonida are arthropods, but their position within Arthropoda has been controversial:

Historically grouped with chelicerates (spiders, scorpions, horseshoe crabs) due to similarities in mouthparts.
Molecular and morphological data have produced conflicting phylogenies. Some studies suggest they may represent a basal or sister lineage to all other extant arthropods, rather than fitting within Chelicerata.

Genetic and Genomic Insights

Recent genomic studies, including the subject of this blog post, reveal gene loss as a significant factor in their evolution.
In particular, the loss of the Hox gene abdominal-A is linked to their unusual body plan. This gene typically regulates the development of abdominal segments in other arthropods, and its absence may explain why sea spiders have no proper abdomen.

Fossil Record

The fossil record of Pycnogonida is sparse due to their soft, poorly mineralised bodies.
Some Cambrian fossils (e.g. Cambropycnogon) show sea-spider-like features, suggesting the group is ancient, possibly dating back over 500 million years.
However, many palaeontologists are cautious about definitive classification of these early forms.

Notable Features & Evolutionary Significance

Pycnogonids show extreme morphological divergence, possibly due to paedomorphosis (retention of juvenile traits in adults).
Their anatomical quirks—such as reduced segmentation, the use of legs for internal organs, and external egg brooding—provide insight into the plasticity of arthropod body plans.
They demonstrate how evolution can proceed not just through addition of genetic information, but also through loss, as supported by gene loss studies.

Key References

BMC Biology (2025): What the sea spider genome reveals about their bizarre anatomy
Arango et al. (2007): “Phylogeny of Sea Spiders (Pycnogonida) Based on Nuclear and Mitochondrial DNA Sequences.”
Brenneis, G. & Scholtz, G. (2014): “The evolution of sea spider body plans: Hox gene expression in Pycnogonida.”
Dunlop, J.A., et al. (2008): “Pycnogonid affinities: a review.”

It is also the subject of a news release from the University of Vienna's Department of Biology.

What the sea spider genome reveals about their bizarre anatomy>
The first high-quality pycnogonid genome provides novel insights in chelicerate evo-devo

An international collaboration featuring the University of Vienna and the University of Wisconsin-Madison (USA) has led to the first-ever chromosome-level genome assembly of a sea spider (Pycnogonum litorale). The genome informs about the development of the characteristic sea spider body plan and constitutes a landmark for revealing the evolutionary history of chelicerates in general. The study was recently published in BMC Biology.
Sea spiders (Pycnogonida) are marine arthropods with highly unusual anatomy: their trunk is very narrow and short, many of their internal organ systems extend into their long legs, and their abdomen is extremely reduced, to the point of being almost unrecognisable. Together with much better known animals such as spiders, scorpions, mites, or horseshoe crabs, sea spiders belong to the group called chelicerates, named after their claw-like mouthparts, the chelicerae. The bizarre body plan of these "no-bodies" raises fascinating questions: what genetic factors underlie its formation? And what can this tell us about the evolutionary history of chelicerates? The answers lie in their genome.

A high-resolution genome

To produce the genome assembly, researchers combined complementary sequencing technologies. First, the genetic material of a single P. litorale individual was obtained using so-called "long-read sequencing", a technology that is able to capture very long stretches of DNA. This facilitates the correct assembly of otherwise challenging repeated or complex genomic regions. Then, the spatial organisation of the genome was assayed in a second P. litorale individual, revealing which DNA pieces lie close to each other in the cell nucleus. By leveraging the distance information, the correct order of the sequenced DNA stretches can be ascertained. This combination of data sources led to the assembly of 57 pseudochromosomes, representing almost the entirety of the sea spider genome in unprecedented resolution. This was additionally supplemented by novel datasets of gene activity in various developmental stages of P. litorale.
The genomes of many non-canonical laboratory organisms are challenging to assemble, and Pycnogonum is no exception. Only the combination of modern high-throughput data sources made a high-quality genome possible. This can now serve as a stepping stone for further research.

"Nikolaos Papadopoulos, first author
Integrative Zoology Unit
Department of Evolutionary Biology
University of Vienna, Vienna, Austria.

Lost genes, visible effects

The research team paid special attention to the so-called Hox cluster – a gene family that is evolutionarily conserved across the animal kingdom.

In arthropods, Hox genes play a central role in the correct specification of the different body segments; but also in many other animal groups they are essential 'master controllers' during body plan development.

Andreas Wanninger, co-corresponding author
Integrative Zoology Unit
Department of Evolutionary Biology
University of Vienna, Vienna, Austria.

The exciting secret of Pycnogonum litorale: a part of the Hox cluster is missing completely from the genome, namely abdominal-A (Abd-A), a gene typically involved in specification and development of the posterior part of the body. Its absence could be linked to the extreme reduction of the pycnogonid abdomen. Similar conditions have been observed in other arthropods with reduced posteriors, such as certain mites and barnacles. Thus, sea spiders offer another example for the well-documented evolutionary relationship between Hox gene loss and body part reduction.

The genome also offers insights into broader evolutionary patterns. Unlike spiders and scorpions, whose genomes show clear signs of ancient whole-genome duplications, no such traces can be found in the genome of P. litorale. As pycnogonids are considered the sister taxon to all chelicerates, this suggests that the genome of the chelicerate ancestor did not already have these duplications; rather, they must have happened much later in evolution, for certain chelicerate sub-groups.

A new reference genome

This newly assembled high-quality genome paves the way for further comparative studies. P. litorale thus becomes a novel valuable reference species in regards to questions about the interrelationships between chelicerates and the evolution of their body plans, as well as the genetic mechanisms that underlie the diversity of arthropods.

From an evolutionary developmental perspective, sea spiders are very interesting: their mode of development may be ancestral for arthropods, but at the same time they boast multiple body plan innovations unique to themselves. Beyond this, they also possess remarkable regenerative abilities. Now that we have the genome and comprehensive datasets on gene activities during development, we can systematically study all of these aspects on the molecular level.

Georg Brenneis, co-corresponding author
Integrative Zoology Unit
Department of Evolutionary Biology
University of Vienna, Vienna, Austria.

The researchers will use the novel reference genome for further studies on gene regulation, development, and regeneration in chelicerates, aiming to better understand the processes underlying the evolutionary success of this group.

Publication:
Nikolaos Papadopoulos ;Siddharth S. Kulkarni; Christian Baranyi; Bastian Fromm; Emily V.W., Setton; Prashant P. Sharma; Andreas Wanninger; and Georg Brenneis.
The genome of a sea spider corroborates a shared Hox cluster motif in arthropods with a reduced posterior tagma. BMC Biol 23, 196 (2025). https://doi.org/10.1186/s12915-025-02276-x

Abstract

Background
Chelicerate evolution is contentiously debated, with recent studies challenging traditional phylogenetic hypotheses and scenarios of major evolutionary events, like terrestrialization. Sea spiders (Pycnogonida) represent the uncontested marine sister group of all other chelicerates, featuring a—likely plesiomorphic—indirect development. Accordingly, pycnogonids hold the potential to provide crucial insight into the evolution of chelicerate genomes and body patterning. Due to the lack of high-quality genomic and transcriptomic resources, however, this potential remains largely unexplored.

Results
We employ long-read sequencing and proximity ligation data to assemble the first near chromosome-level sea spider genome for Pycnogonum litorale, complemented by comprehensive transcriptomic resources. The assembly has a size of 471 Mb in 57 pseudochromosomes, a repeat content of 61.05%, 15,372 predicted protein-coding genes, and robust completeness scores (95.8% BUSCO Arthropoda score, 95.7% of conserved microRNA families). Genome-scale self-synteny and homeobox gene cluster analysis show no evidence of a whole-genome duplication (WGD). We identify a single, intact Hox cluster lacking Abdominal-A (abdA/Hox9), corroborated by the absence of an abdA ortholog in the novel transcriptomic resources.

Conclusions
Our high-quality genomic and transcriptomic resources establish P. litorale as a key research organism for modern studies on chelicerate genome evolution, development, and phylogeny. The lack of WGD signature in P. litorale further strengthens the inference that WGDs are derived traits in the chelicerate tree. The combination of abdA loss with the reduction of the posterior tagma emerges as a common theme in arthropod evolution, as it is shared with other, distantly related arthropod taxa with a vestigial opisthosoma/abdomen.

Background

Chelicerata represents an extremely diverse arthropod lineage boasting more than 120,000 extant species [1]. They inhabit a wide range of habitats and have adopted highly divergent life strategies, as impressively evidenced by extant terrestrial arachnid taxa (such as spiders, scorpions, harvestmen, mites, and ticks). In contrast to most of their chelicerate kin, horseshoe crabs (Xiphosura) and sea spiders (Pycnogonida), plus selected mite taxa, are the only extant groups that inhabit the oceans [2, 3].

Paramount to the chelicerate radiation has been the evolutionary plasticity of their body plan. One of its widely conserved hallmarks is the presence of two tagmata: the prosoma and the opisthosoma [4]. The anterior prosoma comprises the ocular region and typically (but not always) six segments bearing the eponymous raptorial chelicera, followed by the pedipalp and four pairs of legs, all of which have been subject to considerable transformations in different lineages. By contrast, the posterior opisthosoma displays far greater evolutionary plasticity across chelicerates, not only in terms of segment number (up to 12 plus the asegmental terminus, the telson), but also with regard to the presence and function of diverse appendage derivatives (e.g., book gills, book lungs, or spinnerets) [4, 5].

The body organization of sea spiders is a unique variation on the chelicerate theme, characterized by several lineage-specific traits. The prosoma carries a prominent suctorial apparatus (proboscis), a heavily modified first leg (oviger) used for egg-carrying and grooming, and at least four pairs of true walking legs [6]. However, three distantly related pycnogonid taxa even possess five or six leg pairs [7,8,9], which showcases a variability in the segmental composition of the prosoma that is uncharacteristic for the other chelicerate taxa. On the other hand, the opisthosoma of pycnogonids is dramatically reduced and represents only a small posterior protrusion (anal tubercle or “abdomen”) (Fig. 1A). Notably, it remains unclear to what extent vestigial opisthosomal segments contribute to the anal tubercle (e.g., [10]).

Fig. 1

A Adult Pycnogonum litorale specimen (approx. 1 cm body length without proboscis), feeding on the sea anemone Metridium senile. The green arrowhead points to the vestigial opisthosoma/anal tubercle. B Post-embryonic instars V of P. litorale (fluorescing in light blue under UV light exposure; approx. 0.5–0.6 mm in size), feeding on the hydrozoan Clava multicornis. C Chromatin contact map, generated from Omni-C data at 1 kb resolution, showing the 57 pseudochromosomes of the haploid genome (dark squares) and the unscaffolded contigs (bottom right region). Color denotes the number of contacts found in each region.

Notwithstanding their peculiar adult body organization, pycnogonids are the only extant chelicerates that display pronounced indirect development: the primary larva that hatches from the egg features only three appendage-bearing segments and subsequently undergoes anamorphic development with sequential body segment addition at the posterior pole. This trait may not only resemble the ancestral condition of chelicerates but also of arthropods in general [11,12,13]. Accordingly, the study of sea spider development holds the potential to crucially inform debates on the evolutionary trajectories of chelicerate body patterning [14, 15]. This is further underscored by their position in the chelicerate tree of life. After a long phylogenetic odyssey [16], pycnogonids are now robustly established as sister group of all other taxa (= Euchelicerata), rendering them one of the few stable anchors in the historically contentious and still controversial higher-order phylogeny of chelicerates [17,18,19,20,21,22].

This stable pycnogonid-euchelicerate sister group relationship also highlights a key role for sea spiders in interpreting the evolution of chelicerate genomes. For instance, the recently proposed hypothesis that whole-genome duplication (WGD) events in arachnopulmonates (spiders, scorpions and kin) and in xiphosurans [23,24,25,26] represent derived states within chelicerates (e.g., [25]) is based on the lack of extensive duplications in so-called apulmonate taxa (harvestmen, ticks, mites) [27,28,29,30]. However, with euchelicerate interrelationships still in flux and no high-quality genomic resources for pycnogonids, polarization of WGD events and the reconstruction of the ancestral chelicerate condition remain challenging.

Similarly, the current availability of but a single scaffold-level draft genome for Nymphon striatum [31] and a handful of bulk transcriptomes from limited developmental stages [7] has prevented inclusion of pycnogonids in macroevolutionary comparative studies. This includes studies on the composition of the Hox gene cluster, one of the best-known and most extensively discussed syntenic motifs of metazoan genomes (e.g., [32]). In euchelicerates, Hox gene cluster duplications provided the first hints for WGD events (e.g., [24]). Functionally, Hox genes play a crucial role in the specification of segment identity along the anterior–posterior body axis of arthropods [33]. This renders them crucial targets in the study of the genetic underpinnings of some of the unique features of the pycnogonid body plan.

The sea spider species Pycnogonum litorale (Strøm, 1762) from the north Atlantic is an emerging laboratory organism that allows us to bridge this knowledge gap. It can be kept in laboratory cultures (Fig. 1A, B), is long-lived, and displays year-round reproduction with a few thousand eggs per mating [34, 35]. In addition to these favorable characteristics, a solid body of morphological data on ontogeny and adult morphology are available for this species (e.g., [36,37,38,39]). As a result, it has contributed to a number of developmental genetic investigations, both in standalone works as well as comparative studies [10, 40,41,42]. To complement this morphological foundation and overcome the limitations of the molecular resources available for pycnogonids, we present here the first genome of P. litorale together with novel transcriptomes for a closely spaced series of embryonic stages and post-embryonic instars of its life cycle.

The discovery that sea spiders (Pycnogonida) owe their unusual body plan to the loss of a key developmental gene directly undermines a central creationist claim: that evolution cannot proceed through the loss of genetic information. Creationist arguments often hinge on the belief that genetic information can only be degraded, not repurposed or lost in beneficial ways, and that such loss is invariably harmful. However, this study demonstrates the opposite — that gene loss can drive major anatomical innovation, resulting in a radically different, yet functional and evolutionarily successful, body plan.

Moreover, the finding contradicts the creationist assertion that new traits and body structures require the miraculous insertion of "new genetic information" from an intelligent designer. In this case, a strikingly different morphology evolved not by adding complex new genes, but by losing one that controls body segmentation in other arthropods. This is a clear, well-supported example of how natural processes—specifically gene loss—can lead to evolutionary novelty.

Finally, the study exposes the creationist misuse of the Second Law of Thermodynamics. Evolutionary processes, including gene duplication, mutation, and loss, occur within open systems where energy flows (like sunlight or metabolism) allow local increases in order—exactly as thermodynamics permits. The evolutionary story of the sea spider is yet another reminder that nature doesn’t need supernatural shortcuts to innovate; it simply needs time, variation, and selection.

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