Observations on the physiology and integumentary structure of the Antarctic pycnogonid Decolopoda austratis

Abstract

The giant Antarctic pycnogonid Decolopoda australis Eights takes up oxygen by diffusion across the integument, particularly of the legs. The circulatory system is feeble and haemolymph pressure changes are induced by leg movements during locomotion rather than by cardiac action. Heart rate is about30–40 beats min‐1 between 1 and 5 °C; it becomes irregular above 5 °C and ceases (reversibly) at6–7 °C. The integumentary structure appears to facilitate gaseous exchange. Although the cuticle (c. 200 μm thick) is chitinous, it is perforated (over 35% of the internal surface) by circular/ ellipsoidal tissue‐filled pits which are separated from the external environment by thin layers of chitinous cuticle no more than4–6 μm thick. The pits occur in all parts of the body and appendages (except arthrodial membranes). It is suggested that the primary function of the tissue within the pits is to form a route for gaseous diffusion, which bypasses the relatively thick impermeable chitinous layers of the rest of the cuticle. Calculations suggest that the pits reduce resistance to gaseous diffusion by about 90%. To function as a respiratory surface, the general body surface has to be kept clean. The cleaning action of the ovigers appears to be effective, except in the regions between the contiguous lateral processes of the body where detrital material accumulates.

The eyes of a “nobody”,Anoplodactylus petiolatus (Pantopoda; Anoplodactylidae)

“…Since the anatomy of pantopods makes a cleaning of the ocelli with the help of the extremities improbable, the influence of a chemical process is suggested. "Slit glands" which might represent multifunctional organs (Davenport et al, 1987;Tomaschko & Briickmann, 1990;Tomaschko, 1992) are present elsewhere on the cuticle. Pantopods which live in dimmed light conditions use their tapetum to increase light capture.…”

Fine Structure of the ‘Slit Organs’ of the Pycnogonid Anoplodactylus petiolatus (Anoplodactylidae)

“…Hence, they do not suffice to establish a mere gland character for the slit organs. The idea of Tomaschko and Buckmann (1990) and Tomaschko (1992) that, not the cuticle in general (Davenport et al 1987), but the slit organs in particular, are involved in bidirectional transport of small molecules can therefore not be rejected unequivocally. However, an active pore opening mechanism or an 'intake valve' would be required (see above).…”

“…In addition to the pores of the slit organs, extensive cuticular pitting is found in Pantopoda. Davenport et al (1987) have shown that pits, i.e. tissue-filled cavities at the inner side of the cuticle, represent over 35% of the internal surface of Decolopoda australis.…”

“…Tomaschko and Biickmann (1990) and Tomaschko (1992) have shown that excretion and the uptake of small molecules are also accomplished by the integument. They surmised that the slit-like pores are involved in these general secretory and resorbing functions while gaseous diffusion is thought to be carried out by the deadend pits (Davenport et al 1987). In addition to the slit organs specialized cuticular glands, e.g.…”

Why might they be giants? Towards an understanding of polar gigantism

“…In particular, although cold temperatures and plentiful oxygen may have allowed the repeated evolution of gigantic Antarctic pycnogonids, there has been superimposed on those events a process of coadaptation by different, linked components of the respiratory system, and tradeoffs among multiple costs and benefits of having cuticles of some particular permeability. Pycnogonids obtain oxygen via diffusion directly across the chitinous exoskeleton, and diffusion is facilitated through pits and channels that make up >30% of the cuticular surface (Davenport et al, 1987). The lack of any size-dependent effects of hypoxia on performance might be explained by selection acting to reduce the 'excess' oxygen diffusion capacity of small polar pycnogonids, perhaps to increase resistance to predators or buckling forces, or to protect them from excessively high levels of internal oxygen, which could produce damaging reactive oxygen species.…”