Reproduction in the deep sea Flashcards
Past predictions on how deep-sea organisms reproduce.
- Orton’s Rule (1920) – based on constant temperature he predicted that deep-sea species would NOT undergo seasonal periodicities in breeding, breeding would be CONTINUOUS
- Thorson’s rule (1950) – Mode of early development. Predicted that deep-sea species will always have LOW FECUNDITY with little or NO PELAGIC DEVELOPMENT
- low food availability and a long way from the surface – predicted no larvae.
- Held true till the 1980’s
3 main types of reproductive pattern
1.Continuous reproduction – dominant pattern.
- Typified by production of few, large eggs (600µm to 3.4mm diameter), well supplied by yolk. In gonochoric (separate sexes) species the males are in a state of continuous maturity.
- Some hermaphrodites, Paroriza pallens (simultaneous) and Ophiacantha bidentata (protandric)
- Intermediate development – seen in very few species.
- Best described in Ophiomusium lymani (Gage & Tyler, 1982). Egg size c. 420µm and intermediate fecundity (c.104 eggs).
- Divergence form these hypothesis, a middle size eggbut with higher fcundity
- Seasonal reproduction – unexpected pattern for the deep-sea.
- First proposed by Schoener (1968). Characteristic small eggs (c.100µm), high fecundity and SYNCHRONY of gamete development.
- Observed in Ophiura ljumgmani (ophiuroid), Plutonaster bifrons (asteroid) and Echnius affinis (echinoid) (Tyler et al., 1982; Tyler, 1986,1988)
- Distinct seasonall pattern for reproduction
- Requires really good synchrony for gamete production. Seasonal spawners – planktotrophic larvae.
Give an example of brooding in the deep sea
Alpinists lorioli
Irregular echinoid – Antarctica 600m water depth
Also seen in certain species of regular echinoid and holothurian
Brood pouches – higher maternal investment when energy is available (higher food inputs in the (not sure which) Antarctic area.
Describe the reproduction of a brooding mysid.
Gnathophausia ingens – deep water mysid
- Larvae (up to 350- small brood size) are carried in a brood pouch for up to 15 months
- Breeds only once (semelparous)
Females invest almost 75% of life-time energy in egg-laying and brooding
Exponential, not hyperbolic, growth curve, which may be linked to semelparity – fastest rate of increase occurs towards end of life
- Larger size linked to increased fecundity
Low G/High S predictions
Evolved as response to low food?
Data from other animals trends, on fecundity and egg size
9 species of meso- and bathypelagic fishes
- Mesopelagic: small, vertical migrants, slow growth and early, repeated reproduction
- Bathypelagic: larger, non-migratory, fast growth and late reproduction (possibly semelparous)
- Cyclothone sp. – Sea of Japan
- Do not vertically migrate
- C. alba (MP) – small, gonochoristic
- Reproduces after 2 years, spawn once, few 100 eggs
- C. atraria (BP) – larger, protandrous hermaphrodite, females mature at 5-6yrs, repeated spawning of 1000s of eggs
- Duration of egg and larval stages increases with depth
- Buoyant eggs – ontogenetic migration, risky!! – more fecund
Body size - what are the trade-offs in the deep sea?
Become small to reduce nutritional requirements
Become large to improve foraging ability
- These options co-vary with reproductive strategy
- Lower limit for effective abundances – produce numerous smaller
- individuals
- Gigantism is prevalent at abyssal depths
- Large size = greater fecundity or larger eggs, also more mobile adults
- Large size also implies greater longevity = potential for longer period of sexual maturity
Many deep-sea species reach a large size but food beneath the surface is severely limiting
How is this achieved?
Partitioning of resources is often apparent in the disparities of biomass allocated to males and females
Either numerical differences or size differences (dimorphism)
Merrett 1994 – Deep-sea fishes
sex and sexual dimorphism
Mature females more abundant than males among myctophids, macrourids, halosaurs and notocanths
Skewed sex ratio at hatching and greater female longevity
Greater longevity and continuing growth can produce size dimorphism but…….
Many deep-sea species have a true sexual dimorphism of size
Males mature at smaller sizes
Extreme strategies – deep water Anglerfish
All species have dwarf males
Large olfactory organs
May attach briefly during mating
Some may be permanent – ‘parasitic males’
Gonads of unattached males and unparasitsed females don’t develop
Some females may have more than one male
Sperm storage in 5 tubeworm species, including Riftia
Strategy of internal fertilisation followed by zygote release rather than brooding assures a high level of fertilisation without sacrificing dispersal potential
Sperm storage is an ideal strategy in an environment where periodic cues for gametogenesis and spawning synchrony are limited
Small, yolky, and slightly buoyant eggs develop into non-feeding trochophore larvae that disperse in the plankton for up to several weeks (Young et al., 1996; Marsh et al., 2001)
Larvae of Riftia pachyptila can disperse more than 100km over a 5-week period
Summary
- Reproduction is often a balancing act for majority of marine species
- In the deep-ocean organisms have developed a range of different strategies that all appear successful!
- Previous predictions on deep-sea reproduction, based on stability, have been disproved
- Food limitation and low population densities appear to be the major selective pressures driving reproductive adaptation in the deep-oceans
