Synaphobranchidae

3.1 | Synaphobranchidae View in CoL : I. robinsae

Four examples of backward swimming were observed (Table 1; Figure 2). At deployment 1, the eel was feeding on the seafloor with its head beneath the bait support arm of the lander whereupon it escaped backward over approximately two body lengths (Video S1). It then turned to its left by bending the body and swam forward away from the lander. Figure 2 shows one cycle of the backward swimming wave propagating forward from the tail. The head oscillates from side to side over a large amplitude as exemplified by comparison of the image at 0.60 s with the start ( 0.00 s) and end ( 1.44 s) positions. At deployment 2, an individual reversed off the seafloor near the lander for four tail beats before bending its body toward the left to swim forward away from the lander (Video S2). At deployment 3, after attempting to feed on the bait, the eel swam backward, ascending out of view of the camera (Video S3). Possibly, a second species was observed at deployment 4, feeding at the bait and then departing backward for approximately two tail beats before turning by bending

Ref Station Latitude Longitude Depth (m) Family Species Period (s) Hz λ (L)

1 TP2_CR6_4500 0.818 144.303 4500 Synaphobranchidae Ilyophis robinsae 1.44 0.69 0.6 2 TP2_MA6_4500 0.8005 144.299 4484 Synaphobranchidae I. robinsae 1.05 0.95 0.7 3 TP2_OM7_4500 3.9307 144.032 4573 Synaphobranchidae I. robinsae - - 0.75

4 TP3_OM5_4600 6.6435 156.932 4816 Synaphobranchidae Synaphobranchid 1.97 0.51 0.75 5 TP1_OM4_5600 32.0665 148.769 5568 Macrouridae Coryphaenoides yaquinae 1.914 0.52 ≤ 0.5 6 TP2_CR7_4500 3.925 144.014 4588 Macrouridae C. yaquinae 2.81 0.36 0.7

7 TP2_MA5_4900 4.4823 145.883 4937 Macrouridae C. yaquinae 3.46 0.29 0.5 8 TP4_OM2_5400 20.7042 146.272 5445 Macrouridae C. yaquinae 4.85 0.21 0.7 9 BH_CL4_6300 25.142 136.395 6328 Ophidiidae Bassozetus sp. 5.55 0.18 ≤ 0.5

10 TP2_CR7_4500 3.925 144.014 4588 Ophidiidae Bassozetus sp. - - ≤ 0.5 11 TP2_OM5_4900 4.4873 145.866 4944 Ophidiidae Bassozetus sp. 11 0.09 - 12 TP6_CR5_4500 4.6162 146.773 4465 Ophidiidae Bassozetus sp. 2.26 0.44 ≤ 0.5

13 TP2_CR6_4500 0.818 144.303 4500 Ophidiidae Barathrites iris 3.27 0.31 0.7 14 TP2_CR6_4500 0.818 144.303 4500 Ophidiidae B. iris 1.98 0.5 0.6

Note: Period, Hz, and λ refer to the observed backward swimming wave, where λ = wavelength expressed in terms of body length L.

Abbreviations: Ref, reference number of the video; station, lander deployment code.

its body in a C shape to the left and swimming away in a forward direction (Video S4).

Except in deployment 3, where the fish moved out of the field of view, the backward swimming was a short escape or departure maneuver over about two body lengths before the individual turned to swim forward away from the lander. With λ values of approximately 0.7, a complete propulsive wave was evident within the length of the fish.