A coordinated sequence of distinct flagellar waveforms enables a sharp flagellar turn mediated by squid sperm pH-taxis

Abstract

Animal spermatozoa navigate by sensing ambient chemicals to reach the site of fertilization. Generally, such chemicals derive from the female reproductive organs or cells. Exceptionally, squid spermatozoa mutually release and perceive carbon dioxide to form clusters after ejaculation. We previously identified the pH-taxis by which each spermatozoon can execute a sharp turn, but how flagellar dynamics enable this movement remains unknown. Here, we show that initiation of the turn motion requires a swim down a steep proton gradient (a theoretical estimation of ≥0.025 pH/s), crossing a threshold pH value of ~5.5. Time-resolved kinematic analysis revealed that the turn sequence results from the rhythmic exercise of two flagellar motions: a stereotypical flagellar ‘bent-cane’ shape followed by asymmetric wave propagation, which enables a sharp turn in the realm of low Reynolds numbers. This turning episode is terminated by an ‘overshoot’ trajectory that differs from either straight-line motility or turning. As with bidirectional pH-taxes in some bacteria, squid spermatozoa also showed repulsion from strong acid conditions with similar flagellar kinematics as in positive pH-taxis. These findings indicate that squid spermatozoa might have a unique reorientation mechanism, which could be dissimilar to that of classical egg-guided sperm chemotaxis in other marine invertebrates.

Male Alternative Reproductive Tactics and Associated Evolution of Anatomical Characteristics in Loliginid Squid

“…In contrast, consort sperm that diffuse after release from the spermatangium (Figure 1) presumably evolved associated with the changes in male ARTs within Loliginidae. The loss of swarming in consort sperm, maybe through the evolution of shorter sperm tails (see Iida et al, 2017), could be the result of relaxed selection due to the absence of a sperm storage organ in the mantle cavity, or increased selection due to sperm competition. Parallel mating is usually performed near or during spawning (Iwata et al, 2005; Wada et al, 2005; Buresch et al, 2009), and consorts are frequently replaced in some species (Hanlon et al, 2002; Shashar and Hanlon, 2013), so intense sperm release should guarantee a higher number of fertilizations for the consort male as more oocytes leaving the oviduct would be fertilized.…”

“…Much progress has been made in the last 25 years in understanding the squid mating system with the application of approaches including extensive in situ (e.g., Sauer et al, 1997; Hanlon et al, 2002, 2004; Shashar and Hanlon, 2013; Mather, 2016) and ex situ behavioral studies (e.g., Lin and Chiao, 2018), experimental manipulations of captive specimens (e.g., Iwata et al, 2005; Buresch et al, 2009; Saad et al, 2018), paternity analyses (from ex situ samples: Iwata et al, 2005; Buresch et al, 2009; from in situ samples: Shaw and Boyle, 1997; Shaw and Sauer, 2004; Naud et al, 2016), in vitro experimentation of the functioning of spermatophores (e.g., Iwata et al, 2015; Apostólico and Marian, 2017), spermatology (e.g., Iwata et al, 2011; Hirohashi and Iwata, 2013; Hirohashi et al, 2013, 2016a,b; Iida et al, 2017), gonadal/ejaculate expenditure (e.g., Iwata and Sakurai, 2007; Apostólico and Marian, 2018a; Iwata et al, 2018), and age and development (Apostólico and Marian, 2018b).…”

Regularized Stokeslet segments

“…This corresponds to a slender cylinder of length to radius ratio of /r e = 1.031(300) = 309. Some sperm are known to swim both in straight lines and in circular motion [11] depending on the amount of asymmetry in the curvature of the flagellum [22]. We set out to use our method to produce both behaviors.…”

“…n=0 f n,k T k,k+1 n,−3(11) where we use superscript k, k + 1 to emphasize that the terms are for the segment between y k and y k+1 . Denoting y k − y k+1 by v k and L k = v k , the coefficients f n,k aref 0,k = (y k y T k )f k , f 1,k = (y k v T k + v k y T k )f k + (y k y T k )(f k+1 − f k ) f 2,k = (v k v T k )f k + (v k y T k + y k v T k )(f k+1 − f k ) f 3,k = (v k v T k )(f k+1 − f k ).…”

Whole spermatangia within the seminal receptacles of female chokka squid (Loligo reynaudii d’Orbigny, 1839–1841)

“…bleekeri that the flagellum of sneaker sperm, from the spermatangia attached around the seminal receptacle, is longer than that of consort sperm, from spermatangia placed around the oviduct (Iwata et al, 2011). There are no differences in swimming velocity between consort and sneaker sperm (Iwata et al, 2011), but sneaker sperm form clusters (Hirohashi et al, 2013) and display asymmetrical movement, using their long flagella, while moving along CO 2 gradients (Iida et al, 2017), behaviours proposed to be adapted to reach the seminal receptacle through active swimming (Hirohashi et al, 2016). Loligo reynaudii has a similar reproductive strategy to that of H. bleekeri, with morphological dimorphism in spermatangia between consort and sneaker males (Iwata et al, 2018).…”