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

Abstract

Limb anomalies are widespread and diversified in arthropods. From trilobites to insects, they range from the loss to the addition or fusion of legs and may appear congenitally or be induced experimentally (e.g., amputation or injury). Basal chelicerates pycnogonids, or sea spiders, also show deformities. Despite being understudied compared to other arthropods, quite a high diversity of limb malformations has been reported in the literature. The present study reports the leg anomalies of two adult females Ammothea hilgendorfi (Böhm, 1879) observed with duplicated leg podomeres. Both individuals were described ethologically and morphologically. Although the current knowledge on pycnogonids is limited, the anomaly is likely due to a problem in the molting process; the specimens were unable to totally remove their old exuviae, which then stacked after the proximal leg segments. The second specimen also showed other leg deformities, hinting at a problem during the molting process itself. The discussion emphasizes that understanding how pycnogonids normally molt would not only help our understanding of how the abnormal patterns appeared but also put pycnogonids into perspective with other arthropods, a phylum in which they have a key taxonomic position.

Keywords

• pantopoda
• sea spider
• pycnogonid
• morphological variation
• malformation
• molting