Yeti claws: Cheliped sexual dimorphism and symmetry in deep-sea yeti crabs (Kiwaidae)

Abstract

Yeti crabs (Kiwaidae) are deep-sea hydrothermal vent and methane seep dwelling crustaceans that farm chemosynthetic microbes on their bodies. Sexual dimorphism is a common feature of decapod crustaceans, but little is known about its prevalence in species from deep-sea habitats. We address this knowledge deficit by investigating claw sexual dimorphism and symmetry in the hydrothermal-vent endemic 'Hoff crab', Kiwa tyleri. A total of 135 specimens from the East Scotia Ridge were examined, revealing mean asymmetry indices close to zero with respect to propodus length and height, albeit with a significantly larger number of marginally left-dominant individuals with respect to propodus length, possibly indicative of some task specialisation between claws, or a vestigial ancestral trait. Both male and female claws exhibit positive allometry with increasing carapace length, but males possess significantly larger claws compared with females when accounting for carapace size, exhibiting faster growing propodus length, and broader propodus heights throughout the size distribution. This marked difference is indicative of either male-male competition for mate access, sexual selection, or differential energy allocation (growth vs reproduction) between males and females, as observed in other decapod crustaceans. In contrast, a reanalysis of data for the methane seep inhabiting yeti crab Kiwa puravida revealed no significant difference in claw allometry, indicating a possible lack of similar sexual selection pressures, and highlighting potential key differences in the ecological and reproductive strategies of K. tyleri and K. puravida relating to claw function, microbial productivity and population density. Whether sex differences in claw allometry represents the norm or the exception in Kiwaidae will require the examination of other species in the family. This research enhances our understanding of the behaviour, ecology and evolution of yeti crabs, providing a basis for future studies.

Fig 1. Dorsal view of the known yeti crab (Kiwaidae) species, from Roterman et al.

[28]. Scale bars are approximate and represent 10 mm. A) Kiwa puravida, B) Kiwa gemma, C) Kiwa araonae, D) Kiwa hirsuta, E) Kiwa tyleri, F) Kiwa sp. SWIR.

Fig 2

Female (A) and male (B) Kiwa tyleri type specimens, modified from Thatje et al. [25]. Scale bar is approximate and represents 10 mm.

Fig 3. Measurements of the external morphology of Kiwa tyleri.

CL = carapace length, PL = cheliped propodus length, PH = cheliped propodus height. Scale bar = 10 mm (1 cm).

Fig 4. Box and whisker plot showing asymmetry index (AI) values for cheliped propodus length and height in 109 specimens of Kiwa tyleri.

Red dot = mean; thick horizonal line = median. The box represents the interquartile range (middle 50% of the data) and whiskers extend 1.5 * interquartile range from the lower and upper quartiles, respectively. Black dots represent outliers (outside 1.5 * interquartile range).

Fig 5. Scatter plots generated in Rstudio of dominant cheliped propodus dimensions for 135 specimens of Kiwa tyleri collected from from hydrothermal vents on the East Scotia Ridge in the Southern ocean.

A) propodus length vs carapace length and B) log-transformed data. C) propodus height vs carapace length and D) log-transformed data. Blue dots are females and orange triangles are males. Lines in B) and D) are SMA regressions fitted to the data calculated in the smatr R package.

Fig 6. Scatter plots of cheliped propodus dimensions for 206 specimens of Kiwa puravida taken from Azofeifa-Solano et al.

[44]. A) propodus length vs carapace length and B) log-transformed data. C) propodus height vs carapace length and D) log-transformed data. Blue circles are females and orange hollow triangles are males. Lines in B) and D) are SMA regressions fitted to the data calculated in the smatr R package.