A postlarval instar of Phoxichilidium femoratum (Pycnogonida, Phoxichilidiidae) with an exceptional malformation

Abstract

Individuals of the marine chelicerate lineage Pycnogonida (sea spiders) show considerable regenerative capabilities after appendage injury or loss. In their natural habitats, especially the long legs of sea spiders are commonly lost and regenerated, as is evidenced by the frequent encounter of specimens with missing or miniature legs. In contrast to this, the collection of individuals with abnormally developed appendages or trunk regions is comparably rare. Here, we studied a remarkable malformation in a postlarval instar of the species Phoxichilidium femoratum (Rathke, 1799) and describe the external morphology and internal organization of the specimen using a combination of fluorescent histochemistry and scanning electron microscopy. The individual completely lacks the last trunk segment with leg pair 4 and the normally penultimate trunk segment bears only a single aberrant appendage resembling an extension of the anteroposterior body axis. Externally, the proximal units of the articulated appendage are unpaired, but further distally a bifurcation into two equally developed leg‐like branches is found. Three‐dimensional reconstruction of the musculature reveals components of two regular leg muscle sets in several of the proximal articles. This confirms interpretation of the entire appendage as a malformed leg and reveals an externally hidden paired organization along its entire proximodistal axis. To explain the origin of this unique malformation, early pioneering studies on the regenerative potential of pycnogonids are evaluated and (a) an injury‐induced partial fusion of the developing limb buds of leg pair 3, as well as (b) irregular leg regeneration following near complete loss of trunk segments 3 and 4 are discussed. Which of the two hypotheses is more realistic remains to be tested by dedicated experimental approaches. These will have to rely on pycnogonid species with established laboratory husbandry in order to overcome the limitations of the few short‐term regeneration studies performed to date.

The sea spider Pycnogonum litorale overturns the paradigm of the absence of axial regeneration in molting animals

“…3 C ). This transient phenotype shows striking similarities to previous reports of pycnogonids with posteriorly directed bi- or tri-furcating legs ( 30 , 40 ). With the third molt, however, the AT and aberrant leg were shed ( SI Appendix , Fig.…”

“…For visualization of the musculature, F-actin labeling of whole specimens was performed with TRITC-conjugated phalloidin (Sigma-Aldrich, #P1951, 50 μg/mL in PBTx; RRID:AB_2315148) overnight at 4 °C, in adaptation of previous staining protocols for pycnogonids ( 40 ). After inspection of the staining quality by epifluorescence stereomicroscopy, incubation was variably extended to maximally four nights at 4 °C.…”

The visual pathway in sea spiders (Pycnogonida) displays a simple serial layout with similarities to the median eye pathway in horseshoe crabs

“…5.6; FEI Visualization Sciences Group, RRID:SCR_007353). Software tools were applied as previously described (Imaris: [ 143 , 144 ]; Amira: [ 145 ]), including (i) the use of optical clipping planes to remove non-target structures, (ii) the generation of 3D-curved optical sections by combination of several oblique slicers, and (iii) the manual segmentation of structures of interest for improved depiction by selective coloration. Movies were generated in Imaris.…”

Description of Limb Anomalies Resulting from Molt Irregularities in Ammothea hilgendorfi (Pycnogonida: Ammotheidae)

“…Loeb (1905) mentioned a Phoxichilidium femoratum (Rathke, 1799) [50] that regenerated its posterior trunk into a limb-like structure [54], and Scholtz and Brenneis (2016) observed a P. litorale growing an additional leg after an injury to its trunk [55]. Other anomalies in pycnogonids have been documented, including a postlarval instar Nymphonella tapetis Ohshima, 1927 [50], with six legs, including one trifid [56]; a postlarva P. femoratum missing a trunk segment, with the previous one only bearing one abnormal leg [57]; a nine-legged Anoplodactylus petiolatus (Krøyer, 1844) [50] in which the last three legs were fused; a Chaetonymphon spinosum [accepted as Nymphon hirtum Fabricius, 1780 [58]] with a bifurcated abdomen; and another C. spinosum with an aberrant chelifore [24]. Gynandromorphy has also been described [59,60].…”