Electrophysiological and Motor Responses to Chemosensory Stimuli in Isolated Cephalopod Arms
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Denison University Denison Digital Commons Denison Faculty Publications 2020 Electrophysiological and Motor Responses to Chemosensory Stimuli in Isolated Cephalopod Arms Kaitlyn E. Fouke Heather J. Rhodes Follow this and additional works at: https://digitalcommons.denison.edu/facultypubs Part of the Biology Commons Reference: Biol. Bull. 238: 1–11. (February 2020) © 2020 The University of Chicago DOI: 10.1086/707837 Electrophysiological and Motor Responses to Chemosensory Stimuli in Isolated Cephalopod Arms * KAITLYN E. FOUKE AND HEATHER J. RHODES The Grass Lab, Marine Biological Laboratory, 7 MBL Street, Woods Hole, Massachusetts 02543; and Department of Biology, Denison University, 100 West College Street, Granville, Ohio 43023 Abstract. While there is behavioral and anatomical evi- D. pealeii, this technique could be attempted with other ceph- dence that coleoid cephalopods use their arms to “taste” alopod species, as comparative questions remain of interest. substances in the environment, the neurophysiology of chemo- sensation has been largely unexamined. The range and sensi- Introduction tivity of detectable chemosensory stimuli, and the processing of chemosensory information, are unknown. To begin to ad- Coleoid cephalopods, the soft-bodied octopods and deca- dress these issues, we developed a technique for recording pods that make up the vast majority of extant cephalopods, have neurophysiological responses from isolated arms, allowing complex nervous systems with remarkable cognitive and sen- us to test responses to biologically relevant stimuli. We tested sory abilities that evolved separately from vertebrate nervous arms from both a pelagic species (Doryteuthis pealeii) and a systems (Budelmann, 1995, 1996; Mather and Kuba, 2013; benthic species (Octopus bimaculoides) by attaching a suc- Hanlon and Messenger, 2018). Their distinct genetics and neu- tion electrode to the axial nerve cord to record neural activity ral organization, therefore, make them fascinating organisms in response to chemical stimuli. Doryteuthis pealeii arms to study (Albertin et al., 2015; Liscovitch-Brauer et al., 2017; showed anecdotal responses to some stimuli but generally O’Brien et al., 2018). The coleoid cephalopods that have been did not tolerate the preparation; tissue was nonviable within examined for chemosensory abilities, including various species minutes ex vivo. Octopus bimaculoides arms were used suc- of octopus, squid, and cuttlefish, detect chemical cues in their cessfully, with tissue remaining healthy and responsive for environment through multiple chemosensory organs, includ- several hours. Arms responded strongly to fish skin extract, ing olfactory pits located near the eye (Woodhams and Mes- glycine, methionine, and conspecific skin extract but not to senger, 1974; Gilly and Lucero, 1992; Budelmann, 1996; Mobley cephalopod ink or seawater controls. Motor responses were et al., 2008; Polese et al., 2016), chemoreceptors on the lips and also observed in response to detected stimuli. These results mouth parts (Graziadei, 1965; Emery, 1975), and chemorecep- suggest that chemosensory receptor cells on O. bimaculoides tors on the suckers of the arms and tentacles (Graziadei, 1962, arms were able to detect environmentally relevant chemicals 1964b; Santi, 1975). and drive local motor responses within the arm. Further explo- Chemosensation in the arms and tentacles is particularly in- ration of potential chemical stimuli for O. bimaculoides arms, teresting because of the organization of the axial nerve cords as well as investigations into the neural processing within the that run down the length of each. Unlike vertebrate peripheral arm, could enhance our understanding of how this species uses nerves, which consist only of axons, the axial nerve cords that its arms to explore its environment. While not successful in run down each arm or tentacle consist of a continuous series of ganglia (apparent from swellings along the nerve cord), with Received 17 October 2019; Accepted 19 December 2019; Published extensive local circuitry (including sucker ganglia that lie out- online 17 February 2020. side the nerve cord; for diagrams see Graziadei, 1962; Rowell, Both authors contributed equally to this work. 1963), resembling a spinal cord more than a nerve (Graziadei, * To whom correspondence should be addressed. Email: rhodesh@denison 1962; Rowell, 1963; Budelmann and Young, 1985; Mather .edu. Abbreviations: ASW, artificial seawater; FSW; filtered seawater; MBL, and Kuba, 2013; Hanlon and Messenger, 2018). This underlies Marine Biological Laboratory. the relative autonomy of arm function, where local sensory in- Online enhancement: video. formation can evoke motor responses within an arm even 1 2 K. E. FOUKE AND H. J. RHODES when disconnected from the brain or other arms (e.g.,Rowell, the arms or suckers of the males, although no receptor or re- 1963; Nesher et al.,2014).Afferentpathwaysalsosendsen- sponse pathway has been identified yet. Squid touch con- sory information to the brain, where responses can be coordi- specifics with their arms in aggressive and reproductive con- nated across arms and behavioral patterns altered based on texts, as well as touching prey items (Budelmann, 1996; Hanlon additional sensory input, such as from the visual system et al., 2013; Hanlon and Messenger, 2018). The possibility that (Budelmann and Young, 1987; Budelmann, 1995; Hanlon and arm chemoreceptors are used in such interactions has yet to be Messenger, 2018). explored. Ink, which can act as a conspecific alarm cue in ceph- Morphology and function of cephalopod arms and tenta- alopods (Gilly and Lucero, 1992; Wood et al., 2008; Stau- cles vary considerably between species (Budelmann, 1996; dinger et al., 2011; Derby, 2014), has been shown to trigger Hanlon and Messenger, 2018); thus, the extent and function responses in chemosensory receptor cells in the olfactory pits of chemosensation in arms and tentacles surely vary as well. of Doryteuthis opalescens (Lucero et al., 1992); but it is not Cephalopods are diverse, occupying many different ecologi- known whether it is also detected by chemoreceptors on other cal niches within the ocean that place different demands on parts of the body. their sensory systems (Budelmann, 1996; Nixon and Young, Despite initial observations of sucker chemoreceptors 2003; Hanlon and Messenger, 2018). Benthic octopods, such decades ago (Graziadei, 1962, 1964b; Santi, 1975), our un- as Octopus bimaculoides used in this study, forage for food derstanding of how different cephalopods use sucker chemo- by reaching their arms into crevices and under rocks and us- sensation remains sparse. In contrast to research on mechano- ing chemosensation and somatosensation to identify prey reception and muscular control (Gutfreund et al., 2006; (Budelmann, 1996; Hanlon and Messenger, 2018). During for- Sumbre et al., 2006; Kier, 2016), there is little known about aging, cuttlefish are known to manipulate their environment the physiological coding and neural processing of chemical with their arms, such as moving sand around or jetting water information. In this study, the physiological and behavioral to expose prey, which requires recognition of their prey and responses to chemical stimuli were recorded from isolated the associated environment and which could involve chemo- arms from D. pealeii (longfin inshore squid) (formerly known sensation (Mather and Kuba, 2013). Pelagic squid, such as as Loligo pealeii Lesueur, 1821) and O. bimaculoides (Cali- Doryteuthis pealeii studied here, use vision as the dominant fornia two-spot octopus) (Pickford and McConnaughey, 1949). sense for hunting; but they use their arms to grasp prey, as well We tested stimuli inspired by previous studies, including amino as in reproductive interactions (Hanlon et al., 2013; Hanlon acids, fish skin extract, D. pealeii egg casing extract, O. bima- and Messenger, 2018). Thus, the density and specificity of che- culoides skin extract, D. pealeii ink, and O. bimaculoides ink moreceptors would be expected to vary between these species. (Chase and Wells, 1986; Lucero et al., 1992; Mobley et al., Indeed, anatomic descriptions of putative chemoreceptive cells 2008; Wood et al., 2008; Cummins et al., 2011; Walderon on suckers report higher densities of such cells on the suckers et al., 2011; Hassenklover et al., 2012; Nesher et al., 2014). of benthic octopods than pelagic squid (Graziadei, 1962, 1964a; We hoped to establish an isolated arm protocol that would en- Santi, 1975). able further research, and we hypothesized that both cephalo- When vision is unavailable, octopus foraging behavior is pods would sense and respond to environmentally relevant dependent on arm chemotactile sensation, reaching into dark chemicals through arm chemoreceptors. crevices and probing other parts of their surroundings (Budel- mann, 1996; Walderon et al., 2011; Mather and Kuba, 2013). In behavioral chemosensation studies, octopuses have been Materials and Methods shown to distinguish between hydrochloric acid, quinine, and Experimental animals sucrose, indicating “taste-by-touch” chemosensory capabilities (Wells, 1963). Octopuses can also detect and respond to chem- Adult longfin inshore squid (Doryteuthis (Amerigo) pealeii icals dissolved in seawater, although the anatomic location of (Lesueur, 1821)) were collected by trawl from the Vineyard the chemoreceptors is unclear in these studies (Chase and Sound near Woods Hole, Massachusetts, by the Marine Biolog- Wells, 1986; Walderon et al., 2011). Additionally,