Reproduction
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  Embryonic development
  Embryonic development is dealt with in this section, while topics of MATE SELECTION & COPULATION, EGG-LAYING, HATCHING & LARVAL LIFE, SETTLEMENT & METAMORPHOSIS, SETTLEMENT CUES, and ONTOGENETIC DEVELOPMENT OF BEHAVIOUR are considered elsewhere.
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Research study 1
 

photograph of egg string of an aeolid nudibranchThe gelatinous mass surrounding the eggs of nudibranchs and other invertebrates functions to protect the embryos from adverse physical conditions, predators, and parasites. In another way of looking at it, the gel also reduces the period of exposure of free-living eggs and embryos to the risks of planktonic life.  On the negative side, the gel creates potential physical barriers to diffusion of oxygen and metabolic waste products (CO2 and ammonia), and interferes with the escape of the larvae after they hatch.

Eggs of an unidentified aeolid nudibranch 10X

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Research study 2
 

details of embryonic development of sea hares Aplysia californicaDetails of development of Aplysia californica through metamorphosis are shown in the accompanying schematic.  Thirteen stages are described by the author, of which the important ones are Stage 1 (Day 1, newly hatched, shell diameter = 125µm, culture at 22oC), Stage 7 (Day 34-35, metamorphosis, 400µm), Stage 11 (Day 51, rhinophores appear, 1000µm), and Stage 13 (Day 120, maturity: egg-laying, 5000µm).  The author provides information on, and drawings of, torsion/detorsion.  Kriegstein 1977 J Exp Zool 199: 275.

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Research study 3
 

Studies on development within egg masses of several opisthobranch species at Friday Harbor Laboratories, Washington suggest that diffusion of oxygen may be a limiting factor to growth.  One indication of this is that some veligers, especially ones located centrally in globose egg masses, have delayed hatching and hatch with shorter shells.  Also, pH’s are lower in older egg capsules near the centre of the mass, indicating that intracapsular stirring resulting from ciliary beating of the embryos does not compensate for their greater metabolic rates.  The authors suggest that oxygen may become limiting for development before accumulation of wastes becomes limiting.  Strathmann & Strathmann 1995 J Mar Biol Assn UK 75: 413; see also Strathmann & Strathmann 1989 p. 201 In, Reproduction, Genetics and distributions of marine organisms (Ryland & Tyler, eds), Olsen & Olsen, Fredensborg, Denmark; also Chaffee & Strathmann 1984 J Exp Mar Biol Ecol 84: 73.

NOTE  species used in the study are 2 cephalaspideans Melanochlamys diomedea and Haminaea vesicula, and a dorid Doris montereyensis

 

 

 

 

Dorid nudibranch Doris (Archidoris) montereyensis with egg ribbon 1.2X

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Research study 4
 

If the embryos compete for oxygen diffusing into the gel, then greater spacing of the embryos within the gel should enhance delivery of oxygen.  But the gel is costly to manufacture.  A model of the diffusion process developed by scientists at Friday Harbor Laboratories, Washington predicts that gelatinous mass thickness will scale inversely with the square root of embryo density.  Thus, with increase in mass thickness, embryo density must decrease (and gel voume increase) disproportionately if a consistent level of diffusion of oxygen is to be maintained.  A comparison of gelatinous egg masses in several species of gastropods in the San Juan Islands shows, in fact, that thicker egg masses have disproportionately more gel volume relative to density of embryos, as the researchers predict.  Moreover, inclusion of sea-urchin embryos in artificial agarose gels shows that development is faster when there is more gel per embryo.  A trade-off exists, then, between increased protection for eggs and embryos from a thicker gel, and a disproportionately increased cost in parental investment.  Lee & Strathmann 1998 The Amer Nat 151: 293.

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Research study 5
 

graph showing metabolic rate of embryos of the nudibranch Tritonia diomedea both free and in egg capsules at different stages of developmentphotograph of nudibranch Tritonia diomedea courtesy Russ Wyeth and Owen WoodwardThe inclusion of multiple (up to hundreds in some species) embryos in a single capsule may also affect metabolic rate by interference with swimming or by increased competition for oxygen diffusing across the capsule membrane.  By the simple expedient of experimentally freeing embryos from their capsules and comparing their oxygen consumption with embryos freed from the gel but still within their capsules, the diffusion-inhibiting effects of the capsule walls can be estimated.  Such experiments on egg masses of Tritonia diomedea at Friday Harbor Laboratories, Washington indicate that free-living late-stage (veliger) embryos respire at a rate about twice that of embryos still in their capsules (see graph upper Right).

Other experiments by the authors indicate that this does not owe to a diffusion-inhibiting effect of the capsule wall; in fact, oxygen gradients across the capsule wall are negligible.  Rather, they suggest alternative possibilities: 1) intra-capsule competition for space reduces larval activity, 2) some type of activity-suppressing metabolite produced by the encapsulated larvae may not be freely diffusible through the capsule wall, and 3) a substance of a “tranquilising” nature may be present in the egg-capsule fluid.  A fourth possibility, that the embryos compete for a resource, oxygen, that is in short supply, is not mentioned by the authors, and the implication is that although diffusion through the gel is inhibited, theoretically there is ample oxygen available on the inner boundary layer of the capsule for all embryos. 

graph showing distribution of oxygen within an intact egg mass of the nudibranch Tritonia diomedea incomparison with that in isolated capsules and a late veliger stage of developmentFinally, oxygen concentrations in intact egg masses and in freed capsules indicate an almost 3-fold reduction in levels within the egg mass, indicating that oxygen diffusion is greatly restricted through the gel (see graph lower Right).  The authors note that most egg masses (of nudibranchs) are long and coil-shaped (as in T. diomedea), or ribbon-shaped, thus minimising diffusion distances.  A concluding point made by the authors is that suppressed metabolism of embryos within their capsules may be energetically advantageous.  Thus, larvae hatch with higher energy content (i.e., more reserves) than they would if they were to metabolise at high free-larval rates throughout encapsulated development.  Moran & Woods 2007 J Exp Biol 210: 722. Photo of Tritonia courtesy Russ Wyeth and Owen Woodward; photos below courtesy of the authors.


NOTE
  the gelatinous tubes (strings) of an egg mass of T. diomedea are about 3mm in diameter and may be 1m in length.  Each capsule contains 15-150 embryos

 

Multiple embryos of 90um size are contained within each egg capsule (800um diameter) of a 2-day-old egg string.

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