The elegant fish is
an electric fish Principle
of Superposition
The
elegant fish in the photograph is the Gymnarchus
niloticus, a native of Africa found in the Nile
River. Gymnarchus has some interesting traits. It swims gracefully with equal
facility either forward or backward. Instead of propelling itself by slashing
its tail sideways, as most fish do, it keeps its spine straight—not only when
swimming straight ahead, but even when turning. Its propulsion is accomplished
by means of the undulations of the fin along its back. Gymnarchus navigates
with great precision, darting after its prey
and
evading obstacles in its path. What is surprising is that it does so just as
precisely when swimming backward. Furthermore, Gymnarchus is nearly blind; its
eyes respond only to extremely bright light.
How then, is it able to locate its prey in the dim light of a muddy river?
السمكة الأنيقة في الصورة هي Gymnarchus niloticus ، وهي إحدى الأسماك الأفريقية الموجودة في نهر النيل. يمتلك Gymnarchus بعض السمات المثيرة للاهتمام. يسبح برشاقة مع سهولة
متساوية سواء للأمام أو للخلف. بدلاً من دفع نفسها عن طريق ربط ذيلها جانبًا، كما
تفعل معظم الأسماك، فإنها تحافظ على عمودها الفقري مستقيماً - ليس فقط عند السباحة
إلى الأمام، ولكن حتى عند الاستدارة. يتم دفعها عن طريق تموجات الزعنفة على طول
ظهرها. يتنقل Gymnarchus بدقة كبيرة، يندفع وراء
فريسته وتجنب العقبات
في طريقها. ما يثير الدهشة هو أنها تفعل ذلك بنفس الدقة عند السباحة للخلف. علاوة
على ذلك، فإن Gymnarchus يكاد يكون أعمى. عيونه
تستجيب فقط للضوء الساطع للغاية.
كيف إذن، هو قادر على تحديد موقع فريسته في الضوء الخافت للوحل نهر؟ (انظر ص 581 للجواب).
it
possesses an unusually large brain, which is believed to help it interpret the
electrical signals.[3] It can make its tail negatively charged with respect to
its head. This produces a symmetrical electric field around its body. Nearby
objects distort this field, and it can sense the distortion on its skin.
يمتلك دماغًا كبيرًا بشكل غير عادي، ويعتقد أنه يساعد في تفسير الإشارات
الكهربائية. يمكنه جعل ذيله مشحونًا سلبًا فيما يتعلق برأسه. ينتج عن هذا مجال
كهربائي متماثل حول جسمه. الأجسام القريبة تشوه هذا المجال، ويمكن أن تشعر بالتشوه
على جلده.
Application
of Electric Fields: Electrolocation
Long
before scientists learned how to detect and measure electric fields, certain
animals and fish evolved organs to produce and detect electric fields.
Gymnurchus niloticus (see the Chapter Opener) has electrical organs running
along the length of its body; these organs set up an electric field around .
When a nearby object distorts the field lines, Gymnarchus detects the change
through sensory receptors, mostly near the head, and responds accordingly. This
extra sense enables the fish to detect prey or predators in muddy streams where
eyes are less useful.
Since
Gymnarchus relies primarily on electrolocation, where slight changes in the
electric field are interpreted as the presence of nearby objects, it is
important that it be able to create the same electric field over and over. For
this reason, Gymnarchus swims by undulating its long dorsal fin while holding
its body rigid. Keeping the backbone straight keeps the negative and positive
charge centers aligned and at a fixed distance apart. A swishing tail would
cause variation in the electric field and that would make electrolocation much
less accurate.