Hemichordata

Balanoglossus gigas, from here.

Belongs within: Deuterostomia.
Contains: Cephalodiscidae, Graptolithina.

The Hemichordata are marine invertebrates exhibiting deuterostomous development and usually possessing pharyngeal openings but lacking an endostyle or notochord (Bulman 1970).

I can has mutant larvae?
Published 2 August 2007

Let me introduce you (assuming you’ve not already met) to the giant planktonic larva Planctosphaera pelagica Spengel, 1932.

Planctosphaera pelagica, from van der Horst (1936).

‘Giant’ is, of course, a relative term. The roughly spherical Planctosphaera reaches about 10mm in diameter (van der Horst 1936) or even 25mm (Williamson 2001). Compared to its generally believed closest relative, the tornaria larva of Enteropneusta (acorn worms), this is huge—tornariae may be about a millimetre in size (Bourne, 1889). The intriguing point about Planctosphaera is that the adult form has never been identified. The similarity between Planctosphaera and tornariae means that it is almost universally accepted as a member of the Hemichordata (see the comparison to the left between the two, again from van der Horst), but it is different enough that the adult may not be a typical acorn worm (not to mention the size…) Williamson (2001) provides one exception—he maintains that Planctosphaera as currently known is the adult form. However, this interpretation is connected with Williamson’s unusual theory of ‘larval transfer’, which is not widely accepted*, and jibes with the fact that known Planctosphaera do not have any sort of gonads or other reproductive structure (van der Horst 1936).

*Williamson maintains that distinct adult and larval forms in various animals result from hybridisation between animals with distinct bauplans, with one stage in the life cycle resembling one parent and one resembling the other. For instance, caterpillars and other insect larvae would be derived from a hybridisation between a direct-developing winged insect and an onychophoran-like animal. First, try to imagine a butterfly mating with an onychophoran. Then, try to stop imagining a butterfly mating with an onychophoran.

Internal anatomy of Planctosphaera pelagica, from van der Horst (1936).

One imaginative interpretation of Planctosphaera that does have a lot going for it is the idea that Planctosphaera is a normal tornaria larva that has become hypertrophied by a long planktonic period. The reference for this idea is Hart (1994)—unfortunately, I haven’t been able to obtain the paper in question and I am unclear whether this is meant to be an adaptive change, or whether Planctosphaera represents a pathological form of a normal tornaria that has failed to develop in the normal way. Such pathologies are not unknown—Temereva et al. (2006) describe a giant phoronid larva (which even possesses rudimentary gonads!) that they interpret as such, and note the existence of giant larvae of ceriantharians, sipunculids and even fish.

Of course, the adult form of Planctosphara could still be out there somewhere, lurking in the ooze at the bottom of the oceanic abyss. As the relatively recent description of Torquarator Holland et al. (2005) demonstrated, we may have only scratched the surface of enteropneust diversity.

Acanthastus luniewskii
Published 21 January 2010
Acanthastus luniewskii. Plate from Kozłowski (1948).

The five known species of Acanthastus were all described by Roman Kozłowski in his legendary 1949 monograph Les graptolithes et quelques nouveaux groupes d’animaux du Tremadoc de la Pologne which has done more than any other book to make me wish that I could read French. It was in this book that Kozłowski established the currently accepted relationship between the Palaeozoic graptolites and the recent pterobranchs based on his descriptions of early sessile graptolites from the early Ordovician of Poland. The Acanthastus remains were found in the same deposits, but establishing their affinities was just a little more difficult.

Acanthastus was represented by small flattened circular chitinous fossils (up to a few millimetres across) with a central dorsal opening surrounded by a ring of upwards-pointing spines. This central opening was crossed by tubes connected to the spines with a central cavity underlying this opening. Beneath and around this central cavity were a number of further chambers, divided by internal walls from the central cavity. The dorsal surface surrounding the central opening was roughened by a covering of small projections.

Diagram showing the internal structure of an idealised Acanthastus specimen, from Kozłowski (1949).

Kozłowski interpreted Acanthastus as a colonial animal, with each fossil representing a collection of individuals (presumably one in each chamber). I’m not entirely convinced by this—except for the central cavity, each of the chambers appears to have been entirely sealed off from the outside world, nor were the chambers connected in any way that would have allowed for the ready transfer of nutrients from the outside. But then, I’ve never been able to understand how on earth blastoids were able to survive either.

Because all the other taxa described by Kozłowski (1949) were graptolites or likely graptolite relatives, Acanthastus has always been associated with graptolites as well (for instance, appearing on a Wikipedia list of graptolite genera). However, Kozłowski himself was much less confident, noting that “Comme nous ne connaissons aucun organisme fossile ou vivant dont la morphologie ressemblerait à celle des Acanthastida il n’est pas possible de préciser actuellement leur position taxonomique” (“As we do not know any fossil or living organism whose morphology resembles that of Acanthastida it is not possible to currently define their taxonomic position”). After ruling out a relationship with coelenterates, bryozoans or tunicates, Kozłowski tentatively suggested that Acanthastus might be related to graptolites and pterobranchs by a vague similarity in skeletal structure, while admitting that its overall morphology was vastly different. His comment about relationships between Acanthastus and graptolites was that “Le plus qu’on pourrait admettre c’est que les Acanthastida appartiennent au même embranchement que ces derniers” (“The most one could admit is that Acanthastida belong to the same embranchement [phylum?] as the latter”), hardly a ringing endorsement. Unfortunately, no further study has been conducted on Acanthastus since Kozłowski (1949). I don’t know how much of Kozłowski’s original material still remains (see below), nor do any further specimens seem to have been recorded.

As well as being one of the most significant publications in the history of graptolite research, Kozłowski’s 1949 monograph has to have one of the most dramatic publication histories. Though Kozłowski seems to have finished composing it in 1938, its publication was delayed for ten years by a small distraction known as World War II* (Kielan-Jaworowska & Urbanek 1978). The Palaeontology department of the Warsaw University where Kozłowski worked was firebombed by the Germans in 1939 and all the material held in it destroyed. Some of Kozłowski’s collection, as well as the monograph manuscript, was saved because it had been hidden in the basement of the Warsaw Seismological Observatory a few days before the destruction of the Palaeontology Department. When Kozłowski was able to return to the Observatory over a month later (German troops had been occupying it over that time), he found the place ransacked and all his material apparently lost. It wasn’t until a few months later that he found his specimens and part of the manuscript among the ruins of the University, while a colleague found the remainder of the manuscript buried in a snowdrift.

*There’s a fantastic story about British television and World War II. Television broadcasting was halted in Britain after the declaration of war. Supposedly, the first broadcast after the end of the war was introduced with the words “As we were saying before we were so rudely interrupted…”

In 1944, hundreds of thousands of Warsaw’s inhabitants, including Kozłowski, were forced to flee the city. Again, the monograph manuscript was hidden, this time in the central heating pipes of Kozłowski’s house. Kozłowski returned in 1945 to find the house ruined but the manuscript still safely hidden and waiting to be published. Also surviving the war intact were the negative for the monograph’s plates which had been forwarded to Paris shortly before the war’s beginning.

Which all kind of puts any problems you might have with reviewer delays into perspective, doesn’t it?

Systematics of Hemichordata
<==Hemichordata [Enteropneusta, Enteropneusti]
|--+--Pterobranchia [Graptolithoidea]CGS00
| | |--CephalodiscidaeM14
| | `--GraptolithinaM14
| `--HarrimaniidaeC05
| | i. s.: Stereobalanus canadensis (Spengel 1893)C05
| | Xenopleura vivipara Gilchrist 1925C05
| |--+--Harrimania planktophilus Cameron 2002MM13, C05
| | `--ProtoglossusMM13
| | |--P. graveolens Giray & King 1996C05
| | `--P. koehleri (Caullery & Mesnil 1900)C05
| `--+--Saxipendium [Saxipendiidae]MM13
| | `--S. coronatumHC05
| `--Saccoglossus Schimkewitsch 1892MM13, B70
| | i. s.: *S. mereschkowskii (Wagner 1885) [=Balanoglossus mereschkowskii]B70
| | S. pusillus (Ritter 1902)C05
| |--S. cambriensisCGS00
| `--+--S. barkleyiiCGS00
| `--S. kowalevskii (Agassiz 1873)CGS00, C05
`--+--Torquarator Holland, Clague et al. 2005HC05 [TorquaratoridaeTG19]
| `--*T. bullocki Holland, Clague et al. 2005HC05
|--SpengelidaeC05
| |--Spengelia Willey 1898C05
| |--Willeyia Punnett 1903C05
| |--Schizocardium Spengel 1893C05
| | `--S. braziliense (Spengel 1893)C05
| `--Glandiceps Spengel 1893C05
| |--G. abyssicolaHC05
| `--G. talabotiPP64
`--PtychoderidaeC05
|--Ptychodera Eschscholtz 1825MM13, B70
| |--*P. flava Eschscholtz 1825B70
| `--P. bahamensis (Spengel 1893)C05
`--+--GlossobalanusMM13
| |--G. berkeleyi Willey 1931C05
| |--G. minutusGD00
| `--G. polybranchioporusC05
`--Balanoglossus Delle Chiaje 1829MM13, B70
|--*B. clavigerus Delle Chiaje 1829B70
|--B. aurantiacus (Girard 1853)C05
|--B. australiensisHS01
|--B. carnosusCGS00
|--B. gigasC05
|--B. kowalevskiiB89
|--‘Tornaria’ kroehniiB89
|--B. minutusB89
`--B. salmoneus [incl. B. salmoneus var. sarniensis]B89

Hemichordata incertae sedis:
Acanthastus Kozłowski 1949 [Acanthastida]M14
|--*A. luniewskii Kozłowski 1949K49
|--A. czyzewskii Kozłowski 1949K49
|--A. kobyleckii Kozłowski 1949K49
|--A. mazureki Kozłowski 1949K49
`--A. mizerjai Kozłowski 1949K49
Tornaria hubbardiC05
Planctosphaera [Planctosphaeroidea, Planctosphaeromorpha]W01
`--P. pelagica Spengel 1932W01
Megaderaion sinemuriense Arduini et al. 1981B93
Oesia Walcott 1911TG19, H62
`--*O. disjuncta Walcott 1911H62
Spartobranchus Caron, Conway Morris & Cameron 2013TG19, CCMC13
`--*S. tenuis (Walcott 1911) [=Ottoia tenuis]CCMC13

*Type species of generic name indicated

References

[B93] Benton, M. J. 1993. Basal deuterostomes (chaetognaths, hemichordates, calcichordates, cephalochordates and tunicates). In: Benton, M. J. (ed.) The Fossil Record 2 pp. 529–535. Chapman & Hall: London.

[B89] Bourne, G. C. 1889. On a tornaria found in British seas. Journal of the Marine Biological Association of the United Kingdom 2 (1): 63–68, pls 7–8.

[BJ00] Budd, G. E., & S. Jensen. 2000. A critical reappraisal of the fossil record of the bilaterian phyla. Biological Reviews 75: 253–295.

[B70] Bulman, O. M. B. 1970. Graptolithina with sections on Enteropneusta and Pterobranchia. In: Teichert, C. (ed.) Treatise on Invertebrate Paleontology pt V 2nd ed. pp. V1–V149. The Geological Society of America, Inc.: Boulder (Colorado), and the University of Kansas: Lawrence (Kansas).

[C05] Cameron, C. B. 2005. A phylogeny of the hemichordates based on morphological characters. Canadian Journal of Zoology 83: 196–215.

[CGS00] Cameron, C. B., J. R. Garey & B. J. Swalla. 2000. Evolution of the chordate body plan: new insights from phylogenetic analyses of deuterostome phyla. Proceedings of the National Academy of Sciences of the USA 97 (9): 4469–4474.

[CCMC13] Caron, J.-B., S. Conway Morris & C. B. Cameron. 2013. Tubicolous enteropneusts from the Cambrian period. Nature 495: 503–506.

[GD00] Giribet, G., D. L. Distel, M. Polz, W. Sterrer & W. C. Wheeler. 2000. Triploblastic relationships with emphasis on the acoelomates and the position of Gnathostomulida, Cycliophora, Plathelminthes, and Chaetognatha: a combined approach of 18S rDNA sequences and morphology. Systematic Biology 49: 539–562.

Hart, M. W., R. L. Miller, & L. P. Madin. Form and feeding mechanism of a living Planctosphaera pelagica (phylum Hemichordata). Marine Biology 120: 521–533.

[HS01] Hayward, B. W., A. B. Stephenson, M. S. Morley, W. M. Blom, H. R. Grenfell, F. J. Brook, J. L. Riley, F. Thompson & J. J. Hayward. 2001. Marine biota of Parengarenga Harbour, Northland, New Zealand. Records of the Auckland Museum 37: 45–80.

[HC05] Holland, N. D., D. A. Clague, D. P. Gordon, A. Gebruk, D. L. Pawson & M. Vecchione. 2005. ‘Lophenteropneust’ hypothesis refuted by collection and photos of new deep-sea hemichordates. Nature 434: 374–376.

Horst, C. J. van der. 1936. Planctosphaera and Tornaria. Quarterly Journal of Microscopical Science: 605–613.

[H62] Howell, B. F. 1962. Worms. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt W. Miscellanea: Conodonts, Conoidal Shells of Uncertain Affinities, Worms, Trace Fossils and Problematica pp. W144–W177. Geological Society of America, and University of Kansas Press.

Kielan-Jaworowska, Z., & A. Urbanek. 1978. Dedication: Roman Kozłowski (1889–1977). Acta Palaeontologica Polonica 23 (4): 415–425.

[K49] Kozłowski, R. 1949. Les graptolithes et quelques nouveaux groupes d’animaux du Tremadoc de la Pologne. Palaeontologica Polonica 3: i–xii, 1–235.

[M14] Maletz, J. 2014. The classification of the Pterobranchia (Cephalodiscida and Graptolithina). Bulletin of Geosciences 89 (3): 477–540.

[MM13] Mitchell, C. E., M. J. Melchin, C. B. Cameron & J. Maletz. 2013. Phylogenetic analysis reveals that Rhabdopleura is an extant graptolite. Lethaia 46: 34–56.

[PP64] Peres, J. M., & J. Picard. 1964. Nouveau manuel de bionomie benthique de la mer Mediterranee. Recueil des Travaux de la Station Marine d’Endoume, Bulletin 31 (27): 5–137.

Temereva, E. N., V. V. Malakhov & A. N. Chernyshev. 2006. Giant actinotroch, a larva of Phoronida from the South China Sea: the giant larva phenomenon. Doklady Akademii Nauk 410 (5): 712–715 (translated: Doklady Biological Sciences 410: 410–413).

[TG19] Topper, T. P., J. Guo, S. Clausen, C. B. Skovsted & Z. Zhang. 2019. A stem group echinoderm from the basal Cambrian of China and the origins of Ambulacraria. Nature Communications 10: 1366.

[W01] Williamson, D. I. 2001. Larval transfer and the origins of larvae. Zoological Journal of the Linnean Society 131: 111–122.

Williamson, D. I. 2006. Hybridization in the evolution of animal form and life-cycle. Zoological Journal of the Linnean Society 148: 585–602.

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