The Hidden Biology of Sponges and Ctenophores

Traditional views on animal evolution portray it as a progression toward complexity, suggesting different animal groups represent grades of complexity due to traits acquired over time.
Recent phylogenetic analyses challenge this classical hypothesis, indicating that it is mismatched with true evolutionary relationships.
A new hypothesis suggests that traits, particularly complex ones, have been both gained and lost throughout evolution.
To understand this new model, deeper knowledge of key clades like Porifera (sponges) and Ctenophora (comb jellies) is essential, as these groups are often modeled poorly and misrepresented.

There are two key sources for understanding early animal evolution: fossils and existing animal diversity.
Utilizing living diversity to inform about historical evolution necessitates analyzing phylogenetic relationships and character traits across a wide range of living species.

All known animals fit into one of five clades:
- Porifera (sponges)
- Ctenophora (comb jellies)
- Placozoa
- Cnidaria
- Bilateria
Monophyly of each clade is supported, scrutinized through morphological, developmental, genomic, and physiological evidence.
Ctenophora or Porifera may be the sister group to all other animals according to various phylogenetic analyses.

Ctenophores and sponges are among the least-studied animals, resulting in significant gaps in knowledge about their morphology and physiology.
Current knowledge largely reflects bilaterian perspectives, skewing true biodiversity and complexity of traits in non-bilaterians.
The misrepresentation of these groups often leads to assumptions of simplicity compared to bilaterians.
This necessitates further exploration to address fundamental questions regarding the most recent common ancestor of all animals, the mechanisms behind trait acquisition, and how traits may have been gained or lost.

Focus on Bilaterians: The bulk of zoological research has concentrated on bilaterians, leaving non-bilaterians like sponges and ctenophores underexplored.
Complexity of Non-Bilaterian Traits: Shared traits with bilaterians are easier to study than unique non-bilaterian traits, leading to hidden biology.
Diversity Underappreciation: The richness of diversity within both Porifera and Ctenophora is often overlooked, lacking focus on multiple species.
Fragility of Specimens: Many species within these groups are fragile and inhabit challenging marine environments, complicating research efforts.

Historically, Porifera was considered the sister group to all other animals, based partly on the assumed homology between choanoflagellates and choanocytes.
Molecular studies have shifted this understanding, suggesting ctenophores may be the sister group to all Metazoans.
The phylogenetic debate continues regarding placements like Coelenterata (cnidarians and ctenophores as sister taxa) and Acrosomata (ctenophores sister to bilaterians).
Improved data acquisition and phylogenetic techniques are crucial for clarifying these relationships.

Ctenophores are recognized for their eight longitudinal rows of ciliary paddles (combs), aiding in locomotion.
- Approximately 200 known species, with many likely undescribed.
- They inhabit diverse marine environments, reaching depths of at least 7000m and exhibiting carnivorous feeding habits.
Unique genomic characteristics include absence of miRNA, reduced mitochondrial genomes, and unique gross anatomy.
Ctenophores lack traditional muscles and have unique nervous systems, featuring interconnected nerve nets and giant smooth muscles but no standard neurotransmitters found in bilaterians.

Rotational symmetry rather than radial symmetry.
Tentacles with specialized glue cells called colloblasts for prey capture.
Unique structures at the aboral end related to gravity sensing (apical organ).
Misconceptions about ctenophores include their portrayal as primitive and simplistic animals. They exhibit complexities of physiology and biology.

Often depicted upside down, swimming with mouth forward instead of downward.
Similarities with cnidarians are superficial and not indicative of shared ancestry.
Various species of ctenophores include both benthic and pelagic forms, challenging the notion of them being primarily pelagic.
Ctenophores are not the ancestors of all living animals nor simple forms of life.

Sponges are sessile animals that filter water through their porous bodies, utilizing holes called porocytes.
Exhibit extensive symbiotic relationships with various microorganisms.
Over 8500 recognized species predominantly found in marine settings, with diverse habitats ranging from deep-sea to intertidal zones.

Unique aquiferous system that channels water through the body, facilitating feeding and waste removal.
Choanocyte chambers serve both respiration and reproduction.
Possess more complexity than previously understood; examples include the presence of epithelial tissues, gradational contractions, and even signaling pathways without a nervous system.

Frequently claimed to lack tissue organization—sponges indeed possess epithelia capable of sealing and sensory functions.
Misunderstood development processes where cell types undergo significant cellular migrations and genesis.
Contributions of sponges to the understanding of animal evolution are often overshadowed by their unique and non-bilaterian characteristics, misleadingly simplified to ‘primitive’ forms.

Recognizing biases that affect the understanding of non-bilaterians can inform the evolution of traits, such as the interpretation of tissue layers and nervous systems across different lineages.
Ctenophores and sponges possess complex traits that are independent of bilaterian models, underscoring the need to reassess previously accepted views in animal hierarchy and development.

Continued examinations of non-bilaterian biology are essential in forming a clear picture of metazoan evolution and the evolutionary paths of complex traits.
Understanding diverse biological strategies employed by sponges and ctenophores can provide insight into the evolutionary processes that shaped animal life.

The traditional ladder-like view of animal evolution is challenged by evidence of trait gain and loss across various lineages.
Emphasizing the hidden biology of sponges and ctenophores and acknowledging the richness of their traits provides a more nuanced picture of early animal evolution and challenges outdated assumptions about animal complexity.