Читать книгу Life in the Open Ocean - Joseph J. Torres - Страница 109

Siphonophores differ considerably, by suborder, in their ability to move about. The cystonects, which have an apical float but no swimming nectophores to aid in propulsion, are limited to contracting and relaxing their stem for movement. Consequently, their swimming ability is weak at best. However, members of the Cystonectae, particularly Physalia, are drifters par‐excellence (Totton 1960; Woodcock 1971; Mackie 1974), using their float as a sail to cruise the open sea. Physalia has two basic morphs, a left‐hand sailor and right‐hand sailor, mirror images of one another that sail 45° to the left and right of the wind direction (Figure 3.34). The crest of the float provides an important part of the sail; its curvature and stiffness may be adjusted to form a characteristic “sailing posture” for most effective movement (Mackie 1974).

Many of the physonects are capable swimmers, combining a float for buoyancy and a battery of nectophores for propulsion. The genus that has received the most attention is Nanomia, a capable swimmer often observed from submersibles. Mackie et al. (1987) described three swimming modes:

1 Synchronous forward swimming, usually considered an escape response to stimulation of the siphosome, where all the nectophores contract together for one or two cycles and produce a velocity of 20–30 cm s−1, a respectable velocity for any small swimming species.

2 Asynchronous forward swimming, where the nectophores contract in a less coordinated fashion, producing a forward velocity of 8–10 cm s−1. Sometimes the nectophores on each side of a column contract rhythmically to produce a side‐to‐side forward movement.

3 Reverse swimming, usually resulting from a stimulation to the float. In this case contraction of muscles at the velar openings directs the jet anteriorly, propelling the siphonophore to the rear.

Table 3.7 Abundance of siphonophores relative to other gelatinous zooplankton, estimated primarily by direct observation.

Source: Mackie et al. (1987), table 17 (pp. 242–243), with the permission of Academic Press (Elsevier).

Location	Siphonophores	Medusae	Ctenophores	Method	References
Gulf of Mexico (no./10 000 m3)	Physonects	Calycophorans
Spring (4)a	2.0 ± 3.4	0	1.7 ± 2.2	10.2 ± 9.9	Grid	Biggs et al. (1984)
Summer (8)	0	0	0	0	Grid	Biggs et al. (1984)
Fall (11)b	8.8 ± 7.1	<0.8 ± 1.0	<8.7 ± 20.7	36.6 ± 51.0	Grid	Biggs et al. (1984)
Winter (8)c	4.1 ± 2.3	2.6 ± 2.1	3.4 ± 3.5	31.2 ± 53.8	Grid	Biggs et al. (1984)
North Atlantic (no./10 000 m3)	Total	Calycophorans
Temperate (4)	15.2 ± 24.5	27%d	14.2 ± 13.4	38.5 ± 31.9	Grid	Biggs et al. (1981)
Transitional (6)	6.2 ± 10.4	62%d	11.5 ± 9.7	24.7 ± 36.1	Grid	Biggs et al. (1981)
Subtropical (7)	8.0 ± 4.5	90%dd	2.6 ± 3.0	2.1 ± 3.2	Grid	Biggs et al. (1981)
Bahamas (no./10 000 m3)	Total 8.3 ± 13.1	9.0 ± 7.8	52.3 ± 59.9	Grid	Biggs et al. (1981)
Southern California (no./m3)	Polygastric	Eudoxids
Sphaeronectes gracilis	8.3 ± 4.6	6.3 ± 5.4	4.6 ± 1.7	2.5 ± 1.5	Net	Purcell and Kremer (1983)
Muggiaea atlantica	6.7 ± 3.7	17.3 ± 11.5	4.6 ± 1.7	2.5 ± 1.5	Net	Purcell and Kremer (1983)
Apolemia sp. at 450 m	0.2 ± 0.02	—	0.06	0.05 ± 0.02	Submersible	Alldredge et al. (1984)
Strait of Georgia (no./m3)	Polygastric	Eudoxids
Muggiaea atlantica (October)	1.4 ± 0.8	8.0 ± 3.8	0.7 ± 0.5	0.1 ± 0.2	Net	Purcell (1982)
Muggiaea atlantica (November)	2.7 ± 1.8	7.2 ± 4.1	1.4	0.08	Net	Mackie and Mills (1983)
Muggiaea atlantica (November)	2.0 ± 2.6	Common	2.9–10	0.8–5.5	Submersible	Mackie and Mills (1983)

Numbers in parentheses following Gulf of Mexico seasons and North Atlantic zones are the number of SCUBA dives.

^a Agalma, Physophora.

^b Forskalia, Agalma, Athorybia, Cordagalma, Rosacea, Stephanophyes.

^c Forskalia, Agalma, Nanomia, Cordagalma.

^d Diphyes, Chelophyes, Rosacea.

Figure 3.34 Dimorphism of Physalia. (a) Right handed (left sailing); (b) left handed (right sailing).

The calycophorans are the Olympic swimmers of the siphonophores. They lack a pneumophore and are often much smaller than the cystonects and physonects, but are, with a few exceptions, very capable swimmers. In the diphyids particularly, the nectophores have powerful swimming muscles and face directly backward, resembling a miniature spacecraft (Figure 3.27d). Table 3.8 gives the swimming speeds of several siphonophores. The diphyids may cruise using only the smaller posterior nectophore (Bone and Trueman 1982), keeping the larger anterior nectophore in reserve for escape. Keep in mind that for small species such as Chelophyes, the escape swimming velocities are many body lengths per second.