Eye Evolution and Comparative Photoreception (10.1)

Practice fill-in-the-blank flashcards covering the evolutionary origins, structural adaptations, photoreceptor cell lineages, and physiological signaling mechanisms of animal visual systems.

According to Land and Nilsson (2002), ancestral opsin photopigments diverged into two major receptor-cell specializations for improved sensitivity: __________ photoreceptors and rhabdomeric photoreceptors.

ciliary

In organisms possessing ciliary photoreceptors, compound eyes evolved independently in polychaete tubeworms and __________.

arc clams

In organisms with rhabdomeric photoreceptors, single-chambered eyes evolved in cephalopods, snails, polychaete lateral eyes, and __________.

spiders

While the fish eye features an inverse retina, the cephalopod eye possesses an __________ retina.

everse

A major anatomical distinction between fish and cephalopod eyes is that a cornea is present in fish eyes, but __________ in cephalopod eyes.

absent

The lens of a fish eye is formed from layers of transparent fibre cells, whereas the lens of a cephalopod eye is formed by __________.

cellular fusions

In the dark, cation channels in vertebrate photoreceptors remain open, generating a dark current that keeps the cell membrane partially depolarized at $-10$ to __________.

$$-40\,mV$$

When exposed to light, vertebrate photoreceptors hyperpolarize, whereas invertebrate rhabdomeric photoreceptors __________.

depolarize

During vertebrate light reception, closing of cation channels reduces dark current, hyperpolarizes the membrane, and causes neurotransmitter release to be __________.

reduced

The Cambrian explosion, which coincided with the rapid evolution of eyes providing spatial vision, occurred approximately $570$ to __________.

$$530\text{ mya}$$

A conservative theoretical estimate by Nilsson and Pelger calculates that the evolution of a complex lens eye from a flat photoreceptive patch requires approximately __________ generations.

$$360{,}000$$

The observation that different eye designs evolved convergently across unrelated phyla suggests that eye structure is driven primarily by the __________ rather than phylogeny.

need for vision

The simplest functional eye type, exemplified by larval trematode worms, is the eye spot, which consists of a single photoreceptor shielded by a __________.

pigment cell

In turbellarian flatworms such as Beklemischeviella contorta, photoreceptors with microvilli assembled into rhabdoms are housed inside a __________.

pigment cup

Some species of turbellarian worms with pigment pit eyes enhance rudimentary image formation through __________ lensing.

mitochondrial

In molluscan eye evolution, the limpet Patella represents an ancestral form from which abalone and Nautilus followed the pinhole route, while land snails and Littorina followed the __________ route.

lens

The cephalopod Nautilus lives at depths of approximately $300\text{ metres}$ and uses an unlensed __________ eye measuring $10\,mm$ in diameter.

pinhole

In Nautilus, pupil diameter fluctuates in response to ambient light within a range of $0.4\,mm$ to __________.

$$2.8\,mm$$

Because a pinhole aperture severely restricts incoming light, the retinal image in Nautilus is approximately __________ times dimmer than that in a fish eye of comparable dimensions.

$$400$$

At a pupil diameter of $0.4\,mm$, the angular subtense of a photoreceptor in Nautilus is __________, yielding low spatial resolution similar to insect compound eyes.

$$2.3^\circ$$

Box jellyfish (Cubozoa) feature specialized sensory clubs known as rhopalia, each containing a total of __________ eyes.

$$6$$

Each cubozoan rhopalium possesses 4 pigment pit eyes and __________ lens eyes.

$$2$$

To minimize spherical aberration, the lenses of box jellyfish eyes have a graded __________.

refractive index

Box jellyfish are believed to lack color vision because they possess only a single __________.

visual pigment

In the cubozoan rhopalium depicted below, the weighted gravity-sensing organ located below the lower lens eye is the __________.

statolith

As illustrated in the comparative diagram, the octopus retina is direct (everse) with light reaching the __________ photoreceptors directly, whereas the cod retina is inverted.

rhabdomeric