Diversity of hippoidean crabs - considering ontogeny, quantifiable morphology, and phenotypic plasticity

Abstract

Representatives of Hippoidea, often called sand crabs or mole crabs, are an ingroup of Anomala. These marine crustaceans inhabit the tropical and subtropical coasts of the world, yet some also appear in temperate climates. Their adults are specialized for digging and living in sandy substrates. Hippoidean zoea-type larvae are planktic and reach large sizes up to a few centimetres. These larvae transform into megalopa larvae, strongly resembling the adult, mediating the transition to the benthic lifestyle of the adult. We reconstructed outlines in dorsal view of over 80 shields of hippoideans, including representatives of Blepharipodidae (sister group to all others), Albuneidae, and Hippidae and including adults, megalopa-type, and zoea-type larvae from all three ingroups. We conducted a morphological analysis on this data using an elliptic Fourier transformation and principal component analysis. We used the results of the analysis to discuss the life history of hippoideans and the special function of megalopae, which often lack emphasis in current research. Early stage zoea larvae, megalopae, and adults show a linear gradient in their morphological development according to our analysis. However, late stage zoea larvae deviate from this pattern, possibly due to their specialization to a long-lasting planktic life. Lastly, we discuss the influence of phenotypic plasticity in hippoidean zoea larvae.

Morphological diversity in true and false crabs reveals the plesiomorphy of the megalopa phase

“…Material used for this study originated from published images and reconstruction drawings in literature, open databases, collections, and museums. Some of the material was documented and published by the authors (for documentation methods, see Eiler et al 50 ) 40 , 51 , 52 . Our material included extant and fossil species of Meiura.…”

Outline analysis as a new method for investigating development in fossil crabs

“…Here, the shield can be retraced with a vector line to create the outline (Figure 2). To eliminate the influence of left-right asymmetry on the data set, we only reconstructed the left or the right half of the shield and then duplicated and mirrored it in anterior-posterior axis and stitched it together to form an entire symmetric shield (Haug et al, 2020;Braig et al, 2021). Biological asymmetry, such as fluctuating asymmetry, has been found in Carcinidae (Spani and Scalici, 2018), but due to its random nature we disregarded it to reduce noise in the data for this analysis.…”

Genetic differentiation of Hippa adactyla based on CO1 gene sequence in Sundaland

“…The distribution of the Hippidae group in the form of pelagic larvae follows ocean currents. Larvae of the Hippidae group have unique characteristics and environmental specialization [5]. Hippidea is distributed from the western Indo-Pacific region to the Atlantic region [6].…”