Knowing a Body Well; On the Importance of Studying the Histology of a Stalked Jellyfish

 

Journal of Morphology Cover for Miranda et al. 2013

How well known is the anatomy of most animals? It is probably fair to say that they are pretty well known in a general way, but for most species not known in great detail. An enduring technique for getting a really focused, more thorough view of the anatomy of an organism is histology, an investigation of the microscopic details of its cells and tissues. 

One of the groups I work on with a handful of great collaborators comprises 50 or so species of stalked jellies (Cnidaria: Staurozoa). These animals are most often found living on seaweeds or rocks in cold waters of the world’s oceans, and they were only recently placed in their own class in recognition of their unique position withtin the tree of cnidarian life, probably as the earliest diverging lineage of Medusozoa. Stauromedusae are intriguing because they are bottom-dwelling polyps (somewhat like anemones), but their bodies contain characters much like those seen in free-swimming jellyfishes.

Fig. 1 from Miranda et al. 2013. General view of a stauromedusa. A: Lateral view of living specimen of Haliclystus antarcticus, attached to the red alga Iridaea cordata. B: Oral view of living specimen of H. antarcticus. Abbreviations: am, arm; an, anchor; AR, adradii; cl, calyx; gd, gonad; in, infundibulum; IR, interradii; mn, manubrium; pd, peduncle; pp, perradial pocket; PR, perradii; tc, tentacles. Pictures from AC Morandini.

Led by a PhD student from the University of Sao Paulo, Lucilia Souza Miranda, we recently published a paper presenting a detailed histological examination of one stauromedusa species, Haliclystus antarcticus. [As an aside: this is the same species for which we identified an unknown life stage through the use of DNA barcoding.] By embedding specimens of this species in paraffin, then cutting the animals into hundreds of sections placed onto microscope slides, and staining the thin sections with different dyes, we (mostly Lucilia) were able to put together an unprecedented 3-dimensional view of the different tissues and cell types in the adult bodies of these animals. 

Fig. 2 from Miranda et al. 2013. Internal anatomy of Haliclystus antarcticus. I–III: Longitudinal sections of the body, showing the sites where the cross sections were made. I: interradii; II: perradii; III: adradii; IV: orientation of the cross sections (A–Q). A–Q: Cross sections of the body, from the basal region (A) to the apical region (Q). A: peduncle with four perradial chambers and four longitudinal interradial muscles; B: four chambers start to fuse, and connect to a median aperture; C: apical region of peduncle, with the four chambers fused; D: connection region between calyx and peduncle; E: basal region of calyx with gastrodermis enveloping the interradial longitudinal muscle, forming four septa; F: basal region of four infundibula, within septa; G: enlargement of infundibula, which divides interradial longitudinal muscles into two regions in each septum; H: evagination of gastrodermis forming gastric filaments and gonads; I: gastrodermis of adjacent septa fuse, forming gastrodermis of manubrium and perradial pockets; J: epidermis of adjacent septa start to fuse; K: epidermis of adjacent septa fuse, completing the formation of manubrium and perradial pockets, and anchors are visible (perradial anchors connected to perradial pockets, and interradial anchors not connected, because of septa); L: perradial anchors connected to perradial pockes and ostia at interradii, connecting two adjacent perradial pockets; M: perradial and interradial anchors connected to gastrodermis of calyx, and adjacent perradial pockets connected by ostia; N: division of perradial pockets into eight regions, toward the eight arms, and anchors are visible; O–Q: perradial pockets fully divided toward the eight arms, fusion of subumbrellar epidermis with exumbrellar epidermis at the arms region. Legend: epidermis- black; gastrodermis- red; mesogleagray; longitudinal interradial muscles- blue; vesicles of gonads- green.

The paper contains a number of new observations about the finescale anatomy of this species,  places these observatoins within a thorough review of histological studies of other stauromedusae, and raises a number of questions for further research. For instance, we found a region at the base of secondary tentacles containing many unorganized nematocysts of differing types. These nematocysts have no contact with the external environment, suggesting that they are not functional. Is this a region where the nematocysts are developing, or just where they accumulate? You might be surprised to know how little is known about nematocyst development in different cnidarians given that these structures are the key evolutionary innovation of the group.

Part of Fig. 14 from Miranda et al. 2012. K–L: detail of the unorganized group of nematocysts, between mesoglea and epidermis; gt, gastrodermis; ms, mesoglea; mu, muscle; nm, nematocysts.

But why go through all of this painstaking work if it only raises more questions? Well, that's science. This work is really important to our overall goals of thoroughly understanding the biodiversity of this little known group of organisms. This careful histological study provides a clear and stable point of comparison. Lucilia has made histological preparations from many of the other species in the group, and we have been sequencing as many animals as we can get our hands on. We are beginning to have a great understanding of the evolutionary relationships within Stauromedusae, but if we hope to understand the evolution of the morphology of the group then we need descriptive studies like this one.

by Allen G. Collins
 

Further reading:

Miranda LS, Collins AG & AC Marques. 2013. Internal anatomy of Haliclystus antarcticus (Cnidaria, Staurozoa) with a discussion on histological features used in staurozoan taxonomy. Journal of Morphology. doi: 10.1002/jmor.20185