Abstract
This review is a tribute to the remarkable contributions of Thomas Huxley to the biology of tunicates, the likely sister group of vertebrates. In 1851, the great biologist and philosopher published two landmark papers on pelagic tunicates in the Philosophical Transactions of the Royal Society. They were dedicated to the description of the adult anatomy and life cycle of thaliaceans and appendicularians, the pelagic relatives of ascidians. In the first part of this review, we discuss the novel anatomical observations and evolutionary hypotheses made by Huxley, which would have a lasting influence on tunicate biology. We also briefly comment on the more philosophical reflections of Huxley on individuality. In the second part, we stress the originality and relevance of past and future studies of tunicates in the resolution of major biological issues. In particular, we focus on the complex relationship between genotype and phenotype and the phenomenon of developmental system drift. We propose that more than 150 years after Huxley's papers, tunicate embryos are still worth studying in their own right, independently of their evolutionary proximity to vertebrates, as they provide original and crucial insights into the process of animal evolution. Tunicates are still at the forefront of biological research.
Highlights
In 1851, two landmark articles by British zoologist Thomas Huxley were published side by side in the Philosophical Transactions of the Royal Society [1,2]. These pieces of work, one of which is reproduced in this issue, were dedicated to the description of the adult anatomy and life cycle of some of the most enigmatic pelagic invertebrates: the salps, pyrosomes and doliolids, composing the thaliaceans, and the appendicularians
Huxley proposed for the first time that, to salps [3], appendicularians are closely related to ascidians, and belong to the tunicates, considered to be the sister group of vertebrates
Since Thomas Huxley, the focus of tunicate biology has been very productively displaced towards sessile ascidians, as clearly follows from the preceding discussion
Summary
In 1851, two landmark articles by British zoologist Thomas Huxley were published side by side in the Philosophical Transactions of the Royal Society [1,2]. Appendicularians had previously been described by Chamisso [10], Mertens [11] and Quoy & Gaimard [12] who had not recognized the similitude to the other tunicates In his two 1851 articles, written shortly after the return of the Rattlesnake, Huxley provides exquisitely detailed descriptions of the adult anatomy of salps and pyrosomes [1], and of appendicularians and doliolids [2], thereby shedding light on several tunicate organs and their functioning. He in particular described a novel organ, the endostyle, which he thought was a ‘very remarkable distinctive character of the Tunicata’ [1]. A pelagic ascidian ancestor, followed by the independent convergent acquisition 4 of sessility in phlebobranchs and stolidobranchs, can not be formally excluded, evidence for a sessile ancestor for thaliaceans as proposed by Huxley has accumulated over time, and appears most likely [23]
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