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Pearls
Pearls Venturing further out into the lagoon I discovered areas populated with colonies of Lambis lambis – all rather large, long-spined, and richly colored specimens – and in other parts Murex ramosus, which were, however, generally badly eroded and not worth taking. Once, coming across a funny little knob in a sandy hollow near some coral, I dug in with my hand – to come up with a live 5-inch Cypraecassis rufa, the first one I had ever found myself anywhere.
On following days I concentrated more on the intriguing mid-lagoon channel which was studded on both sides with remarkably large coral boulders rising to within a few feet of lowest low-tide level from depths of 25 to 30 feet. A prize find was a large, 100mm Cymatium gutturnium, alive and in perfect condition, that had been hiding inside a narrow crevice. Here I also found several Drupina lobata, the curious Indian Ocean form of D. grossularia, which may or may not be a full-fledged species of its own.. They always seemed to occur in pairs. Lambis scorpius was lying exposed on the coral rubble covered sides of the steeply sloping channel in about 20 feet – the limit of my skin-diving prowess – while L. crocata, and L. arthritica were rare and limited to the outer reef ramparts accessible only during minus tides. Such low tides occurred only a few times during my stay. These were the "sea shell days": on the flat, solid reef-plateau rising from the sea for only 15 to 30 minutes Cypraea histrio, C. vitellus, C. carneola, and C. helvola were quite common. C. chinensis I found both under coral in the lagoon and on the reef, all with a beautiful, deep coloring of the base and sides. Most sought-after were C. mappa and C. stolida. I found none myself, but native fishermen had a few for sale, and some fellow holidayers found 3 or 4 C. stolida under coral blocks in the lagoon. There was one specimen that tallied in all respects, for me, with the description of subspecies fluctuans Iredale as given by Messrs. C. C. Woltz and D. B. Becher in their valuable little guide pamphlet "Collecting sea shells in Dar-Es-Salaam." I was unfortunately not able to obtain this specimen from its finder, but could inspect it very thoroughly, and found it to be of apparently sub-adult age.
Certain other sea shells, notably C. teres and C. nucleus, seemed to be more widespread outside the reef. SCUBA divers brought up specimens from 10 to 20 meters depth. I once accompanied them on a diving excursion, but having only goggles, had to remain near the surface above a large coral plateau that rose to within 4 or 5 meters of the surface from surrounding greater depths. It was a breathtaking experience for me. Never had I seen fish in such numbers. Above and between the canyons and slopes of the bizzarely shaped coral gardens literally thousands of fish in all sizes were moving about in an unbelievable pandemonium of color. Once an immense swarm of perhaps one to one-and-a-half foot long silvery fish completely enveloped me, parting in two streams only feet away from my face, and joining again behind my back. In one place, I discovered a large leopard trigger fish which let me approach on short dives to within a few feet. Museum aquaria gladly pay 500 US-Dollars for this rare beauty. The few coral plates I managed to turn over yielded, however, no shells, while the SCUBA divers working in deeper water nearby brought up quite a lot of sea shells.
Pearls When the cilia which fringe the velum are operative, currents are set up which
attract particles to the food groove. These particles are swept clockwise along
the food groove to the mouth and down the esophagus to the style sac. Algal
particles can be seen rotating within the style sac as they are digested by
enzymes from the larval digestive gland. Waste materials are pushed along the
intestine and exit into the mantle cavity behind the head region. The larval heart
aids in the circulation of body fluids. Eyes are located at the bases of the
cephalic tentacles and probablv do little more than perceive light and
dark regions. Many presetllement protoconchs have a beak, which prevents
the mantle cavity from becoming clogged with particles.
In short, this type of information interests a broad spectrum of specialists.
The term 'veliger' denotes a molluscan larval stage found in bivalves and gastropods. The veliger has a velum (L., a veil), a circular or lobed diaphanous fold of tissue located on either side of the head. Around the periphery of the velum beat cilia, which set up currents which effect both food-gathering and locomotion. Bivalves have larval shells which are also bivalved; there are no tentacles in the head region lateral to the eyes, and the velum is usually circular, although it can be bi-lobed or even four-lobed, as in Pinna, the paper oyster.
In gastropods, a variety of protoconch shapes and colors, foot shapes, velar shapes and 'faces' obtain. Some gastropod veligers (certain nudibranchs) do not have exterior protoconchs. Some veligers (bubble shells) do not have cephalic tentacles.
It is important to stress that the veliger stage is only one arbitrarily defined developmental stage in a continuum: egg/sperm –– zygote –– early cleavage stages –– non-motile embryo –– trochophore –– veliger –– veliconcha –– juvenile –– adult –– egg /sperm. Generally, gastropods deposit fertilized eggs in egg capsules on some substratum. In many species, all development through metamorphosis is passed in the capsule and the young hatch as miniature adults. By Thorson's estimate2, 75-85 percent of tropical and subtropical species of gastropods have planktonic veliger stages. Because so many gastropods have planktonic larval stages, the species can extend their geographic ranges and exploit new food supplies and shelter. Thus, the chances of survival of the species is increased. After hatching, veligers may remain planktonic for periods ranging from a few minutes to two or more months.
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